Weirdly changing status codes

Question

I have a simple program to test exit codes:

int main() { 
    return 2000;
}

In the terminal if I run the program then type:

echo $?

the terminal replies with 208. What? Why would it do that?

Exit codes only range through 256? Why force main to return `int` then? — sircodesalot, Oct 01 '13 at 02:23
Because the language says so... (the language requires `main` to return an `int`) — Mysticial, Oct 01 '13 at 02:24
@Mystical I think the OP was getting at why the _standard_ says so. — Cole Tobin, Oct 01 '13 at 02:29
Inherited from 'C', for the same reason that "toupper" etc take an int when the values are actually chars. 'int' was at one point a catch-all, generic, "a number". It's gotten a lot bigger since then. — kfsone, Oct 01 '13 at 02:58

score 6 · Accepted Answer · edited May 23 '17 at 11:57

While the return value of main is an int (generally 32 bits), only 8 bits of the exit code are communicated out to the rest of the operating system.

As Mystical pointed out, 2000 % 256 = 208.

In Linux, a parent process retrieves the status of a child process via a call to wait or waitpid (usually after the child exits). That call provides a status value - encoded in this int is the exit code of the child, along with several other fields, indicating if/how the child exited, etc. The exit code is extracted via the WEXITSTATUS macro.

int status;
waitpid(childpid, &status, 0);
if (WIFEXITED(status))
    printf("Child exited with code %d\n", WEXITSTATUS(status));

It's interesting to note that I don't really see this mentioned in any of the man pages, though!

But it's very apparent if we take a look at the kernel:

In kernel/exit.c, we see where the exit system call is defined:

888  SYSCALL_DEFINE1(exit, int, error_code)
889  {
890      do_exit((error_code&0xff)<<8);
891  }

You can see right there, that the int error code is truncated to eight bits, and stuffed into bits 15:8 of the code passed to do_exit.

Related questions:

Any benefit in using WEXITSTATUS macro in C over division by 256 on exit() status?

score 2 · Answer 2 · edited Oct 01 '13 at 02:29

2

UNIX error codes are a byte big, so your value is chopped down to its remainder mod 256, namely 208.

edited Oct 01 '13 at 02:29

Cole Tobin

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answered Oct 01 '13 at 02:24

Jakob Weisblat

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score 2 · Answer 3 · answered Oct 01 '13 at 02:42

The C standard only defines three return codes: 0, EXIT_SUCCESS and EXIT_FAILURE. (Normally, EXIT_SUCCESS is 0, but I don't think that is technically required.)

If the status return is 0 or EXIT_SUCCESS, then "an implementation-defined form of the status successful termination is returned [to the host environment]". If it's EXIT_FAILURE, then "unsuccessful termination" is returned.

Any other return code is implementation-defined. Your implementation defines that a status return of 2000 returns 208 to the host-environment. It's completely within it's rights to do so.

Weirdly changing status codes

3 Answers3