waitid（）错误：参数无效

Question

I have this code from APUE book and exercise in which I need to replace wait() with waitid():

#include "apue.h"
#include <sys/wait.h>

int main(void)
{
    pid_t   pid;
    int     status;

    if ((pid = fork()) < 0)
        err_sys("fork error");
    else if (pid == 0)              /* child */
        exit(7);

    if (wait(&status) != pid)       /* wait for child */
        err_sys("wait error");
    pr_exit(status);                /* and print its status */

    if ((pid = fork()) < 0)
        err_sys("fork error");
    else if (pid == 0)              /* child */
        abort();                    /* generates SIGABRT */

    if (wait(&status) != pid)       /* wait for child */
        err_sys("wait error");
    pr_exit(status);                /* and print its status */

    if ((pid = fork()) < 0)
        err_sys("fork error");
    else if (pid == 0)              /* child */
        status /= 0;                /* divide by 0 generates SIGFPE */

    if (wait(&status) != pid)       /* wait for child */
        err_sys("wait error");
    pr_exit(status);                /* and print its status */

    exit(0);
}

I tried this:

id_t    pid;
siginfo_t info;
pid = fork();
// ...

waitid(P_PID, pid, &info, WNOHANG) // also tried with WNOWAIT

and got waitid error: Invalid argument. When I tried: waitid(P_PID, pid, &info, WEXITED) I got Signal number: 17 for all three waitid() calls where original code's output are signals 7, 6 and 8 respectively. Why am I getting "invalid argument" and how can I force system to generate signals 7, 6 and 8?

Answer 1

waitid(P_ALL, 0, &info, WEXITED) , where info is of type siginfo_t , collects the same child processes as wait(&status) does. They differ in

1. the siginfo_t * argument of waitid() and its interpretation vs . the int * argument of wait() and its interpretation, and
2. the meaning of the two functions' return values.

You appear to want to use waitid() to wait for a particular child each time, however, and that would be this: waitid(P_PID, pid, &info, WEXITED) .

Note that whereas wait() returns the pid of the child process it collected on success, waitid() returns 0 on success.

Note also that although the siginfo_t structure has a member named si_status , it is not equivalent to the value wait() provides to the caller via its second argument. siginfo_t.si_status is the process's actual exit code, whereas the status provided by wait() is a bitmask of several different fields. You get the exit code from the latter via the WEXITSTATUS() macro, though you would be wise to check whether it in fact terminated normally ( WIFEXITED() ) vs. being signaled ( WIFSIGNALED() ).

Answer 2

From man waitid:

   The child state changes to wait for are specified by ORing one or more of the following flags in options:

   WEXITED     Wait for children that have terminated.

   WSTOPPED    Wait for children that have been stopped by delivery of a signal.

   WCONTINUED  Wait for (previously stopped) children that have been resumed by delivery of SIGCONT.

   The following flags may additionally be ORed in options:

   WNOHANG     As for waitpid().

   WNOWAIT     Leave the child in a waitable state; a later wait call can be used to again retrieve the child status information.

And:

   EINVAL The options argument was invalid.

So, you should probably provide WEXITED.

Edit: I added a couple of defines to your code:

#define pr_exit(n) printf("%d\n", n)
#define err_sys perror

and #includes to provide declarations and what I see is exit statuses of 0x700, 0x86 and 0x88, which AFAICS is perfectly correct.

Those exit statuses are {normal exit status of 7}, {exit with signal 6}, and {exit with signal 8}, respectively. Note that exit status of 7 is not the same as exiting due to signal 7.

(Signal 17 is SIGCHLD in linux. I can't see why your children would be exiting with signal 17. Either they or the parent would have to be setting up a SIGCHLD handler in order to cause that. SIGCHLD has specific and "unusual" behavior.)

Answer 3

At least one of WCONTINUED, WEXITED, or WSTOPPED must be specified in the options argument.

from APUE book