From Andrew Gierth <andrew@erlenstar.demon.co.uk>
Organization none at all
Date 20 Apr 1997 21:44:05 +0100
Summary This posting contains answers to frequently-asked questions regarding
programming in the Unix environment.
Keywords unix faq
Newsgroups comp.unix.programmer,comp.answers,news.answersMessage-ID <auto-cup-faq-000005-19970420214405@erlenstar.demon.co.uk>
Archive-Name: unix-faq/programmer/faq
Comp-unix-programmer-Archive-Name: faq
URL: http://www.erlenstar.demon.co.uk/unix/faq_toc.html
Posting-Frequency: every 2 weeks
Copyright: Collection Copyright (C) 1997 Andrew Gierth.
Last-Modified: 1997/04/20 07:18:05
Version: 1.12
==============================================================================
About this FAQ
**************
$Id: rawfaq.texi,v 1.12 1997/04/20 07:18:05 andrew Exp $
This FAQ was originally begun by Patrick Horgan in May 1996; I took it over
after it had been lying idle for several months. I've reorganised it a bit
and added some stuff; I still regard it as 'under development'.
Comments, suggestions, additions, corrections etc. should be sent to the
maintainer at: <andrew@erlenstar.demon.co.uk>.
A hypertext version of this document is available on the WWW. The home site
is located at `http://www.erlenstar.demon.co.uk/unix/faq_toc.html'.
This document is available by FTP from the news.answers archives at
rtfm.mit.edu and its many mirror sites worldwide. The official archive name
is `unix-faq/programmer/faq'. Sites which also archive *.answers posts by
group should also carry the file under the `comp.unix.programmer' directory.
Other sources of information are not listed here. You can find pointers to
other FAQs, books, source code etc. in the regular [READ ME FIRST] posting
that should appear weekly in comp.unix.programmer. Administrivia regarding
newsgroup conduct, etc., are also found there; I want to reserve this
document specifically for technical Q's and A's.
All contributions have been edited by the maintainer, therefore any errors
or omissions are my responsibility rather than that of the contributor.
This FAQ is now maintained as Texinfo source; I'm generating a raw text
version for Usenet using the `makeinfo' program, and an HTML version using
`texi2html'.
Contributors in no particular order:
Andrew Gierth <andrew@erlenstar.demon.co.uk>
Patrick J. Horgan <patrick@broadvision.com>
Stephen Baynes <baynes@ukpsshp1.serigate.philips.nl>
James Raynard <jraynard@dial.pipex.com>
Michael F. Quigley <michael@cybernetics.net>
Daniel Brandt <dbrandt@crl.com>
Steve Badrich <sbadrich@mcimail.com>
Ken Pizzini <ken@halcyon.com>
Thamer Al-Herbish <shadows@whitefang.com>
Nick Kew <nick.kew@pobox.com>
Dan Abarbanel <dabarban@madge.com>
Billy Chambless <billy@cast.msstate.edu>
Walter Briscoe <walter@wbriscoe.demon.co.uk>
Jim Buchanan <jbuchana@iquest.net>
Dave Plonka <Dave.Plonka@mfa.com>
Collection Copyright (C) 1997 Andrew Gierth. This document may be
distributed freely on Usenet or by email; it may be archived on FTP or WWW
sites that mirror the news.answers archives, provided that all reasonable
efforts are made to ensure that the archive is kept up-to-date. (This
permission may be withdrawn on an individual basis.) It may not be
published in any other form, whether in print, on the WWW, on CD-ROM, or in
any other medium, without the express permission of the maintainer.
List of Questions
*****************
1. Process Control
1.1 Creating new processes: fork()
1.1.1 What does fork() do?
1.1.2 What's the difference between fork() and vfork()?
1.1.3 Why use _exit rather than exit in the child branch of a fork?
1.2 Environment variables
1.2.1 How can I get/set an environment variable from a program?
1.2.2 How can I read the whole environment?
1.3 How can I sleep for less than a second?
1.4 How can I get a finer-grained version of alarm()?
1.5 How can a parent and child process communicate?
1.6 How do I get rid of zombie processes?
1.6.1 What is a zombie?
1.6.2 How do I prevent them from occuring?
1.7 How do I get my program to act like a daemon?
1.8 How can I look at process in the system like ps does?
1.9 Given a pid, how can I tell if it's a running program?
1.10 What's the return value of system/pclose/waitpid?
1.11 How do I find out about a process' memory usage?
1.12 Why do processes only ever increase in size?
1.13 How do I change the name of my program (as seen by 'ps')?
1.14 How can I find a process' executable file?
2. General File handling (including pipes and sockets)
2.1 How to manage multiple connections?
2.1.1 How do I use select()?
2.1.2 How do I use poll()?
2.1.3 Can I use SysV IPC at the same time as select or poll?
2.2 How can I tell when the other end of a connection shuts down?
2.3 Best way to read directories?
2.4 How can I find out if someone else has a file open?
2.5 How do I 'lock' a file?
2.6 How do I find out if a file has been updated by another process?
2.7 How does the 'du' utility work?
2.8 How do I find the size of a file?
2.9 How do I expand '~' in a filename like the shell does?
2.10 What can I do with named pipes (FIFOs)?
2.10.1 What is a named pipe?
2.10.2 How do I create a named pipe?
2.10.3 How do I use a named pipe?
2.10.4 Can I use a named pipe across NFS?
2.10.5 Can multiple processes write to the pipe simultaneously?
2.10.6 Using named pipes in applications
3. Terminal I/O
3.1 How can I make my program not echo input?
3.2 How can I read single characters from the terminal?
3.3 How can I check and see if a key was pressed?
3.4 How can I move the cursor around the screen?
3.5 What are pttys?
3.6 How to handle a serial port or modem?
4. System Information
4.1 How can I tell how much memory my system has?
4.2 How do I check a user's password?
4.2.1 How do I get a user's password?
4.2.2 How do I get shadow passwords by uid?
4.2.3 How do I verify a user's password?
5. Miscellaneous programming
5.1 How do I compare strings using wildcards?
5.1.1 How do I compare strings using filename patterns?
5.1.2 How do I compare strings using regular expressions?
5.2 What's the best way to send mail from a program?
5.2.1 The simple method: /bin/mail
5.2.2 Invoking the MTA directly: /usr/lib/sendmail
5.2.2.1 Supplying the envelope explicitly
5.2.2.2 Allowing sendmail to deduce the recipients
6. Use of tools
6.1 How can I debug the children after a fork?
6.2 How to build library from other libraries?
6.3 How to create shared libraries / dlls?
6.4 Can I replace objects in a shared library?
1. Process Control
******************
1.1 Creating new processes: fork()
==================================
1.1.1 What does fork() do?
--------------------------
#include <sys/types.h>
#include <unistd.h>
pid_t fork(void);
The `fork()' function is used to create a new process from an existing
process. The new process is called the child process, and the existing
process is called the parent. You can tell which is which by checking the
return value from `fork()'. The parent gets the child's pid returned to
him, but the child gets 0 returned to him. Thus this simple code
illustrate's the basics of it.
pid_t pid;
switch (pid = fork())
{
case -1:
/* Here pid is -1, the fork failed */
/* Some possible reasons are that you're */
/* out of process slots or virtual memory */
perror("The fork failed!");
break;
case 0:
/* pid of zero is the child */
/* Here we're the child...what should we do? */
/* ... */
/* but after doing it, we should do something like: */
_exit(0);
default:
/* pid greater than zero is parent getting the child's pid */
printf("Child's pid is %d\n",pid);
}
Of course, one can use `if()... else...' instead of `switch()', but the
above form is a useful idiom.
