WAIT(2)             Linux Programmer's Manual             WAIT(2)

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.

       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(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 state-
               ment 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 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.)

       EINVAL  if the options argument was invalid.

       ERESTARTSYS
               if  WNOHANG was not set and an unblocked signal or
               a SIGCHLD was caught. This error  is  returned  by
               the  system  call.   The  library interface is not
               allowed to return  ERESTARTSYS,  but  will  return
               EINTR.

NOTES
       The  Single  Unix Specification describes a flag SA_NOCLD-
       WAIT (not present 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.

CONFORMING TO
       SVr4, POSIX.1

SEE ALSO
       signal(2), wait4(2), signal(7)

Linux                      23 June 1997                         1