WAIT(T) Linux Programmer's Manual WAIT(T) NAME wait, waitpid - wait for process termination SYNOPSIS #include <sys/types.h> #include <sys/wait.h> pid_t wait(int *status); pid_t waitpid(pid_t pid, int *status, int options); DESCRIPTION The wait function suspends execution of the current pro- cess until a child has exited, or until a signal is deliv- ered whose action is to terminate the current process or to call a signal handling function. If a child has already exited by the time of the call (a so-called "zom- bie" process), the function returns immediately. Any sys- tem resources used by the child are freed. The waitpid function suspends execution of the current process until a child as specified by the pid argument has exited, or until a signal is delivered whose action is to terminate the current process or to call a signal handling function. If a child as requested by pid has already exited by the time of the call (a so-called "zombie" pro- cess), the function returns immediately. Any system resources used by the child are freed. The value of pid can be one of: < -1 which means to wait for any child process whose process group ID is equal to the absolute value of pid. -1 which means to wait for any child process; this is the same behaviour which wait exhibits. 0 which means to wait for any child process whose process group ID is equal to that of the calling process. > 0 which means to wait for the child whose process ID is equal to the value of pid. The value of options is an OR of zero or more of the fol- lowing constants: WNOHANG which means to return immediately if no child has exited. WUNTRACED which means to also return for children which are stopped, and whose status has not been reported. (For Linux-only options, see below.) If status is not NULL, wait or waitpid store status infor- mation in the location pointed to by status. This status can be evaluated with the following macros (these macros take the stat buffer (an int) as an argument -- not a pointer to the buffer!): WIFEXITED(D) is non-zero if the child exited normally. WEXITSTATUS(S) evaluates to the least significant eight bits of the return code of the child which terminated, which may have been set as the argument to a call to exit() or as the argument for a return statement in the main program. This macro can only be evalu- ated if WIFEXITED returned non-zero. WIFSIGNALED(D) returns true if the child process exited because of a signal which was not caught. WTERMSIG(G) returns the number of the signal that caused the child process to terminate. This macro can only be evaluated if WIFSIGNALED returned non-zero. WIFSTOPPED(D) returns true if the child process which caused the return is currently stopped; this is only possible if the call was done using WUNTRACED. WSTOPSIG(G) returns the number of the signal which caused the child to stop. This macro can only be evaluated if WIFSTOPPED returned non-zero. Some versions of Unix (e.g. Linux, Solaris, but not AIX, SunOS) also define a macro WCOREDUMP(P) to test whether the child process dumped core. Only use this enclosed in #ifdef WCOREDUMP ... #endif. RETURN VALUE The process ID of the child which exited, or zero if WNO- HANG was used and no child was available, or -1 on error (in which case errno is set to an appropriate value). ERRORS ECHILD if the process specified in pid does not exist or is not a child of the calling process. (This can happen for one's own child if the action for SIGCHLD is set to SIG_IGN. See also the NOTES sec- tion about threads.) EINVAL if the options argument was invalid. EINTR if WNOHANG was not set and an unblocked signal or a SIGCHLD was caught. NOTES The Single Unix Specification describes a flag SA_NOCLD- WAIT (not supported under Linux) such that if either this flag is set, or the action for SIGCHLD is set to SIG_IGN (which, by the way, is not allowed by POSIX), then chil- dren that exit do not become zombies and a call to wait() or waitpid() will block until all children have exited, and then fail with errno set to ECHILD. LINUX NOTES In the Linux kernel, a kernel-scheduled thread is not a distinct construct from a process. Instead, a thread is simply a process that is created using the Linux-unique clone(e) system call; other routines such as the portable pthread_create(e) call are implemented using clone(e). Before Linux 2.4, a thread was just a special case of a process, and as a consequence one thread could not wait on the children of another thread, even when the latter belongs to the same thread group. However, POSIX pre- scribes such functionality, and since Linux 2.4 a thread can, and by default will, wait on children of other threads in the same thread group. The following Linux-specific options are for use with children created using clone(e). __WCLONE Wait for "clone" children only. If omitted then wait for "non-clone" children only. (A "clone" child is one which delivers no signal, or a signal other than SIGCHLD to its parent upon termination.) This option is ignored if __WALL is also specified. __WALL (Since Linux 2.4) Wait for all children, regardless of type ("clone" or "non-clone"). __WNOTHREAD (Since Linux 2.4) Do not wait for children of other threads in the same thread group. This was the default before Linux 2.4. CONFORMING TO SVr4, POSIX.1 SEE ALSO clone(e), signal(l), wait4(4), pthread_create(e), sig- nal(l) Linux 2000-07-24 WAIT(T)