Of help when doing this is knowing just what is and is not inherited by the
child. This list can vary depending on Unix implementation, so take it
with a grain of salt. Note that the child gets COPIES of these things, not
the real thing.
Inherited by the child from the parent:
* process credentials (real/effective/saved UIDs and GIDs)
* environment
* stack
* memory
* open file descriptors (note that the underlying file positions are
shared between the parent and child, which can be confusing)
* close-on-exec flags
* signal handling settings
* nice value
* scheduler class
* process group ID
* session ID
* current working directory
* root directory
* file mode creation mask (umask)
* resource limits
* controlling terminal
Unique to the child:
* process ID
* different parent process ID
* Own copy of file descriptors and directory streams.
* process, text, data and other memory locks are NOT inherited.
* process times, in the tms struct
* resource utilizations are set to 0
* pending signals initialized to the empty set
* timers created by timer_create not inherited
* asynchronous input or output operations not inherited
1.1.2 What's the difference between fork() and vfork()?
-------------------------------------------------------
Some systems have a system call `vfork()', which was originally designed as
a lower-overhead version of `fork()'. Since `fork()' involved copying the
entire address space of the process, and was therefore quite expensive, the
`vfork()' function was introduced (in 3.0BSD).
*However*, since `vfork()' was introduced, the implementation of `fork()'
has improved drastically, most notably with the introduction of
'copy-on-write', where the copying of the process address space is
transparently faked by allowing both processes to refer to the same
physical memory until either of them modify it. This largely removes the
justification for `vfork()'; indeed, a large proportion of systems now lack
the functionality of `vfork()' completely. For compatibility, though, there
may still be a `vfork()' call present, that simply calls `fork()' without
attempting to emulate the `vfork()' semantics.
As a result, it is *very* unwise to actually make use of any of the
differences between `fork()' and `vfork()'. Indeed, it is probably unwise
to use `vfork()' at all, unless you know exactly *why* you want to.
The basic difference between the two is that when a new process is created
with `vfork()', the parent process is temporarily suspended, and the child
process borrows the parent's address space. This strange state of affairs
continues until the child process either exits, or calls `execve()', at
which point the parent process continues.
This means that the child process of a `vfork()' must be *extremely*
careful to avoid unexpectedly modifying variables of the parent process. In
particular, the child process must *not* return from the function
containing the `vfork()' call, and it must *not* call `exit()' (if it needs
to exit, it should use `_exit()'; actually, this is also true for the child
of a normal `fork()').
(There are some deviant systems, e.g. SCO OpenServer, that have a `vfork()'
that is distinct from `fork()', but only emulates part of the original
`vfork()' - in this case, it suspends the parent process, but doesn't share
the address space with the child. I have no idea why they thought this
would be useful.)
1.1.3 Why use _exit rather than exit in the child branch of a fork?
-------------------------------------------------------------------
There are a few differences between `exit()' and `_exit()' that become
significant when `fork()', and especially `vfork()', is used.
The basic difference between `exit()' and `_exit()' is that the former
performs clean-up related to user-mode constructs in the library, and calls
user-supplied cleanup functions, whereas the latter performs only the
kernel cleanup for the process.
In the child branch of a `fork()', it is normally incorrect to use
`exit()', because that can lead to stdio buffers being flushed twice, and
temporary files being unexpectedly removed. In C++ code the situation is
worse, because destructors for static objects may be run incorrectly.
(There are some unusual cases, like daemons, where the *parent* should call
`_exit()' rather than the child; the basic rule, applicable in the
overwhelming majority of cases, is that `exit()' should be called only once
for each entry into `main'.)
In the child branch of a `vfork()', the use of `exit()' is even more
dangerous, since it will affect the state of the *parent* process.
1.2 Environment variables
=========================
1.2.1 How can I get/set an environment variable from a program?
---------------------------------------------------------------
Getting the value of an environment variable is done by using `getenv()'.
#include <stdlib.h>
char *getenv(const char *name);
Setting the value of an environment variable is done by using `putenv()'.
#include <stdlib.h>
int putenv(char *string);
The string passed to putenv must *not* be freed or made invalid, since a
pointer to it is kept by `putenv()'. This means that it must either be a
static buffer or allocated off the heap. The string can be freed if the
environment variable is redefined or deleted via another call to `putenv()'.
Remember that environment variables are inherited; each process has a
separate copy of the environment. As a result, you can't change the value
of an environment variable in another process, such as the shell.
Suppose you wanted to get the value for the `TERM' environment variable.
You would use this code:
char *envvar;
envvar=getenv("TERM");
printf("The value for the environment variable TERM is "
if(envvar)
{
printf("%s\n",envvar);
}
else
{
printf("not set.\n");
}
Now suppose you wanted to create a new environment variable called `MYVAR',
with a value of `MYVAL'. This is how you'd do it.
static char envbuf[256];
sprintf(envbuf,"MYVAR=%s","MYVAL");
if(putenv(envbuf))
{
printf("Sorry, putenv() couldn't find the memory for %s\n",envbuf);
/* Might exit() or something here if you can't live without it */
}
1.2.2 How can I read the whole environment?
-------------------------------------------
If you don't know the names of the environment variables, then the
`getenv()' function isn't much use. In this case, you have to dig deeper
into how the environment is stored.
A global variable, `environ', holds a pointer to an array of pointers to
environment strings, each string in the form `"NAME=value"'. A `NULL'
pointer is used to mark the end of the array. Here's a trivial program to
print the current environment (like `printenv'):
#include <stdio.h>
extern char **environ;
int main()
{
char **ep = environ;
char *p;
while ((p = *ep++))
printf("%s\n", p);
return 0;
}
In general, the `environ' variable is also passed as the third, optional,
parameter to `main()'; that is, the above could have been written:
#include <stdio.h>
int main(int argc, char **argv, char **envp)
{
char *p;
while ((p = *envp++))
printf("%s\n", p);
return 0;
}
However, while pretty universally supported, this method isn't actually
defined by the POSIX standards. (It's also less useful, in general.)
1.3 How can I sleep for less than a second?
===========================================
The `sleep()' function, which is available on all Unixes, only allows for a
duration specified in seconds. If you want finer granularity, then you need
to look for alternatives:
* Many systems have a function `usleep()'
* You can use `select()' or `poll()', specifying no file descriptors to
test; a common technique is to write a `usleep()' function based on
either of these (see the comp.unix.questions FAQ for some examples)
* If your system has itimers (most do), you can roll your own `usleep()'
using them (see the BSD sources for `usleep()' for how to do this)
* If you have POSIX realtime, there is a `nanosleep()' function
Whichever route you choose, it is important to realise that you may be
constrained by the timer resolution of the system (some systems allow very
short time intervals to be specified, others have a resolution of, say,
10ms and will round all timings to that). Also, as for `sleep()', the delay
you specify is only a *minimum* value; after the specified period elapses,
there will be an indeterminate delay before your process next gets
scheduled.
