When a program exits, it can return to the parent process a small
amount of information about the cause of termination, using the
exit status. This is a value between 0 and 255 that the exiting
process passes as an argument to exit.
Normally you should use the exit status to report very broad information about success or failure. You can’t provide a lot of detail about the reasons for the failure, and most parent processes would not want much detail anyway.
There are conventions for what sorts of status values certain programs should return. The most common convention is simply 0 for success and 1 for failure. Programs that perform comparison use a different convention: they use status 1 to indicate a mismatch, and status 2 to indicate an inability to compare. Your program should follow an existing convention if an existing convention makes sense for it.
A general convention reserves status values 128 and up for special purposes. In particular, the value 128 is used to indicate failure to execute another program in a subprocess. This convention is not universally obeyed, but it is a good idea to follow it in your programs.
Warning: Don’t try to use the number of errors as the exit status. This is actually not very useful; a parent process would generally not care how many errors occurred. Worse than that, it does not work, because the status value is truncated to eight bits. Thus, if the program tried to report 256 errors, the parent would receive a report of 0 errors—that is, success.
For the same reason, it does not work to use the value of errno
as the exit status—these can exceed 255.
Portability note: Some non-POSIX systems use different
conventions for exit status values. For greater portability, you can
use the macros EXIT_SUCCESS and EXIT_FAILURE for the
conventional status value for success and failure, respectively. They
are declared in the file stdlib.h.
int EXIT_SUCCESS ¶This macro can be used with the exit function to indicate
successful program completion.
On POSIX systems, the value of this macro is 0. On other
systems, the value might be some other (possibly non-constant) integer
expression.
int EXIT_FAILURE ¶This macro can be used with the exit function to indicate
unsuccessful program completion in a general sense.
On POSIX systems, the value of this macro is 1. On other
systems, the value might be some other (possibly non-constant) integer
expression. Other nonzero status values also indicate failures. Certain
programs use different nonzero status values to indicate particular
kinds of "non-success". For example, diff uses status value
1 to mean that the files are different, and 2 or more to
mean that there was difficulty in opening the files.
Don’t confuse a program’s exit status with a process’ termination status.
There are lots of ways a process can terminate besides having its program
finish. In the event that the process termination is caused by program
termination (i.e., exit), though, the program’s exit status becomes
part of the process’ termination status.