Avoid arbitrary limits on the length or number of any data structure, including file names, lines, files, and symbols, by allocating all data structures dynamically. In most Unix utilities, “long lines are silently truncated”. This is not acceptable in a GNU utility.
Utilities reading files should not drop NUL characters, or any other nonprinting characters including those with codes above 0177. The only sensible exceptions would be utilities specifically intended for interface to certain types of terminals or printers that can't handle those characters. Whenever possible, try to make programs work properly with sequences of bytes that represent multibyte characters, using encodings such as UTF-8 and others.
Check every system call for an error return, unless you know you wish to
ignore errors. Include the system error text (from perror or
equivalent) in every error message resulting from a failing
system call, as well as the name of the file if any and the name of the
utility. Just “cannot open foo.c” or “stat failed” is not
sufficient.
Check every call to malloc or realloc to see if it
returned zero. Check realloc even if you are making the block
smaller; in a system that rounds block sizes to a power of 2,
realloc may get a different block if you ask for less space.
In Unix, realloc can destroy the storage block if it returns
zero. GNU realloc does not have this bug: if it fails, the
original block is unchanged. Feel free to assume the bug is fixed. If
you wish to run your program on Unix, and wish to avoid lossage in this
case, you can use the GNU malloc.
You must expect free to alter the contents of the block that was
freed. Anything you want to fetch from the block, you must fetch before
calling free.
If malloc fails in a noninteractive program, make that a fatal
error. In an interactive program (one that reads commands from the
user), it is better to abort the command and return to the command
reader loop. This allows the user to kill other processes to free up
virtual memory, and then try the command again.
Use getopt_long to decode arguments, unless the argument syntax
makes this unreasonable.
When static storage is to be written in during program execution, use explicit C code to initialize it. Reserve C initialized declarations for data that will not be changed.
Try to avoid low-level interfaces to obscure Unix data structures (such
as file directories, utmp, or the layout of kernel memory), since these
are less likely to work compatibly. If you need to find all the files
in a directory, use readdir or some other high-level interface.
These are supported compatibly by GNU.
The preferred signal handling facilities are the BSD variant of
signal, and the posix sigaction function; the
alternative USG signal interface is an inferior design.
Nowadays, using the posix signal functions may be the easiest way
to make a program portable. If you use signal, then on GNU/Linux
systems running GNU libc version 1, you should include
bsd/signal.h instead of signal.h, so as to get BSD
behavior. It is up to you whether to support systems where
signal has only the USG behavior, or give up on them.
In error checks that detect “impossible” conditions, just abort. There is usually no point in printing any message. These checks indicate the existence of bugs. Whoever wants to fix the bugs will have to read the source code and run a debugger. So explain the problem with comments in the source. The relevant data will be in variables, which are easy to examine with the debugger, so there is no point moving them elsewhere.
Do not use a count of errors as the exit status for a program. That does not work, because exit status values are limited to 8 bits (0 through 255). A single run of the program might have 256 errors; if you try to return 256 as the exit status, the parent process will see 0 as the status, and it will appear that the program succeeded.
If you make temporary files, check the TMPDIR environment
variable; if that variable is defined, use the specified directory
instead of /tmp.
In addition, be aware that there is a possible security problem when creating temporary files in world-writable directories. In C, you can avoid this problem by creating temporary files in this manner:
fd = open(filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
or by using the mkstemps function from libiberty.
In bash, use set -C to avoid this problem.