1.4 How can I get a finer-grained version of alarm()?
=====================================================
Modern Unixes tend to implement alarms using the `setitimer()' function,
which has a higher resolution and more options than the simple `alarm()'
function. One should generally assume that `alarm()' and
`setitimer(ITIMER_REAL)' may be the same underlying timer, and accessing it
both ways may cause confusion.
Itimers can be used to implement either one-shot or repeating signals;
also, there are generally 3 separate timers available:
`ITIMER_REAL'
counts real (wall clock) time, and sends the `SIGALRM' signal
`ITIMER_VIRTUAL'
counts process virtual (user CPU) time, and sends the `SIGVTALRM'
signal
`ITIMER_PROF'
counts user and system CPU time, and sends the `SIGPROF' signal; it is
intended for interpreters to use for profiling.
Itimers, however, are not part of many of the standards, despite having
been present since 4.2BSD. The POSIX realtime extensions define a similar,
but different, function: `settimer()'. There are also functions defined to
query the resolution of POSIX timers.
1.5 How can a parent and child process communicate?
===================================================
A parent and child can communicate through any of the normal inter-process
communication schemes (pipes, sockets, message queues, shared memory), but
also have some special ways to communicate that take advantage of their
relationship as a parent and child.
One of the most obvious is that the parent can get the exit status of the
child.
Since the child inherits file descriptors from its parent, the parent can
open both ends of a pipe, fork, then the parent close one end and the child
close the other end of the pipe. This is what happens when you call the
`popen()' routine to run another program from within yours, i.e. you can
write the the file descriptor returned from popen() and the child process
sees it as its stdin, and you can read from the file descriptor and see
what the program wrote to it's stdout.
Also, the child process inherits memory segments mmapped anonymously (or by
mmapping the special file `/dev/zero') by the parent; these shared memory
segments are not accessible from unrelated processes.
1.6 How do I get rid of zombie processes?
=========================================
1.6.1 What is a zombie?
-----------------------
When a program forks and the child finishes before the parent, the kernel
still keeps some of its information about the child in case the parent
might need it - for example, the parent may need to check the child's exit
status. To be able to get this information, the parent calls `wait()'; when
this happens, the kernel can discard the information.
In the interval between the child terminating and the parent calling
`wait()', the child is said to be a "zombie" (if you do 'ps', the child
will have a 'Z' in its status field to indicate this). Even though it's not
running, it's still taking up an entry in the process table. (It consumes
no other resources, but some utilities may show bogus figures for e.g. CPU
usage; this is because some parts of the process table entry have been
overlaid by accounting info to save space.)
This is not good, as the process table has a fixed number of entries and it
is possible for the system to run out of them. Even if the system doesn't
run out, there is a limit on the number of processes each user can run,
which is usually smaller than the system's limit. This is one of the
reasons why you should always check if `fork()' failed, by the way!
If the parent terminates without calling wait(), the child is "adopted" by
`init', which handles the work necessary to cleanup after the child. (This
is a special system program with process ID 1 - it's actually the first
program to run after the system boots up).
1.6.2 How do I prevent them from occuring?
------------------------------------------
You need to ensure that your parent process calls `wait()' (or `waitpid()',
`wait3()', etc.) for every child process that terminates; or, on some
systems, you can instruct the system that you are uninterested in child
exit states.
Another approach is to `fork()' *twice*, and have the immediate child
process exit straight away. This causes the grandchild process to be
orphaned, so the init process is responsible for cleaning it up. For code
to do this, see the function `fork2()' in the examples section.
To ignore child exit states, you need to do the following (check your
system's manpages to see if this works):
struct sigaction sa;
sa.sa_handler = SIG_IGN;
#ifdef SA_NOCLDWAIT
sa.sa_flags = SA_NOCLDWAIT;
#else
sa.sa_flags = 0;
#endif
sigemptyset(&sa.sa_mask);
sigaction(SIGCHLD, &sa, NULL);
If this is successful, then the `wait()' functions are prevented from
working; if any of them are called, they will wait until *all* child
processes have terminated, then return failure with `errno == ECHILD'.
The other technique is to catch the SIGCHLD signal, and have the signal
handler call `waitpid()' or `wait3()'. See the examples section for a
complete program.
1.7 How do I get my program to act like a daemon?
=================================================
Here's the steps to become a daemon.
1. `fork()' so the parent can exit, this returns control to the command
line or shell invoking your program. This step is required so that
the new process is guaranteed not to be a process group leader. The
next step, `setsid()', fails if you're a process group leader.
2. `setsid()' to become a process group and session group leader. Since a
controlling terminal is associated with a session, and this new
session has not yet acquired a controlling terminal our process now
has no controlling terminal, which is a Good Thing for daemons.
3. `fork()' again so the parent, (the session group leader), can exit.
This means that we, as a non-session group leader, can never regain a
controlling terminal.
4. `chdir("/")' to ensure that our process doesn't keep any directory in
use. Failure to do this could make it so that an administrator
couldn't unmount a filesystem, because it was our current directory.
[Equivalently, we could change to any directory containing files
important to the daemon's operation.]
5. `umask(0)' so that we have complete control over the permissions of
anything we write. We don't know what umask we may have inherited.
[This step is optional]
6. `close()' fds 0, 1, and 2. This releases the standard in, out, and
error we inherited from our parent process. We have no way of knowing
where these fds might have been redirected to. Note that many daemons
use `sysconf()' to determine the limit `_SC_OPEN_MAX'. `_SC_OPEN_MAX'
tells you the maximun open files/process. Then in a loop, the daemon
can close all possible file descriptors. You have to decide if you
need to do this or not. If you think that there might be
file-descriptors open you should close them, since there's a limit on
number of concurrent file descriptors.
7. Establish new open descriptors for stdin, stdout and stderr. Even if
you don't plan to use them, it is still a good idea to have them open.
The precise handling of these is a matter of taste; if you have a
logfile, for example, you might wish to open it as stdout or stderr,
and open `/dev/null' as stdin; alternatively, you could open
`/dev/console' as stderr and/or stdout, and `/dev/null' as stdin, or
any other combination that makes sense for your particular daemon.
Almost none of this is necessary (or advisable) if your daemon is being
started by `inetd'. In that case, stdin, stdout and stderr are all set up
for you to refer to the network connection, and the `fork()'s and session
manipulation should *not* be done (to avoid confusing `inetd'). Only the
`chdir()' and `umask()' steps remain as useful.
1.8 How can I look at process in the system like ps does?
=========================================================
You really *don't* want to do this.
The most portable way, by far, is to do `popen(pscmd, "r")' and parse the
output. (pscmd should be something like `"ps -ef"' on SysV systems; on BSD
systems there are many possible display options: choose one.)
In the examples section, there are two complete versions of this; one for
SunOS 4, which requires root permission to run and uses the `kvm_*'
routines to read the information from kernel data structures; and another
for SVR4 systems (including SunOS 5), which uses the `/proc' filesystem.
It's even easier on systems with an SVR4.2-style `/proc'; just read a
psinfo_t structure from the file `/proc/PID/psinfo' for each PID of
interest. However, this method, while probably the cleanest, is also
perhaps the least well-supported. (On FreeBSD's /proc, you read a
semi-undocumented printable string from `/proc/PID/status'; I don't know
about Linux.)
1.9 Given a pid, how can I tell if it's a running program?
==========================================================
Use `kill()' with 0 for the signal number.
There are four possible results from this call:
* `kill()' returns 0
- this implies that a process exists with the given PID, and the
system would allow you to send signals to it. It is
system-dependent whether the process could be a zombie.
