What is 4 (0x00000004)

EINTR

If a signal handler is invoked while a system call or library function call is blocked, then either:

• the call is automatically restarted after the signal handler returns; or
• the call fails with the error EINTR.

Which of these two behaviors occurs depends on the interface and whether or not the signal handler was established using the SA_RESTART flag. The details vary across UNIX systems; below, the details for Linux.

If a blocked call to one of the following interfaces is interrupted by a signal handler, then the call is automatically restarted after the signal handler returns if the SA_RESTART flag was used; otherwise the call fails with the error EINTR:

• read(2), readv(2), write(2), writev(2), and ioctl(2) calls on "slow" devices. A "slow" device is one where the I/O call may block for an indefinite time, for example, a terminal, pipe, or socket. If an I/O call on a slow device has already transferred some data by the time it is interrupted by a signal handler, then the call will return a success status (normally, the number of bytes transferred). Note that a (local) disk is not a slow device according to this definition; I/O operations on disk devices are not interrupted by signals.
• open(2), if it can block (e.g., when opening a FIFO; see fifo(7)).
• wait(2), wait3(2), wait4(2), waitid(2), and waitpid(2).
• Socket interfaces: accept(2), connect(2), recv(2), recvfrom(2), recvmmsg(2), recvmsg(2), send(2), sendto(2), and sendmsg(2), unless a timeout has been set on the socket (see below).
• File locking interfaces: flock(2) and the F_SETLKW and F_OFD_SETLKW operations of fcntl(2)
• POSIX message queue interfaces: mq_receive(3), mq_timedreceive(3), mq_send(3), and mq_timedsend(3).
• futex(2) FUTEX_WAIT (since Linux 2.6.22; beforehand, always failed with EINTR).
• getrandom(2).
• pthread_mutex_lock(3), pthread_cond_wait(3), and related APIs.
• futex(2) FUTEX_WAIT_BITSET.
• POSIX semaphore interfaces: sem_wait(3) and sem_timedwait(3) (since Linux 2.6.22; beforehand, always failed with EINTR).
• read(2) from an inotify(7) file descriptor (since Linux 3.8; beforehand, always failed with EINTR).