waitpid(S)

waitpid -- wait for child process to change state

Synopsis

   #include <sys/types.h>
   #include <sys/wait.h>
   
   pid_t waitpid(pid_t pid, int stat_loc, int options);

Description

waitpid suspends the calling process until one of its children changes state; if a child process changed state prior to the call to waitpid, return is immediate. pid specifies a set of child processes for which status is requested.

If pid is equal to (pid_t)-1, status is requested for any child process.
If pid is greater than (pid_t)0, it specifies the process ID of the child process for which status is requested.
If pid is equal to (pid_t)0 status is requested for any child process whose process group ID is equal to that of the calling process.
If pid is less than (pid_t)-1, status is requested for any child process whose process group ID is equal to the absolute value of pid.

If waitpid returns because the status of a child process is available, then that status may be evaluated with the macros defined by wstat(M). If the calling process had specified a non-zero value of stat_loc, the status of the child process will be stored in the location pointed to by stat_loc.

The options argument is constructed from the bitwise inclusive OR of zero or more of the following flags, defined in the header file sys/wait.h:

WCONTINUED: the status of any continued child process specified by pid, whose status has not been reported since it continued (from a job control stop), shall also be reported to the calling process.
WNOHANG: waitpid will not suspend execution of the calling process if status is not immediately available for one of the child processes specified by pid.
WNOWAIT: keep the process whose status is returned in stat_loc in a waitable state. The process may be waited for again with identical results.
WUNTRACED: the status of any child processes specified by pid^ that are stopped, and whose status has not yet been reported since they stopped, shall also be reported to the calling process.

waitpid with options equal to WUNTRACED and pid^ equal to (pid_t)-1 is identical to a call to wait(S).

Return values

If waitpid returns because the status of a child process is available, it returns the process ID of the child process for which status is reported. If waitpid was invoked with WNOHANG set in options, it has at least one child process specified by pid for which status is not available, and status is not available for any process specified by pid, waitpid returns 0. Otherwise, waitpid returns -1 and sets errno to identify the error.

Errors

In the following conditions, waitpid fails and sets errno to:

EINTR: waitpid was interrupted due to the receipt of a signal sent by the calling process.
EINVAL: An invalid value was specified for options.
ECHILD: The process or process group specified by pid does not exist or is not a child of the calling process or can never be in the states specified by options.

References

exec(S), exit(S), fork(S), intro(S), pause(S), ptrace(S), signal(S), sigaction(S), siginfo(M), wait(S), waitid(S), wstat(M)

Notices

Considerations for threads programming

While one thread is blocked, siblings might still be executing.