* `kill()' returns -1, `errno == ESRCH'
- either no process exists with the given PID, or security
enhancements are causing the system to deny its existence. (On
some systems, the process could be a zombie.)
* `kill()' returns -1, `errno == EPERM'
- the system would not allow you to kill the specified process.
This means that either the process exists (again, it could be a
zombie) or draconian security enhancements are present (e.g. your
process is not allowed to send signals to *anybody*).
* `kill()' returns -1, with some other value of `errno'
- you are in trouble!
The most-used technique is to assume that success or failure with `EPERM'
implies that the process exists, and any other error implies that it
doesn't.
1.10 What's the return value of system/pclose/waitpid?
======================================================
The return value of `system()', `pclose()', or `waitpid()' doesn't
seem to be the exit value of my process... or the exit value is
shifted left 8 bits... what's the deal?
The man page is right, and so are you! If you read the man page for
`waitpid()' you'll find that the return code for the process is encoded.
The value returned by the process is normally in the top 16 bits, and the
rest is used for other things. You can't rely on this though, not if you
want to be portable, so the suggestion is that you use the macros provided.
These are usually documented under `wait()' or `wstat'.
Macros defined for the purpose (in `<sys/wait.h>') include (stat is the
value returned by `waitpid()'):
`WIFEXITED(stat)'
Non zero if child exited normally.
`WEXITSTATUS(stat)'
exit code returned by child
`WIFSIGNALED(stat)'
Non-zero if child was terminated by a signal
`WTERMSIG(stat)'
signal number that terminated child
`WIFSTOPPED(stat)'
non-zero if child is stopped
`WSTOPSIG(stat)'
number of signal that stopped child
`WIFCONTINUED(stat)'
non-zero if status was for continued child
`WCOREDUMP(stat)'
If WIFSIGNALED(stat) non-zero this is non-zero if core dumped
1.11 How do I find out about a process' memory usage?
=====================================================
todo
1.12 Why do processes only ever increase in size?
=================================================
When you free memory back to the heap with free(), on almost all systems
that *doesn't* reduce the memory usage of your program. The memory 'free'd
is still part of the process' address space, and will be used to satisfy
future `malloc()' requests.
If you really need to free memory back to the system, look at using
`mmap()' to allocate private anonymous mappings. When these are unmapped,
the memory really is released back to the system.
Of course, if your program increases in size when you think it shouldn't,
you may have a 'memory leak' - a bug in your program that results in unused
memory not being freed.
1.13 How do I change the name of my program (as seen by 'ps')?
==============================================================
On BSDish systems, the `ps' program actually looks into the address space
of the running process to find the current `argv[]', and displays that.
That enables a program to change its 'name' simply by modifying `argv[]'.
On SysVish systems, the command name and usually the first 80 bytes of the
parameters are stored in the process' u-area, and so can't be directly
modified. There may be a system call to change this (unlikely), but
otherwise the only way is to perform an `exec()', or write into kernel
memory (dangerous, and only possible if running as root).
Some systems (notably Solaris) may have two separate versions of `ps', one
in `/usr/bin/ps' with SysV behaviour, and one in `/usr/ucb/ps' with BSD
behaviour. On these systems, if you change `argv[]', then the BSD version
of `ps' will reflect the change, and the SysV version won't.
Check to see if your system has a function `setproctitle()'.
1.14 How can I find a process' executable file?
===============================================
This would be a good candidate for a list of "Frequently Unanswered
Questions", because the fact of asking the question usually means that the
design of the program is flawed :-)
You can make a "best guess" by looking at the value of `argv[0]'. If this
contains a `/', then it is probably the absolute or relative (to the
current directory at program start) path of the executable. If it does not,
then you can mimic the shell's search of the `PATH' variable, looking for
the program. However, success is not guaranteed, since it is possible to
invoke programs with arbitrary values of `argv[0]', and in any case the
executable may have been renamed or deleted since it was started.
The most common reason people ask this question is in order to locate
configuration files with their program. This is considered to be bad form;
directories containing executables should contain *nothing* except
executables, and administrative requirements often make it desirable for
configuration files to be located on different filesystems to executables.
A less common, but more legitimate, reason to do this is to allow the
program to call `exec()' *on itself*; this is a method used (e.g. by some
versions of `sendmail') to completely reinitialise the process (e.g. if a
daemon receives a `SIGHUP').
2. General File handling (including pipes and sockets)
******************************************************
See also the Sockets FAQ, available at:
`http://kipper.york.ac.uk/~vic/sock-faq/'
`ftp://rtfm.mit.edu/pub/usenet/news.answers/unix-faq/socket'
2.1 How to manage multiple connections?
=======================================
I have to monitor more than one (fd/connection/stream) at a time. How
do I manage all of them?
Use `select()' or `poll()'.
Note: `select()' was introduced in BSD, whereas `poll()' is an artifact of
SysV STREAMS. As such, there are portability issues; pure BSD systems may
still lack `poll()', whereas some older SVR3 systems may not have
`select()'. SVR4 added `select()', and the Posix.1g standard defines both.
`select()' and `poll()' essentially do the same thing, just differently.
Both of them examine a set of file descriptors to see if specific events
are pending on any, and then optionally wait for a specified time for an
event to happen.
[Important note: neither `select()' nor `poll()' do anything useful when
applied to plain files; they are useful for sockets, pipes, ptys, ttys &
possibly other character devices, but this is system-dependent.]
There the similarity ends....
2.1.1 How do I use select()?
----------------------------
The interface to `select()' is primarily based on the concept of an
`fd_set', which is a set of FDs (usually implemented as a bit-vector). In
times past, it was common to assume that FDs were smaller than 32, and just
use an int to store the set, but these days, one usually has more FDs
available, so it is important to use the standard macros for manipulating
fd_sets:
fd_set set;
FD_ZERO(&set); /* empties the set */
FD_SET(fd,&set) /* adds FD to the set */
FD_CLR(fd,&set); /* removes FD from the set */
FD_ISSET(fd,&set) /* true if FD is in the set */
In most cases, it is the system's responsibility to ensure that fdsets can
handle the whole range of file descriptors, but in some cases you may have
to predefine the `FD_SETSIZE' macro. *This is system-dependent*; check your
`select()' manpage. Also, some systems have problems handling more than
1024 file descriptors in `select()'.
The basic interface to select is simple:
int select(int nfds, fd_set *readset,
fd_set *writeset,
fd_set *exceptset, struct timeval *timeout);
where
`nfds'
the number of FDs to examine; this must be greater than the largest FD
in any of the fdsets, *NOT* the actual number of FDs specified
`readset'
the set of FDs to examine for readability
`writeset'
the set of FDs to examine for writability
`exceptfds'
the set of FDs to examine for exceptional status (note: errors are
*NOT* exceptional statuses)
`timeout'
NULL for infinite timeout, or points to a timeval specifying the
maximum wait time (if `tv_sec' and `tv_usec' both == 0, then the
status of the FDs is polled, but the call never blocks)
The call returns the number of 'ready' FDs found, and the three fdsets are
modified in-place, with only the ready FDs left in the sets. Use the
`FD_ISSET' macro to test the returned sets.
