EINTR

EINTR, short for "Interrupted system call," is an errno value used on Unix-like systems to indicate that a system call was interrupted by a signal before it could complete. It is surfaced by the C library when a blocking operation is interrupted by a signal handler.

When a process is blocked in a system call and a signal is delivered, the kernel may terminate the call and return with EINTR. Depending on the signal action, the interrupted call may be automatically restarted (via the SA_RESTART flag) or may fail with EINTR. The restart behavior can vary by system and by the specific call being interrupted.

Commonly observed with blocking operations such as read, write, accept, poll, select, nanosleep, and wait, EINTR signals that a signal was handled mid-operation. If EINTR is returned, portable code typically retries the call, taking care to handle any partial results and to avoid busy looping. The exact handling depends on the function and the surrounding logic.

Portability and values: The numeric value of EINTR is system dependent; on many Unix-like systems it is 4. Windows uses a different error-reporting mechanism, so EINTR is not used there. In portable code, programmers often check for EINTR and retry, while accounting for the possibility that the call was not restartable or that other error conditions occurred.

See also: EAGAIN, EWOULDBLOCK, SA_RESTART, errno. EINTR remains a common source of subtle bugs in event-driven and I/O-heavy programs, making correct signal handling and robust retry logic important in system programming.