How does waitpid work

The third reason for the waitpid function is to permit independent sections of a process to spawn and wait for children without interfering with each other.

For example, the following problem occurs in developing a portable shell, or command interpreter:. On all historical implementations, the final pclose fails to reap the wait status of the popen. The status values are retrieved by macros, rather than given as specific bit encodings as they are in most historical implementations and thus expected by existing programs. This was necessary to eliminate a limitation on the number of signals an implementation can support that was inherent in the traditional encodings.

Some of the macro names were adopted from 4. These macros syntactically operate on an arbitrary integer value. However, that did not follow the conventions of other specifications in this volume of IEEE Std The extension that affects wait and waitpid and is common in historical implementations is the ptrace function.

Most applications do not need to concern themselves with such extensions because they have control over what extensions they or their children use. However, applications, such as command interpreters, that invoke arbitrary processes may see this behavior when those arbitrary processes misuse such extensions.

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What does waitpid do? Ask Question. Asked 11 years, 2 months ago. Active 8 years, 6 months ago. Viewed 11k times. The wait system call suspends execution of the current process until one of its children terminates. The waitpid system call suspends execution of the current process until a child specified by pid argument has changed state.

By default, waitpid waits only for terminated children, but this behaviour is modifiable via the options argument, as described below. For Linux-only options, see below. If status is not NULL, wait and waitpid store status information in the int to which it points. This integer can be inspected with the following macros which take the integer itself as an argument, not a pointer to it, as is done in wait and waitpid!

The waitid system call available since Linux 2. A child that terminates, but has not been waited for becomes a "zombie". The kernel maintains a minimal set of information about the zombie process PID, termination status, resource usage information in order to allow the parent to later perform a wait to obtain information about the child. It seems like the child dies before the parent calls waitpid , but I thought it the child just became a zombie until it is collected by the waitpid.

Does anyone have any clues? It looks though I didn't compile and run the program that the problem is not that the child exits to quickly, but that the parent gets to waitpid before the child process has even started up, thus the error message "No child processes". This is the problem. Hamilton inet: j