Here's a simple example of testing a single FD for readability:
int isready(int fd)
{
int rc;
fd_set fds;
struct timeval tv;
FD_ZERO(&fds);
FD_SET(fd,&fds);
tv.tv_sec = tv.tv_usec = 0;
rc = select(fd+1, &fds, NULL, NULL, &tv);
if (rc < 0)
return -1;
return FD_ISSET(fd,&fds) ? 1 : 0;
}
Note that we can pass `NULL' for fdsets that we aren't interested in
testing.
2.1.2 How do I use poll()?
--------------------------
Warning: SVR4 man page advises that any polling on "Non-STREAMS" drivers
should use `chpoll()'. [Editorial note: I'm not entirely sure what this is
supposed to mean.]
Poll accepts a pointer to a list of `struct pollfd', in which the
descriptors and the events you wish to poll for are stored. The events are
specified via a bitwise mask in the events field of the structure. The
instance of the structure will later be filled in and returned to you with
any events which occured. Macros defined by `poll.h' on SVR4 (probably
older versions as well), are used to specify the events in the field. A
timeout may be specified in milliseconds, only the type provided is an
integer which is quite perplexing. A timeout of 0 causes `poll()' to return
immediately, a value of -1 will suspend poll till an event is found to be
true.
struct pollfd {
int fd; /* The descriptor. */
short events; /* The event(s) is/are specified here. */
short revents; /* Events found are returned here. */
};
Alot like select, the return value if positive reflects how many
descriptors were found to satisfy the events requested. A zero return value
is returned if the timeout period is reached before any of the events
specified have occured. A negative value should immediately be followed by
a check of errno, since it signifies an error.
If no events are found, revent is cleared, so there's no need for you to do
this yourself.
The returned events are tested to contain the event.
Here's an example:
/* Poll on two descriptors for Normal data, or High priority data.
If any found call function handle() with appropriate descriptor
and priority. Dont timeout, only giveup if error, or one of the
descriptors hangs up. */
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <stropts.h>
#include <poll.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#define NORMAL_DATA 1
#define HIPRI_DATA 2
int poll_two_normal(int fd1,int fd2)
{
struct pollfd poll_list[2];
int retval;
poll_list[0].fd = fd1;
poll_list[1].fd = fd2;
poll_list[0].events = POLLIN|POLLPRI;
poll_list[1].events = POLLIN|POLLPRI;
while(1)
{
retval = poll(poll_list,(unsigned long)2,-1);
/* Retval will always be greater than 0 or -1 in this case.
Since we're doing it while blocking */
if(retval < 0)
{
fprintf(stderr,"Error while polling: %s\n",strerror(errno));
return -1;
}
if(((poll_list[0].revents&POLLHUP) == POLLHUP) ||
((poll_list[0].revents&POLLERR) == POLLERR) ||
((poll_list[0].revents&POLLNVAL) == POLLNVAL) ||
((poll_list[1].revents&POLLHUP) == POLLHUP) ||
((poll_list[1].revents&POLLERR) == POLLERR) ||
((poll_list[1].revents&POLLNVAL) == POLLNVAL))
return 0;
if((poll_list[0].revents&POLLIN) == POLLIN)
handle(poll_list[0].fd,NORMAL_DATA);
if((poll_list[0].revents&POLLPRI) == POLLPRI)
handle(poll_list[0].fd,HIPRI_DATA);
if((poll_list[1].revents&POLLIN) == POLLIN)
handle(poll_list[1].fd,NORMAL_DATA);
if((poll_list[1].revents&POLLPRI) == POLLPRI)
handle(poll_list[1].fd,HIPRI_DATA);
}
}
2.1.3 Can I use SysV IPC at the same time as select or poll?
------------------------------------------------------------
*No.* (Except on AIX, which has an incredibly ugly kluge to allow this.)
In general, trying to combine the use of `select()' or `poll()' with using
SysV message queues is troublesome. SysV IPC objects are not handled by
file descriptors, so they can't be passed to `select()' or `poll()'. There
are a number of workarounds, of varying degrees of ugliness:
- Abandon SysV IPC completely :-)
- `fork()', and have the child process handle the SysV IPC,
communicating with the parent process by a pipe or socket, which the
parent process can `select()' on.
- As above, but have the child process do the `select()', and
communicate with the parent by message queue.
- Arrange for the process that sends messages to you to send a signal
after each message. *Warning:* handling this right is non-trivial;
it's very easy to write code that can potentially lose messages or
deadlock using this method.
(Other methods exist.)
2.2 How can I tell when the other end of a connection shuts down?
=================================================================
If you try to read from a pipe, socket, FIFO etc. when the writing end of
the connection has been closed, you get an end-of-file indication (`read()'
returns 0 bytes read). If you try and write to a pipe, socket etc. when the
reading end has closed, then a `SIGPIPE' signal will be delivered to the
process, killing it unless the signal is caught. (If you ignore or block
the signal, the `write()' call fails with `EPIPE'.)
2.3 Best way to read directories?
=================================
While historically there have been several different interfaces for this,
the only one that really matters these days the the Posix.1 standard
`<dirent.h>' functions.
The function `opendir()' opens a specified directory; `readdir()' reads
directory entries from it in a standardised format; `closedir()' does the
obvious. Also provided are `rewinddir()', `telldir()' and `seekdir()' which
should also be obvious.
If you are looking to expand a wildcard filename, then most systems have
the `glob()' function; also check out `fnmatch()' to match filenames
against a wildcard, or `ftw()' to traverse entire directory trees.
2.4 How can I find out if someone else has a file open?
=======================================================
This is another candidate for "Frequently Unanswered Questions" because, in
general, your program should never be interested in whether someone else
has the file open. If you need to deal with concurrent access to the file,
then you should be looking at advisory locking.
This is, in general, too hard to do anyway. Tools like `fuser' and `lsof'
that find out about open files do so by grovelling through kernel data
structures in a most unhealthy fashion. You can't usefully invoke them from
a program, either, because by the time you've found out that the file
is/isn't open, the information may already be out of date.
2.5 How do I 'lock' a file?
===========================
There are three main file locking mechanisms available. All of them are
"advisory"[*], which means that they rely on programs co-operating in order
to work. It is therefore vital that all programs in an application should
be consistent in their locking regime, and great care is required when your
programs may be sharing files with third-party software.
[*] Well, actually some Unices permit mandatory locking via the sgid bit -
RTFM for this hack.
Some applications use lock files - something like "filename.lock". Simply
testing for the existence of such files is inadequate though, since a
process may have been killed while holding the lock. The method used by
UUCP (probably the most notable example: it uses lock files for controlling
access to modems, remote systems etc.) is to store the PID in the lockfile,
and test if that pid is still running. Even this isn't enough to be sure
(since PIDs are recycled); it has to have a backstop check to see if the
lockfile is old, which means that the process holding the lock must update
the file regularly. Messy.
The locking functions are:
flock();
lockf();
fcntl();
`flock()' originates with BSD, and is now available in most (but not all)
Unices. It is simple and effective on a single host, but doesn't work at
all with NFS. It locks an entire file. Perhaps rather deceptively, the
popular Perl programming language implements its own `flock()' where
necessary, conveying the illusion of true portability.
`fcntl()' is the only POSIX-compliant locking mechanism, and is therefore
the only truly portable lock. It is also the most powerful, and the
hardest to use. For NFS-mounted file systems, `fcntl()' requests are
passed to a daemon (`rpc.lockd'), which communicates with the lockd on the
server host. Unlike `flock()' it is capable of record-level locking.
