Other Aliaswait, waitpid, wait3
SYNOPSISIn sys/types.h In sys/wait.h Ft pid_t Fn wait int *status In sys/time.h In sys/resource.h Ft pid_t Fn waitpid pid_t wpid int *status int options Ft pid_t Fn wait3 int *status int options struct rusage *rusage Ft pid_t Fn wait4 pid_t wpid int *status int options struct rusage *rusage
DESCRIPTIONThe Fn wait function suspends execution of its calling process until Fa status information is available for a terminated child process, or a signal is received. On return from a successful Fn wait call, the Fa status area contains termination information about the process that exited as defined below.
The Fn wait4 system call provides a more general interface for programs that need to wait for certain child processes, that need resource utilization statistics accumulated by child processes, or that require options. The other wait functions are implemented using Fn wait4 .
The Fa wpid argument specifies the set of child processes for which to wait. If Fa wpid is -1, the call waits for any child process. If Fa wpid is 0, the call waits for any child process in the process group of the caller. If Fa wpid is greater than zero, the call waits for the process with process id Fa wpid . If Fa wpid is less than -1, the call waits for any process whose process group id equals the absolute value of Fa wpid .
The Fa status argument is defined below.
The Fa options argument contains the bitwise OR of any of the following options. The WCONTINUED option indicates that children of the current process that have continued from a job control stop, by receiving a SIGCONT signal, should also have their status reported. The WNOHANG option is used to indicate that the call should not block if there are no processes that wish to report status. If the WUNTRACED option is set, children of the current process that are stopped due to a SIGTTIN , SIGTTOU , SIGTSTP or SIGSTOP signal also have their status reported. The WSTOPPED option is an alias for WUNTRACED The WNOWAIT option keeps the process whose status is returned in a waitable state. The process may be waited for again after this call completes.
If Fa rusage is non-zero, a summary of the resources used by the terminated process and all its children is returned (this information is currently not available for stopped or continued processes).
When the WNOHANG option is specified and no processes wish to report status, Fn wait4 returns a process id of 0.
The Fn waitpid function is identical to Fn wait4 with an Fa rusage value of zero. The older Fn wait3 call is the same as Fn wait4 with a Fa wpid value of -1.
The following macros may be used to test the manner of exit of the process. One of the first three macros will evaluate to a non-zero (true) value:
- Fn WIFCONTINUED status
- True if the process has not terminated, and has continued after a job control stop. This macro can be true only if the wait call specified the WCONTINUED option).
- Fn WIFEXITED status
- True if the process terminated normally by a call to _exit2 or exit(3).
- Fn WIFSIGNALED status
- True if the process terminated due to receipt of a signal.
- Fn WIFSTOPPED status
- True if the process has not terminated, but has stopped and can be restarted. This macro can be true only if the wait call specified the WUNTRACED option or if the child process is being traced (see ptrace(2)).
Depending on the values of those macros, the following macros produce the remaining status information about the child process:
- Fn WEXITSTATUS status
- If Fn WIFEXITED status is true, evaluates to the low-order 8 bits of the argument passed to _exit2 or exit(3) by the child.
- Fn WTERMSIG status
- If Fn WIFSIGNALED status is true, evaluates to the number of the signal that caused the termination of the process.
- Fn WCOREDUMP status
- If Fn WIFSIGNALED status is true, evaluates as true if the termination of the process was accompanied by the creation of a core file containing an image of the process when the signal was received.
- Fn WSTOPSIG status
- If Fn WIFSTOPPED status is true, evaluates to the number of the signal that caused the process to stop.
NOTESSee sigaction(2) for a list of termination signals. A status of 0 indicates normal termination.
If a parent process terminates without waiting for all of its child processes to terminate, the remaining child processes are assigned the parent process 1 ID (the init process ID).
If a signal is caught while any of the Fn wait calls are pending, the call may be interrupted or restarted when the signal-catching routine returns, depending on the options in effect for the signal; see discussion of SA_RESTART in sigaction(2).
The implementation queues one SIGCHLD signal for each child process whose status has changed, if Fn wait returns because the status of a child process is available, the pending SIGCHLD signal associated with the process ID of the child process will be discarded. Any other pending SIGCHLD signals remain pending.
If SIGCHLD is blocked, Fn wait returns because the status of a child process is available, the pending SIGCHLD signal will be cleared unless another status of the child process is available.
RETURN VALUESIf Fn wait returns due to a stopped, continued, or terminated child process, the process ID of the child is returned to the calling process. Otherwise, a value of -1 is returned and errno is set to indicate the error.
If Fn wait4 , Fn wait3 , or Fn waitpid returns due to a stopped, continued, or terminated child process, the process ID of the child is returned to the calling process. If there are no children not previously awaited, -1 is returned with errno set to Er ECHILD . Otherwise, if WNOHANG is specified and there are no stopped, continued or exited children, 0 is returned. If an error is detected or a caught signal aborts the call, a value of -1 is returned and errno is set to indicate the error.
ERRORSThe Fn wait function will fail and return immediately if:
- Bq Er ECHILD
- The calling process has no existing unwaited-for child processes.
- Bq Er ECHILD
- No status from the terminated child process is available because the calling process has asked the system to discard such status by ignoring the signal SIGCHLD or setting the flag SA_NOCLDWAIT for that signal.
- Bq Er EFAULT
- The Fa status or Fa rusage argument points to an illegal address. (May not be detected before exit of a child process.)
- Bq Er EINTR
- The call was interrupted by a caught signal, or the signal did not have the SA_RESTART flag set.
STANDARDSThe Fn wait and Fn waitpid functions are defined by POSIX; Fn wait4 and Fn wait3 are not specified by POSIX. The Fn WCOREDUMP macro and the ability to restart a pending Fn wait call are extensions to the POSIX interface.
HISTORYThe Fn wait function appeared in AT&T System v6 .