On Fri, Feb 04, 2022 at 23:11:29 +0100, Jakub Wilk wrote: > https://www.sourceware.org/ml/libc-alpha/2018-09/msg00270.html hints that > you can use poll() to query for open fds. That should be efficient, portable > and async-signal-safe.
I just saw this report, and happened to have some code to do that. I am attaching it for reference. Compared to closefrom_procfs(), there would be no efficient way to see file descriptors above the limit. With Linux having added close_range() a while ago, this may all be moot. With poll(), a fall-back is needed, to deal with the "Unable to allocate memory for kernel data structures" case, and the bizarre "The nfds value exceeds the RLIMIT_NOFILE value". A system call to return the next open descriptor, equal to or greater than a specified value, would be useful; ConnectServerInfo() can do this on QNX. I wonder whether anything is known to depend on this async-signal-safety or large-descriptor-handling. Hypothetically, libbsd's non-preservation of errno could also cause a portability problem. (Regarding the referenced mailing list discussion, it would be better to have multi-threaded processes never call fork(); they should have forked while still single-threaded, creating a "fork server" to call fork() and exec() as necessary. Firefox does this. It avoids problems relating to signals, memory [over-]commitments, and the file descriptor table.) -- Michael
/* Permission is hereby granted, irrevocably and free of charge, to any * person obtaining a copy of this software and associated documentation * files (the "Software"), to deal in the Software without restriction, * including without limitation the rights to use, copy, modify, merge, * publish, distribute, sublicense, and/or sell copies of the Software, * and to permit persons to whom the Software is furnished to do so. * * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. * * In addition to the above license, the author(s) have dedicated * all copyright and related and neighboring rights to this work * to the public domain worldwide, to the extent possible under law. * For details: git cat-file blob 0e259d42c996742e9e3cba14c677129b2c1b6311 * or see: https://creativecommons.org/publicdomain/zero/1.0/legalcode */ #ifndef HDRINCL_UTIL_FD_ITER_H #define HDRINCL_UTIL_FD_ITER_H 20241002 #include <errno.h> #include <fcntl.h> #include <poll.h> #include <stdint.h> struct fd_iter { unsigned iter_next, iter_end, map_end; uint8_t map[32]; }; static _Bool fd_is_valid(int fd) { int flags; do { flags = fcntl(fd, F_GETFD); } while ((-1 == flags) && (EINTR == errno)); return -1 != flags; } static inline void fd_iter_init(struct fd_iter *iter, int low_fd, int high_fd) { *iter = (struct fd_iter){0}; iter->iter_next = (low_fd >= 0) ? (unsigned)low_fd : -1u; if (high_fd > low_fd) { iter->iter_end = 1 + (unsigned)high_fd; } } static inline _Bool X_fd_iter_refresh_map(struct fd_iter *iter) { struct pollfd poll_fds[8 * sizeof iter->map] = {{.events=0}}; if (iter->map_end > -256u) { // avoid poll() till the counter wraps to 0 if (++iter->map_end) { return 0; } } // use poll() to look for valid file descriptors { nfds_t nfds = 0; int rv; while (nfds < 8 * sizeof iter->map) { unsigned ufd = iter->iter_next + (unsigned)nfds; if (ufd >= iter->iter_end) { break; } poll_fds[nfds++].fd = (int)ufd; } rv = poll(&poll_fds[0], nfds, 0 /*timeout*/); if (rv < 0) { // avoid using poll() for the next several calls iter->map_end = -4u; return 0; } } // store the poll() results in iter->map { unsigned u; uint8_t mask = 1; for (u = 0; u < sizeof iter->map; ++u) { iter->map[u] = 0; } for (u = 0; u < 8 * sizeof iter->map; ++u) { if (0 == (poll_fds[u].revents & POLLNVAL)) { iter->map[u / 8] |= mask; } // rotate mask left if (mask & 0x80) { mask = 1; } else { mask <<= 1; } } iter->map_end = iter->iter_next + u; } return 1; } static inline _Bool X_fd_iter_advance(int *fd_out, struct fd_iter *iter, _Bool consume) { for (; iter->iter_next < iter->iter_end; ++iter->iter_next) { unsigned map_start = iter->map_end - (unsigned)(8 * sizeof iter->map); unsigned idx = iter->iter_next; if ((idx < map_start) || (idx >= iter->map_end)) { if (X_fd_iter_refresh_map(iter)) { map_start = iter->iter_next; } else { if (fd_is_valid((int)iter->iter_next)) { goto found; } continue; } } idx -= map_start; if (( iter->map[idx / 8] & (1u << (idx & 7)) )) { goto found; } } return 0; found: *fd_out = (int)iter->iter_next; if (consume) iter->iter_next += 1; return 1; } static inline _Bool fd_iter_peek(int *fd_out, struct fd_iter *iter) { return X_fd_iter_advance(fd_out, iter, 0); } static inline _Bool fd_iter_next(int *fd_out, struct fd_iter *iter) { return X_fd_iter_advance(fd_out, iter, 1); } #endif // !defined HDRINCL_UTIL_FD_ITER_H // originally written by Michael Gold <[email protected]>
/* Permission is hereby granted, irrevocably and free of charge, to any * person obtaining a copy of this software and associated documentation * files (the "Software"), to deal in the Software without restriction, * including without limitation the rights to use, copy, modify, merge, * publish, distribute, sublicense, and/or sell copies of the Software, * and to permit persons to whom the Software is furnished to do so. * * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. * * In addition to the above license, the author(s) have dedicated * all copyright and related and neighboring rights to this work * to the public domain worldwide, to the extent possible under law. * For details: git cat-file blob 0e259d42c996742e9e3cba14c677129b2c1b6311 * or see: https://creativecommons.org/publicdomain/zero/1.0/legalcode */ #include <stdio.h> #include "fd_iter.h" int main(int argc, char **argv) { int fd; struct fd_iter iter; fd_iter_init(&iter, 0, 1023); while (fd_iter_next(&fd, &iter)) { printf("%d\n", fd); } } // originally written by Michael Gold <[email protected]>
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