`lockf()' is merely a simplified programming interface to the locking
functions of `fcntl()'.
Whatever locking mechanism you use, it is important to sync all your file
IO while the lock is active:
lock(fd);
write_to(some_function_of(fd));
flush_output_to(fd); /* NEVER unlock while output may be buffered */
unlock(fd);
do_something_else; /* another process might update it */
lock(fd);
seek(fd, somewhere); /* because our old file pointer is not safe */
do_something_with(fd);
...
A few useful `fcntl()' locking recipes (error handling omitted for
simplicity) are:
#include <fcntl.h>
#include <unistd.h>
read_lock(int fd) /* a shared lock on an entire file */
{
fcntl(fd, F_SETLKW, file_lock(F_RDLCK, SEEK_SET));
}
write_lock(int fd) /* an exclusive lock on an entire file */
{
fcntl(fd, F_SETLKW, file_lock(F_WRLCK, SEEK_SET));
}
append_lock(int fd) /* a lock on the _end_ of a file - other
processes may access existing records */
{
fcntl(fd, F_SETLKW, file_lock(F_WRLCK, SEEK_END));
}
The function file_lock used by the above is
struct flock* file_lock(short type, short whence)
{
static struct flock ret ;
ret.l_type = type ;
ret.l_start = 0 ;
ret.l_whence = whence ;
ret.l_len = 0 ;
ret.l_pid = getpid() ;
return &ret ;
}
2.6 How do I find out if a file has been updated by another process?
====================================================================
This is close to being a Frequently Unanswered Question, because people
asking it are often looking for some notification from the system when a
file or directory is changed, and there is no portable way of getting this.
(IRIX has a non-standard facility for monitoring file accesses, but I've
never heard of it being available in any other flavour.)
In general, the best you can do is to use `fstat()' on the file. (Note: the
overhead on `fstat()' is quite low, usually much lower than the overhead of
`stat()'.) By watching the mtime and ctime of the file, you can detect when
it is modified, or deleted/linked/renamed. This is a bit kludgy, so you
might want to rethink *why* you want to do it.
2.7 How does the 'du' utility work?
===================================
`du' simply traverses the directory structure calling `stat()' (or more
accurately, `lstat()') on every file and directory it encounters, adding up
the number of blocks consumed by each.
If you want more detail about how it works, then the simple answer is:
Use the source, Luke!
Source for BSD systems (FreeBSD, NetBSD and OpenBSD) is available as
unpacked source trees on their FTP distribution sites; source for GNU
versions of utilities is available from any of the GNU mirrors, but you
have to unpack the archives yourself.
2.8 How do I find the size of a file?
=====================================
Use `stat()', or `fstat()' if you have the file open.
These calls fill in a data structure containing all the information about
the file that the system keeps track of; that includes the owner, group,
permissions, size, last access time, last modification time, etc.
The following routine illustrates how to use `stat()' to get the file size.
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
int get_file_size(char *path,off_t *size)
{
struct stat file_stats;
if(stat(path,&file_stats))
return -1;
*size = file_stats.st_size;
return 0;
}
2.9 How do I expand '~' in a filename like the shell does?
==========================================================
The standard interpretation for `~' at the start of a filename is: if alone
or followed by a `/', then substitute the current user's `HOME' directory;
if followed by the name of a user, then substitute that user's `HOME'
directory. If no valid expansion can be found, then shells will leave the
filename unchanged.
Be wary, however, of filenames that actually start with the `~' character.
Indiscriminate tilde-expansion can make it very difficult to specify such
filenames to a program; while quoting will prevent the shell from doing the
expansion, the quotes will have been removed by the time the program sees
the filename. As a general rule, do not try and perform tilde-expansion on
filenames that have been passed to the program on the command line or in
environment variables. (Filenames generated within the program, obtained by
prompting the user, or obtained from a configuration file, are good
candidates for tilde-expansion.)
Here's a piece of C++ code (using the standard string class) to do the job:
string expand_path(const string& path)
{
if (path.length() == 0 || path[0] != '~')
return path;
const char *pfx = NULL;
string::size_type pos = path.find_first_of('/');
if (path.length() == 1 || pos == 1)
{
pfx = getenv("HOME");
if (!pfx)
{
// Punt. We're trying to expand ~/, but HOME isn't set
struct passwd *pw = getpwuid(getuid());
if (pw)
pfx = pw->pw_dir;
}
}
else
{
string user(path,1,(pos==string::npos) ? string::npos : pos-1);
struct passwd *pw = getpwnam(user.c_str());
if (pw)
pfx = pw->pw_dir;
}
// if we failed to find an expansion, return the path unchanged.
if (!pfx)
return path;
string result(pfx);
if (pos == string::npos)
return result;
if (result.length() == 0 || result[result.length()-1] != '/')
result += '/';
result += path.substr(pos+1);
return result;
}
2.10 What can I do with named pipes (FIFOs)?
============================================
2.10.1 What is a named pipe?
----------------------------
A "named pipe" is a special file that is used to transfer data between
unrelated processes. One (or more) process(es) writes to it, while another
process reads from it. Named pipes are visible in the file system and may
be viewed with ls like any other file. (Named pipes are also called fifo's,
which stands for "First In, First Out".)
Named pipes may be used to pass data between unrelated processes, while
normal (unnamed) pipes can only connect parent/child processes (unless you
try *very* hard).
Named pipes are strictly unidirectional, even on systems where anonymous
pipes are bidirectional (full-duplex).
2.10.2 How do I create a named pipe?
------------------------------------
To create a named pipe interactively, you'll use either `mknod' or
`mkfifo'. On some systems, mknod will be found in /etc. In other words, it
might not be on your path. See your man pages for details.
To make a named pipe within a C program use `mkfifo()':
/* set the umask explicitly, you don't know where it's been */
umask(0);
if (mkfifo("test_fifo", S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP))
{
perror("mkfifo");
exit(1);
}
If you don't have `mkfifo()', you'll have to use `mknod()':
/* set the umask explicitly, you don't know where it's been */
umask(0);
if (mknod("test_fifo",
S_IFIFO | S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP,
0))
{
perror("mknod");
exit(1);
}
2.10.3 How do I use a named pipe?
---------------------------------
To use the pipe, you open it like a normal file, and use `read()' and
`write()' just as though it was a plain pipe.
However, the `open()' of the pipe may block. The following rules apply:
* If you open for both reading and writing (O_RDWR), then the open will
not block.
* If you open for reading (O_RDONLY), the open will block until another
process opens the FIFO for writing, unless O_NONBLOCK is specified, in
which case the open succeeds.
* If you open for writing (O_WRONLY), the open will block until another
process opens the FIFO for reading, unless O_NONBLOCK is specified, in
which case the open fails.
When reading and writing the FIFO, the same considerations apply as for
regular pipes and sockets, i.e. `read()' will return EOF when all writers
have closed, and `write()' will raise SIGPIPE when there are no readers (if
SIGPIPE is blocked or ignored, the call fails with EPIPE).
2.10.4 Can I use a named pipe across NFS?
-----------------------------------------
No, you can't. There is no facility in the NFS protocol to do this. (You
may be able to use a named pipe on an NFS-mounted filesystem to communicate
between processes on the same client, though.)
