From: Yann Sionneau &lt;ysionneau(a)kalray.eu&gt; Avoid calling select with empty sets which hangs the process This makes uClibc-ng act like glibc and musl Without this fix the test_poll of python3 testsuite hangs forever Scenario of the issue: If you call poll with only invalid file descriptors, like in python3 testsuite (https://github.com/python/cpython/blob/master/Lib/test/test_poll.py#L83) You will go through uClibc poll emulation code, which is based on select syscall. Your first call to select will fail, it will return -1 and errno will be set to EBADF: https://github.com/wbx-github/uclibc-ng/blob/master/libc/sysdeps/linux/comm… Then you will go through the for loop which tests individually each file descriptor by calling select on each one: https://github.com/wbx-github/uclibc-ng/blob/master/libc/sysdeps/linux/comm… each call will also return -1 with errno being equal to EBADF. Therefore all pollfd will have the POLLNVAL flag in their respective revents field. And, the most important, rset/wset/xset will stay empty. Then the for loop ends, the "continue" makes the while loop run again. The following select() is run again: https://github.com/wbx-github/uclibc-ng/blob/master/libc/sysdeps/linux/comm… But this time the sets are empty. If the poll was called with timeout set to -1, this select will hang forever because there is no timeout and the sets are empty so no event will ever wake it up. test program: int main(void) { struct pollfd pfd; int ret; int pipe_fds[2]; pipe(pipe_fds); close(pipe_fds[0]); close(pipe_fds[1]); pfd.fd = pipe_fds[0]; pfd.events = POLLIN | POLLOUT | POLLPRI; pfd.revents = 0; ret = poll(&pfd, 1, -1); printf("ret: %d\n", ret); if (ret < 0) printf("error: %s", strerror(errno)); else { puts("revents: "); if (pfd.revents & POLLERR) printf(" POLLERR"); if (pfd.revents & POLLHUP) printf(" POLLHUP"); if (pfd.revents & POLLNVAL) printf(" POLLNVAL"); puts(""); } return 0; } This hangs on uClibc-ng aarch64 and Kalray's arch (kv3) but does the following on musl and glibc: " ret: 1 revents: POLLNVAL " strace output of this program with uClibc *without* the patch applied: pselect6(4, [3], [3], [3], NULL, NULL) = -1 EBADF (Bad file descriptor) pselect6(4, [3], [3], [3], {tv_sec=0, tv_nsec=0}, NULL) = -1 EBADF (Bad file descriptor) pselect6(0, 0x7ffffffb80, 0x7ffffffb68, 0x7ffffffb50, NULL, NULL (never finishes) strace output of this program with uClibc *with* the patch applied: pselect6(4, [3], [3], [3], NULL, NULL) = -1 EBADF (Bad file descriptor) pselect6(4, [3], [3], [3], {tv_sec=0, tv_nsec=0}, NULL) = -1 EBADF (Bad file descriptor) write(1, "ret: 1\n", 7ret: 1 ) = 7 write(1, "revents: \n", 10revents: ) = 10 write(1, " POLLNVAL\n", 10 POLLNVAL ) = 10 exit_group(0) = ? +++ exited with 0 +++ --- libc/sysdeps/linux/common/poll.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/libc/sysdeps/linux/common/poll.c b/libc/sysdeps/linux/common/poll.c index d1f1f17..3d46d5b 100644 --- a/libc/sysdeps/linux/common/poll.c +++ b/libc/sysdeps/linux/common/poll.c @@ -53,6 +53,7 @@ int __NC(poll)(struct pollfd *fds, nfds_t nfds, int timeout) fd_set *rset, *wset, *xset; struct pollfd *f; int ready; + int error_num; int maxfd = 0; int bytes; @@ -142,6 +143,7 @@ int __NC(poll)(struct pollfd *fds, nfds_t nfds, int timeout) /* Reset the return value. */ ready = 0; + error_num = 0; for (f = fds; f < &fds[nfds]; ++f) if (f->fd != -1 && (f->events & (POLLIN|POLLOUT|POLLPRI)) @@ -178,8 +180,13 @@ int __NC(poll)(struct pollfd *fds, nfds_t nfds, int timeout) ++ready; } else if (errno == EBADF) + { f->revents |= POLLNVAL; + error_num++; + } } + if (ready == 0) + return error_num; /* Try again. */ continue; } -- 1.8.3.1