What int values are relevant for exit() in C?

问题

On TutorialsPoint.com, exit is passed the value 0, while people often pass it 1. I've even seen exit(3);

What do the different values mean?

回答1:

By convention, a program that exits successfully calls exit (or returns from main) with a value of 0. Shell programs (most programs, actually) will look for this to determine if a program ran successfully or not.

Any other value is considered an abnormal exit. What each of those values mean is defined by the program in question.

On Unix and similar systems, only the lower 8 bits of the exit value are used as the exit code of the program and are returned to the parent process on a call to wait. Calling exit(n) is equivalent to calling exit(n & 0xff)

From the man page:

The exit() function causes normal process termination and the value of status & 0377 is returned to the parent (see wait(2)).

回答2:

The only portable values to pass to exit are 0, EXIT_SUCCESS, and EXIT_FAILURE. The latter two are macros defined in <stdlib.h>, the same header that declares the exit function.

Both 0 and EXIT_SUCCESS conventionally indicate that the program succeeded. EXIT_FAILURE indicates that it failed somehow. (EXIT_SUCCESS is almost certainly defined as 0.)

For UNIX-like systems, EXIT_FAILURE is defined as 1, and exit(1) is also common (though a bit less portable). Some operating systems might use a different convention; for example OpenVMS uses even values for failure and odd values for success, with some special-case code to map exit(0) to a failure status.

Other values may be used by some programs to indicate different kinds of failure. For example, the grep command uses 0 if a match was found, 1 if no match was found, and 2 if some other error occurred.

回答3:

int main()
{
   exit(0);
}

is the same as

int main()
{
   return 0;
}

The return values are basically error codes:

0 (or macro EXIT_SUCCESS defined in "stdlib.h") means successful program termination
1 (or macro EXIT_FAILURE defined in "stdlib.h") means program termination because of failure

Other error codes are also possible, but they are system dependent thus not part of the C standard, i.e. they are not portable.

回答4:

Exit values are program dependent. The biggest consideration is probably that most (all?) shells consider a return value of zero to mean success. Any other value indicates failure.

来源：https://stackoverflow.com/questions/36904391/what-int-values-are-relevant-for-exit-in-c

标签

exit