2.10.5 Can multiple processes write to the pipe simultaneously?
---------------------------------------------------------------
If each piece of data written to the pipe is less than PIPE_BUF in size,
then they will not be interleaved. However, the boundaries of writes are
not preserved; when you read from the pipe, the read call will return as
much data as possible, even if it originated from multiple writes.
The value of PIPE_BUF is guaranteed (by Posix) to be at least 512. It may
or may not be defined in `<limits.h>', but it can be queried for individual
pipes using `pathconf()' or `fpathconf()'.
2.10.6 Using named pipes in applications
----------------------------------------
How can I implement two way communication between one server and
several clients?
It is possible that more than one client is communicating with your server
at once. As long as each command they send to the server is smaller than
PIPE_BUF (see above), they can all use the same named pipe to send data to
the server. All clients can easily know the name of the server's incoming
fifo.
However, the server can not use a single pipe to communicate with the
clients. If more than one client is reading the same pipe, there is no way
to ensure that the appropriate client receives a given response.
A solution is to have the client create its own incoming pipe before
sending data to the server, or to have the server create its outgoing pipes
after receiving data from the client.
Using the client's process ID in the pipe's name is a common way to
identify them. Using fifo's named in this manner, each time the client
sends a command to the server, it can include its PID as part of the
command. Any returned data can be sent through the appropriately named pipe.
3. Terminal I/O
***************
3.1 How can I make my program not echo input?
=============================================
How can I make my program not echo input, like login does when asking
for your password?
There is an easy way, and a slightly harder way:
The easy way, is to use `getpass()', which is probably found on almost all
Unices. It takes a string to use as a prompt. It will read up to an EOF or
newline and returns a pointer to a static area of memory holding the string
typed in.
The harder way is to use `tcgetattr()' and `tcsetattr()', both use a
`struct termios' to manipulate the terminal. The following two routines
should allow echoing, and non-echoing mode.
#include <stdlib.h>
#include <stdio.h>
#include <termios.h>
#include <string.h>
static struct termios stored;
void echo_off(void)
{
struct termios new;
tcgetattr(0,&stored);
memcpy(&new, &stored, sizeof(struct termios));
new.c_lflag &= (~ECHO);
tcsetattr(0,TCSANOW,&new);
return;
}
void echo_on(void)
{
tcsetattr(0,TCSANOW,&stored);
return;
}
Both routines used, are defined by the POSIX standard.
3.2 How can I read single characters from the terminal?
=======================================================
How can I read single characters from the terminal? My program is
always waiting for the user to press `<RETURN>'.
Terminals are usually in canonical mode, where input is read in lines after
it is edited. You may set this into non-canonical mode, where you set how
many characters should be read before input is given to your program. You
also may set the timer in non-canonical mode terminals to 0, this timer
flushs your buffer at set intervals. By doing this, you can use `getc()' to
grab the key pressed immediately by the user. We use `tcgetattr()' and
`tcsetattr()' both of which are defined by POSIX to manipulate the
`termios' structure.
#include <stdlib.h>
#include <stdio.h>
#include <termios.h>
#include <string.h>
static struct termios stored;
void set_keypress(void)
{
struct termios new;
tcgetattr(0,&stored);
memcpy(&new,&stored,sizeof(struct termios));
/* Disable canonical mode, and set buffer size to 1 byte */
new.c_lflag &= (~ICANON);
new.c_cc[VTIME] = 0;
new.c_cc[VMIN] = 1;
tcsetattr(0,TCSANOW,&new);
return;
}
void reset_keypress(void)
{
tcsetattr(0,TCSANOW,&stored);
return;
}
3.3 How can I check and see if a key was pressed?
=================================================
How can I check and see if a key was pressed? On DOS I use the
`kbhit()' function, but there doesn't seem to be an equivalent?
If you set the terminal to single-character mode (see previous answer),
then (on most systems) you can use `select()' or `poll()' to test for
readability.
3.4 How can I move the cursor around the screen?
================================================
How can I move the cursor around the screen? I want to do full screen
editing without using curses.
Seriously, you probably *don't* want to do this. Curses knows about how to
handle all sorts of oddities that different terminal types exhibit; while
the termcap/terminfo data will tell you whether any given terminal type
possesses any of these oddities, you will probably find that correctly
handling all the combinations is a *huge* job.
However, if you insist on getting your hands dirty (so to speak), look into
the `termcap' functions, particularly `tputs()', `tparm()' and `tgoto()'.
3.5 What are pttys?
===================
Pseudo-teletypes (pttys, ptys, other variant abbreviations) are
pseudo-devices that have two parts: the "master" side, which can be thought
of as the 'user', and the "slave" side, which behaves like a standard tty
device.
They exist in order to provide a means to emulate the behaviour of a serial
terminal under the control of a program. For example, `telnet' uses a
pseudo-terminal on the remote system; the remote login shell sees the
behaviour it expects from a tty device, but the master side of the
pseudo-terminal is being controlled by a daemon that forwards all data over
the network. They are also used by programs such as `xterm', `expect',
`script', `screen', `emacs', and many others.
3.6 How to handle a serial port or modem?
=========================================
todo
4. System Information
*********************
4.1 How can I tell how much memory my system has?
=================================================
This is another "Frequently Unanswered Question". In most cases, you should
not even *attempt* to find out.
(todo)
4.2 How do I check a user's password?
=====================================
4.2.1 How do I get a user's password?
-------------------------------------
Traditionally user passwords were kept in the `/etc/passwd' file, on most
UNIX flavours. Which is usually of this format:
username:password:uid:gid:gecos field:home directory:login shell
Though this has changed with time, now user information may be kept on
other hosts, or not necessarily in the `/etc/passwd' file. Modern
implementations also made use of "shadow" password files which hold the
password, along with sensitive information. This file would be readable
only by privileged users.
The password is usually not in clear text, but encrypted due to security
concerns.
POSIX defines a suite of routines which can be used to access this database
for queries. The quickest way to get an individual record for a user is
with the `getpwnam()' and `getpwuid()' routines. Both return a pointer to a
struct passwd, which holds the users information in various members.
`getpwnam()' accepts a string holding the user's name, `getpwuid()' accepts
a uid (type `uid_t' as defined by POSIX). Both return NULL if they fail.
However, as explained earlier, a shadow database exists on most modern
systems to hold sensitive information, namely the password. Some systems
only return the password if the calling uid is of the superuser, others
require you to use another suite of functions for the shadow password
database. If this is the case you need to make use of `getspnam()', which
accepts a username and returns a struct spwd. Again, in order to
successfully do this, you will need to have privileges.
4.2.2 How do I get shadow passwords by uid?
-------------------------------------------
My system uses the getsp* suite of routines to get the sensitive user
information. However I do not have `getspuid()', only `getspnam()'.
How do I work around this, and get by uid?
The work around is relatively painless. The following routine should go
straight into your personal utility library:
#include <stdlib.h>
#include <stdio.h>
#include <pwd.h>
#include <shadow.h>
struct spwd *getspuid(uid_t pw_uid)
{
struct spwd *shadow;
struct passwd *ppasswd;
if( ((ppasswd = getpwuid(pw_uid)) == NULL)
|| ((shadow = getspnam(ppasswd->pw_name)) == NULL))
return NULL;
return shadow;
}
The problem is, that some systems do not keep the uid, or other information
in the shadow database.
