Package: manpages-dev Version: 6.03-2 Dear Maintainer,
I saw that the manual for exit(3) claimed "the least significant byte of
status (i.e., status & 0xFF) is returned to the parent". This surprised
me because I was pretty sure that POSIX required the full status to made
available. It does:
"the full value shall be available from waitid() and in the siginfo_t
passed to a signal handler for SIGCHLD."
https://pubs.opengroup.org/onlinepubs/9699919799/functions/exit.html
And the wait(2) manual page documents it as such:
"waitid()
…
si_status
Either the exit status of the child, as given to _exit(2) (or
exit(3)), or the signal that caused the child to terminate,
stop, or continue. The si_code field can be used to determine
how to interpret this field."
However, this doesn't actually work on Linux. Here's a test program:
#include <sys/wait.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
int main() {
pid_t pid=fork();
if(pid<0){perror("fork"); exit(1);}
if(pid==0)_exit(12345);
siginfo_t si={0};
pid=waitid(P_PID,pid,&si,WEXITED);
if(pid<0)perror("waitid");
printf("pid=%d, status=%d\n",pid,si.si_status);
}
It prints status=57 (12345 & 255), which is not "as given". Until Linux
is changed to follow the recent POSIX requirement, the manual should say
that si_status is the low 8 bits. It might also be helpful to note this
as a deviation from the 2018 edition of POSIX.
By the way, running the test with "strace -f" shows that the child gives
the full status code to the kernel, and the waitid() syscall returns the
truncated version.
- Michael
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