WAIT
Section: Linux Programmer's Manual (2)
Updated: 24 July 1993
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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 process until a child 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
has already exited by the time of the call (a so-called "zombie"
process), the function returns immediately. Any system 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"
process), 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
- -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 exclusive OR of zero or more of the following 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.
If
status
is not
NULL,
wait
or
waitpid
store status information in the location pointed to by
statloc.
This status can be evaluated with the following macros (these macros take
the stat buffer as an argument --- not a pointer to the buffer!):
- WIFEXITED(status)
-
is non -zero if the child exited normally.
- WEXITSTATUS(status)
-
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 evaluated if
WIFEXITED
returned non-zero.
- WIFSIGNALED(status)
-
returns true if the child process exited because of a signal which was
not caught.
- WTERMSIG(status)
-
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(status)
-
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(status)
-
returns the number of the signal which caused the child to stop. This
macro can only be evaluated if
WIFSTOPPED
returned non-zero.
RETURN VALUE
The process ID of the child which exited, -1 on error or zero if
WNOHANG
was used and no child was available (in which case,
errno
is set to an appropriate value).
ERRORS
- ECHILD
-
if the child process specified in
pid
does not exist.
- EPERM
-
if the effective userid of the calling process does not match that of
the process being waited for, and the effective userid of the calling
process it not that of the superuser.
- ERESTARTSYS
-
if
WNOHANG
was not set and an unblocked signal or a
SIGCHLD
was caught; this is an extension to the POSIX.1 standard.
CONFORMS TO
POSIX.1
SEE ALSO
signal(2), wait4(2), signal(7)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMS TO
-
- SEE ALSO
-
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