4.2.3 How do I verify a user's password?
----------------------------------------
The fundamental problem here is, that various authentication systems exist,
and passwords aren't always what they seem. Also with the traditional one
way encryption method used by most UNIX flavours (out of the box), the
encryption algorithm may differ, some systems use a one way DES encryption,
others like the international release of FreeBSD use MD5.
The most popular way is to have a one way encryption algorithm, where the
password cannot be decrypted. Instead the password is taken in clear text
from input, and encrypted and checked against the encrypted password in the
database. The details of how to encrypt should really come from your man
page for `crypt()', but here's a usual version:
/* given a plaintext password and an encrypted password, check if
* they match; returns 1 if they match, 0 otherwise.
*/
int check_pass(const char *plainpw, const char *cryptpw)
{
return strcmp(crypt(plainpw,cryptpw), cryptpw) == 0;
}
This works because the salt used in encrypting the password is stored as an
initial substring of the encrypted value.
5. Miscellaneous programming
****************************
5.1 How do I compare strings using wildcards?
=============================================
The answer to *that* depends on what exactly you mean by "wildcards".
There are two quite different concepts that qualify as "wildcards". They
are:
*Filename patterns*
These are what the shell uses for filename expansion ("globbing")
*Regular Expressions*
These are used by editors, `grep', etc. for matching text, but they
normally *aren't* applied to filenames
5.1.1 How do I compare strings using filename patterns?
-------------------------------------------------------
Unless you are unlucky, your system should have a function `fnmatch()' to
do filename matching. This generally allows only the Bourne shell style of
pattern; i.e. it recognises `*', `[...]' and `?', but probably won't
support the more arcane patterns available in the Korn and Bourne-Again
shells.
If you don't have this function, then rather than reinvent the wheel, you
are probably better off snarfing a copy from the BSD or GNU sources.
Also, for the common cases of matching actual filenames, look for `glob()',
which will find all existing files matching a pattern.
5.1.2 How do I compare strings using regular expressions?
---------------------------------------------------------
There are a number of slightly different syntaxes for regular expressions;
most systems use at least two: the one recognised by `ed', sometimes known
as "Basic Regular Expressions", and the one recognised by `egrep',
"Extended Regular Expressions". Perl has it's own slightly different
flavour, as does Emacs.
To support this multitude of formats, there is a corresponding multitude of
implementations. Systems will generally have regexp-matching functions
(usually `regcomp()' and `regexec()') supplied, but be wary; some systems
have more than one implementation of these functions available, with
different interfaces. In addition, there are many library implementations
available. (It's common, BTW, for regexps to be compiled to an internal
form before use, on the assumption that you may compare several separate
strings against the same regexp.)
One library available for this is the `rx' library, available from the GNU
mirrors. This seems to be under active development, which may be a good or
a bad thing depending on your point of view :-)
5.2 What's the best way to send mail from a program?
====================================================
There are several ways to send email from a Unix program. Which is the best
method to use in a given situation varies, so I'll present two of them. A
third possibility, not covered here, is to connect to a local SMTP port (or
a smarthost) and use SMTP directly; see RFC 821.
5.2.1 The simple method: /bin/mail
----------------------------------
For simple applications, it may be sufficient to invoke `mail' (usually
`/bin/mail', but could be `/usr/bin/mail' on some systems).
*WARNING:* Some versions of UCB Mail may execute commands prefixed by `~!'
or `~|' given in the message body even in non-interactive mode. This can be
a security risk.
Invoked as `mail -s 'subject' recipients...' it will take a message body on
standard input, and supply a default header (including the specified
subject), and pass the message to `sendmail' for delivery.
This example mails a test message to `root' on the local system:
#include <stdio.h>
#define MAILPROG "/bin/mail"
int main()
{
FILE *mail = popen(MAILPROG " -s 'Test Message' root", "w");
if (!mail)
{
perror("popen");
exit(1);
}
fprintf(mail, "This is a test.\n");
if (pclose(mail))
{
fprintf(stderr, "mail failed!\n");
exit(1);
}
}
If the text to be sent is already in a file, then one can do:
system(MAILPROG " -s 'file contents' root </tmp/filename");
These methods can be extended to more complex cases, but there are many
pitfalls to watch out for:
* If using system() or popen(), you must be very careful about quoting
arguments to protect them from filename expansion or word splitting
* Constructing command lines from user-specified data is a common source
of buffer-overrun errors and other security holes
* This method does not allow for CC: or BCC: recipients to be specified
(some versions of /bin/mail may allow this, some do not)
5.2.2 Invoking the MTA directly: /usr/lib/sendmail
--------------------------------------------------
The `mail' program is an example of a "Mail User Agent", a program intended
to be invoked by the user to send and receive mail, but which does not
handle the actual transport. A program for transporting mail is called an
"MTA", and the most commonly found MTA on Unix systems is called
`sendmail'. There are other MTAs in use, such as `MMDF', but these
generally include a program that emulates the usual behaviour of `sendmail'.
Historically, `sendmail' has usually been found in `/usr/lib', but the
current trend is to move library programs out of `/usr/lib' into
directories such as `/usr/sbin' or `/usr/libexec'. As a result, one
normally invokes `sendmail' by its full path, which is system-dependent.
To understand how `sendmail' behaves, it's useful to understand the concept
of an "envelope". This is very much like paper mail; the envelope defines
who the message is to be delivered to, and who it is from (for the purpose
of reporting errors). Contained in the envelope are the "headers", and the
"body", separated by a blank line. The format of the headers is specified
primarily by RFC 822; see also the MIME RFCs.
There are two main ways to use `sendmail' to originate a message: either
the envelope recipients can be explicitly supplied, or `sendmail' can be
instructed to deduce them from the message headers. Both methods have
advantages and disadvantages.
5.2.2.1 Supplying the envelope explicitly
.........................................
The recipients of a message can simply be specified on the command line.
This has the drawback that mail addresses can contain characters that give
`system()' and `popen()' considerable grief, such as single quotes, quoted
strings etc. Passing these constructs successfully through shell
interpretation presents pitfalls. (One can do it by replacing any single
quotes by the sequence single-quote backslash single-quote single-quote,
then surrounding the entire address with single quotes. Ugly, huh?)
Some of this unpleasantness can be avoided by eschewing the use of
`system()' or `popen()', and resorting to `fork()' and `exec()' directly.
This is sometimes necessary in any event; for example, user-installed
handlers for SIGCHLD will usually break `pclose()' to a greater or lesser
extent.
Here's an example:
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sysexits.h>
/* #include <paths.h> if you have it */
#ifndef _PATH_SENDMAIL
#define _PATH_SENDMAIL "/usr/lib/sendmail"
#endif
/* -oi means "dont treat . as a message terminator"
* remove ,"--" if using a pre-V8 sendmail (and hope that no-one
* ever uses a recipient address starting with a hyphen)
* you might wish to add -oem (report errors by mail)
*/
#define SENDMAIL_OPTS "-oi","--"
/* this is a macro for returning the number of elements in array */
#define countof(a) ((sizeof(a))/sizeof((a)[0]))
/* send the contents of the file open for reading on FD to the
* specified recipients; the file is assumed to contain RFC822 headers
* & body, the recipient list is terminated by a NULL pointer; returns
* -1 if error detected, otherwise the return value from sendmail
* (which uses <sysexits.h> to provide meaningful exit codes)
*/
int send_message(int fd, const char **recipients)
{
static const char *
