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WAIT !!!WAIT NAME SYNOPSIS DESCRIPTION RETURN VALUE ERRORS NOTES CONFORMING TO SEE ALSO ---- !!NAME wait, waitpid - wait for process termination !!SYNOPSIS __#include __ #include __ __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 __ 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 '' The value of ''pid'' can be one of: 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. 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 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 ''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__ 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. Some versions of Unix (e.g. Linux, Solaris, but not AIX, SunOS) also define a macro __WCOREDUMP(__''status''__)__ 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 __WNOHANG__ 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 section 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_NOCLDWAIT (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 children 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. 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(2) system call; other routines such as the portable pthread_create(3) call are implemented using clone(2). Thus, if two threads A and B are siblings, then thread A cannot wait on any processes forked by thread B or its descendents, because an uncle cannot wait on his nephews. In some other Unix-like systems, where multiple threads are implemented as belonging to a single process, thread A can wait on any processes forked by sibling thread B; you will have to rewrite any code that makes this assumption for it to work on Linux. !!CONFORMING TO SVr4, POSIX.1 !!SEE ALSO clone(2), signal(2), wait4(2), pthread_create(3), signal(7) ----
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