wait(2)
NAME
wait, wait3 − wait for process to terminate
SYNTAX
#include <sys/wait.h>
pid = wait(status)
int pid;
union wait *status;
pid = wait((union wait*)0)
int pid;
#include <sys/time.h>
#include <sys/resource.h>
pid = wait3(status, options, rusage)
int pid;
union wait *status;
int options;
struct rusage *rusage;
DESCRIPTION
The wait system call causes its caller to delay until a signal is received or one of its child processes terminates. If any child has died since the last wait, return is immediate, returning the process id and exit status of one of the terminated children. If there are no children, return is immediate with the value −1 returned.
On return from a successful wait call, if status is nonzero, the high byte of status contains the low byte of the argument to exit supplied by the child process; the low byte of status contains the termination status of the process. A more precise definition of the status word is given in <sys/wait.h>.
The wait3 system call provides an alternate interface for programs which must not block when collecting the status of child processes. The status parameter is defined as above. The options parameter is used to indicate the call should not block if there are no processes which wish to report status (WNOHANG), and/or that only children of the current process which are stopped due to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP signal should have their status reported (WUNTRACED). If rusage is non-zero, a summary of the resources used by the terminated process and all its children is returned (this information is not available for stopped processes).
When the WNOHANG option is specified and no processes wish to report status, wait3 returns a pid of 0. The WNOHANG and WUNTRACED options may be combined by or’ing the two values.
See sigvec() for a list of termination statuses (signals). A 0 status indicates normal termination. A special status (0177) is returned for a process stopped by the process tracing mechanism, ptrace(.). If the 0200 bit of the termination status is set, a core image of the process was produced by the system.
If the parent process terminates without waiting on its children, the initialization process (process ID = 1) inherits the children.
The wait and wait3 system calls are automatically restarted when a process receives a signal while awaiting termination of a child process.
RETURN VALUE
If wait returns due to a stopped 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.
The wait3 system call returns −1 if there are no children not previously waited for; 0 is returned if WNOHANG is specified and there are no stopped or exited children.
ENVIRONMENT
When your program is complied using the System V environment, when the SIGCLD signal is being ignored, wait continues until all children terminate. SIGCLD is the same as SIGCHLD.
Also, when using the System V environment, status is of type int *.
DIAGNOSTICS
The wait system call will fail and return immediately if one or more of the following are true:
[ECHILD] The calling process has no existing unwaited-for child processes.
[EFAULT] The status or rusage arguments point to an illegal address.