c - When will eventfd_read() block? -

i wonder under situation eventfd_read() block?

i read manpage doesn't mention anything.

i created file descriptor through eventfd(0,0).

thanks in advance.

from eventfd(2) man page read() call:

if eventfd counter 0 @ time of call read(2), call either blocks until counter becomes nonzero (at time, read(2) proceeds described above) or fails error eagain if file descriptor has been made nonblocking.

and eventfd_read() , eventfd_write() functions:

the functions perform read , write operations on eventfd file descriptor, returning 0 if correct number of bytes transferred, or -1 otherwise.

so eventfd_read() wrapper read() , blocks when read() blocks, i.e. when eventfd counter 0 , o_nonblock not set descriptor (using fcntl(2) or efd_nonblock).

you can verify in glibc sources:

 int eventfd_read (int fd, eventfd_t *value) {   return __read (fd, value, sizeof (eventfd_t)) != sizeof (eventfd_t) ? -1 : 0; }