NAME

wait, waitpid - wait for a child process to stop or terminate

 SYNOPSIS

#include <sys/types.h>
#include <sys/wait.h>

pid_t wait(int *stat_loc);
pid_t waitpid(pid_t pid, int *stat_loc, int options);

 DESCRIPTION

The wait() and waitpid() functions allow the calling process to obtain status information pertaining to one of its child processes. Various options permit status information to be obtained for child processes that have terminated or stopped. If status information is available for two or more child processes, the order in which their status is reported is unspecified.

The wait() function will suspend execution of the calling thread until status information for one of its terminated child processes is available, or until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process. If more than one thread is suspended in wait() or waitpid() awaiting termination of the same process, exactly one thread will return the process status at the time of the target process termination. If status information is available prior to the call to wait(), return will be immediate.

The waitpid() function will behave identically to wait(), if the pid argument is (pid_t)-1 and the options argument is 0. Otherwise, its behaviour will be modified by the values of the pid and options arguments.

The pid argument specifies a set of child processes for which status is requested. The waitpid() function will only return the status of a child process from this set:

• If pid is equal to (pid_t)-1, status is requested for any child process. In this respect, waitpid() is then equivalent to wait().

• If pid is greater than 0, it specifies the process ID of a single child process for which status is requested.

• If pid is 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.

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

WCONTINUED

The waitpid() function will report the status of any continued child process specified by pid whose status has not been reported since it continued from a job control stop.

WNOHANG

The waitpid() function will not suspend execution of the calling thread if status is not immediately available for one of the child processes specified by pid.

WUNTRACED

The status of any child processes specified by pid that are stopped, and whose status has not yet been reported since they stopped, will also be reported to the requesting process.

If the calling process has SA_NOCLDWAIT set or has SIGCHLD set to SIG_IGN, and the process has no unwaited for children that were transformed into zombie processes, the calling thread will block until all of the children of the process containing the calling thread terminate, and wait() and waitpid() will fail and set errno to [ECHILD].

If wait() or waitpid() return because the status of a child process is available, these functions will return a value equal to the process ID of the child process. In this case, if the value of the argument stat_loc is not a null pointer, information will be stored in the location pointed to by stat_loc. If and only if the status returned is from a terminated child process that returned 0 from main() or passed 0 as the status argument to _exit() or exit(), the value stored at the location pointed to by stat_loc will be 0. Regardless of its value, this information may be interpreted using the following macros, which are defined in <sys/wait.h> and evaluate to integral expressions; the stat_val argument is the integer value pointed to by stat_loc.

WIFEXITED(stat_val)

Evaluates to a non-zero value if status was returned for a child process that terminated normally.

WEXITSTATUS(stat_val)

If the value of WIFEXITED(stat_val) is non-zero, this macro evaluates to the low-order 8 bits of the status argument that the child process passed to _exit() or exit(), or the value the child process returned from main().

WIFSIGNALED(stat_val)

Evaluates to non-zero value if status was returned for a child process that terminated due to the receipt of a signal that was not caught (see <signal.h>).

WTERMSIG(stat_val)

If the value of WIFSIGNALED(stat_val) is non-zero, this macro evaluates to the number of the signal that caused the termination of the child process.

WIFSTOPPED(stat_val)

Evaluates to a non-zero value if status was returned for a child process that is currently stopped.

WSTOPSIG(stat_val)

If the value of WIFSTOPPED(stat_val) is non-zero, this macro evaluates to the number of the signal that caused the child process to stop.

WIFCONTINUED(stat_val)

Evaluates to a non-zero value if status was returned for a child process that has continued from a job control stop.

If the information pointed to by stat_loc was stored by a call to waitpid() that specified the WUNTRACED flag  and did not specify the WCONTINUED flag, exactly one of the macros WIFEXITED(*stat_loc), WIFSIGNALED(*stat_loc), and WIFSTOPPED(*stat_loc), will evaluate to a non-zero value.

If the information pointed to by stat_loc was stored by a call to waitpid() that specified the WUNTRACED  and WCONTINUED flags, exactly one of the macros WIFEXITED(*stat_loc), WIFSIGNALED(*stat_loc), WIFSTOPPED(*stat_loc),  and WIFCONTINUED(*stat_loc), will evaluate to a non-zero value.

If the information pointed to by stat_loc was stored by a call to waitpid() that did not specify the WUNTRACED  or WCONTINUED flags, or by a call to the wait() function, exactly one of the macros WIFEXITED(*stat_loc) and WIFSIGNALED(*stat_loc) will evaluate to a non-zero value.

If the information pointed to by stat_loc was stored by a call to waitpid() that did not specify the WUNTRACED flag  and specified the WCONTINUED flag, or by a call to the wait() function, exactly one of the macros WIFEXITED(*stat_loc), WIFSIGNALED(*stat_loc),  and WIFCONTINUED(*stat_loc), will evaluate to a non-zero value.

There may be additional implementation-dependent circumstances under which wait() or waitpid() report status. This will not occur unless the calling process or one of its child processes explicitly makes use of a non-standard extension. In these cases the interpretation of the reported status is implementation-dependent.

If a parent process terminates without waiting for all of its child processes to terminate, the remaining child processes will be assigned a new parent process ID corresponding to an implementation-dependent system process.

 RETURN VALUE

If wait() or waitpid() returns because the status of a child process is available, these functions will return a value equal to the process ID of the child process for which status is reported. If wait() or waitpid() returns due to the delivery of a signal to the calling process, -1 will be returned and errno will be set to [EINTR]. 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, 0 will be returned. Otherwise, (pid_t)-1 will be returned, and errno will be set to indicate the error.

 ERRORS

The wait() function will fail if:

[ECHILD]

The calling process has no existing unwaited-for child processes.

[EINTR]

The function was interrupted by a signal. The value of the location pointed to by stat_loc is undefined.

The waitpid() function will fail if:

[ECHILD]

The process or process group specified by pid does not exist or is not a child of the calling process.

[EINTR]

The function was interrupted by a signal. The value of the location pointed to by stat_loc is undefined.

[EINVAL]

The options argument is not valid.

 EXAMPLES

None.

 APPLICATION USAGE

None.

 FUTURE DIRECTIONS

None.

 SEE ALSO

exec, exit(), fork(), wait3(), waitid(), <sys/types.h>, <sys/wait.h>.

DERIVATION

Derived from Issue 1 of the SVID.