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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).