ECHILD

ECHILD is an errno value used by POSIX-compliant systems to indicate that a process has no child processes to wait for. It is defined in errno.h and is commonly reported by wait, waitpid, and waitid when there are no unwaited-for child processes left for the calling process.

The typical meaning of ECHILD is that the calling process either has no children at all or has already waited for all of its children. In this case, a wait-related system call returns −1 and sets errno to ECHILD. This differs from ESRCH, which means that a specific process identifier does not exist or is not a child of the caller. In other words, ECHILD is about the absence of waiting targets (children), not the absence of the process indicated by a particular PID.

Strerror(ECHILD) on most systems yields "No child processes." The value of ECHILD is implementation-defined and may differ across platforms, but its purpose remains the same in errno-based error reporting. Developers typically check for ECHILD to detect that there are no more child processes to reap and to handle such situations gracefully, for example by stopping a loop that waits for child state changes or by performing alternate work.

In summary, ECHILD signals that there are no child processes left to wait for in the calling process, distinguishing it from errors that indicate an invalid or nonexistent target.