Thus spake Peter Meerwald-Stadler (pme...@pmeerw.net): > > I am trying to add inotify support to my tail implementation (for -F). > > This is what happens: > > > > inotify_init() = 4 > > inotify_add_watch(4, "/tmp/foo", IN_MODIFY) = 1 > > inotify_rm_watch(4, 1) = -1 EINVAL (Invalid argument) > > inotify_add_watch(4, "/tmp/foo", IN_MODIFY) = 2 > > > > There is also some polling, some reading and some statting going on here, > > but > > those are on other descriptors than 4 so they should not matter). > > > > Can somebody explain the EINVAL I'm getting from inotify_rm_watch to me? > > This is a stock kernel 4.5.0.
> #include <stdio.h> > #include <sys/inotify.h> > int main() { > int fd, i, j; > printf("init %d\n", fd=inotify_init()); // 3 > printf("add %d\n", i=inotify_add_watch(fd, "/tmp/foo", IN_MODIFY)); // 1 > printf("rm %d\n", inotify_rm_watch(fd, i)); // 0 > printf("add %d\n", j=inotify_add_watch(fd, "/tmp/foo", IN_MODIFY)); // 2 > return 0; > } > Ubuntu kernel x86_64 4.4.0-21, seems to work here > so we have to guess what's going on between _add and _rm? Wait! It just occurred to me that this does not make any sense at all. You use the name of the file with inotify_add_watch, not the descriptor to the file. Why would closing the file matter? My "load generator" test program is: #include <unistd.h> #include <assert.h> #include <fcntl.h> #include <stdio.h> int main() { int fd=open("/tmp/foo",O_WRONLY|O_CREAT|O_TRUNC,0600); assert(fd>-1); sleep(1); write(fd,"1\n",2); sleep(1); write(fd,"2\n",2); int fd2=open("/tmp/bar",O_WRONLY|O_CREAT|O_TRUNC,0600); assert(fd>-1); write(fd2,"3\n",2); rename("/tmp/bar","/tmp/foo"); close(fd); sleep(1); write(fd2,"4\n",2); close(fd2); } I touch /tmp/foo first, then I run my inotify tail -F on it, and I expect the output to be 1\n2\n3\n4\n It is. Then I press Ctrl-C. Here is the strace of the tail: execve("./bin-x86_64/tail", ["./bin-x86_64/tail", "-F", "/tmp/foo"], [/* 57 vars */]) = 0 arch_prctl(ARCH_SET_FS, 0x7fff1b1e2920) = 0 rt_sigaction(SIGPIPE, {SIG_IGN, [PIPE], SA_RESTORER|SA_NODEFER, 0x4018d0}, {SIG_DFL, [], 0}, 8) = 0 open("/tmp/foo", O_RDONLY) = 3 fstat(3, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f251cf9f000 read(3, "", 32768) = 0 inotify_init() = 4 inotify_add_watch(4, "/tmp/foo", IN_MODIFY) = 1 fstat(3, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0 stat("/tmp/foo", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0 poll([{fd=4, events=POLLIN}], 1, 1000) = 0 (Timeout) fstat(3, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0 stat("/tmp/foo", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0 poll([{fd=4, events=POLLIN}], 1, 1000) = 1 ([{fd=4, revents=POLLIN}]) read(4, "\1\0\0\0\2\0\0\0\0\0\0\0\0\0\0\0", 2048) = 16 fstat(3, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0 stat("/tmp/foo", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0 poll([{fd=4, events=POLLIN}], 1, 1000) = 1 ([{fd=4, revents=POLLIN}]) read(4, "\1\0\0\0\2\0\0\0\0\0\0\0\0\0\0\0", 2048) = 16 fstat(3, {st_mode=S_IFREG|0644, st_size=2, ...}) = 0 read(3, "1\n", 8192) = 2 write(1, "1\n", 2) = 2 read(3, "", 8192) = 0 poll([{fd=4, events=POLLIN}], 1, 1000) = 1 ([{fd=4, revents=POLLIN}]) read(4, "\1\0\0\0\2\0\0\0\0\0\0\0\0\0\0\0", 2048) = 16 fstat(3, {st_mode=S_IFREG|0644, st_size=4, ...}) = 0 read(3, "2\n", 8192) = 2 write(1, "2\n", 2) = 2 read(3, "", 8192) = 0 poll([{fd=4, events=POLLIN}], 1, 1000) = 0 (Timeout) fstat(3, {st_mode=S_IFREG|0644, st_size=4, ...}) = 0 stat("/tmp/foo", {st_mode=S_IFREG|0600, st_size=4, ...}) = 0 close(3) = 0 inotify_rm_watch(4, 1) = -1 EINVAL (Invalid argument) open("/tmp/foo", O_RDONLY) = 3 inotify_add_watch(4, "/tmp/foo", IN_MODIFY) = 2 open("/tmp", O_RDONLY) = 5 read(3, "3\n4\n", 8192) = 4 write(1, "3\n4\n", 4) = 4 read(3, "", 8192) = 0 poll([{fd=4, events=POLLIN}], 1, 1000) = 1 ([{fd=4, revents=POLLIN}]) read(4, "\1\0\0\0\0\200\0\0\0\0\0\0\0\0\0\0", 2048) = 16 fstat(3, {st_mode=S_IFREG|0600, st_size=4, ...}) = 0 stat("/tmp/foo", {st_mode=S_IFREG|0600, st_size=4, ...}) = 0 poll([{fd=4, events=POLLIN}], 1, 1000) = 0 (Timeout) fstat(3, {st_mode=S_IFREG|0600, st_size=4, ...}) = 0 stat("/tmp/foo", {st_mode=S_IFREG|0600, st_size=4, ...}) = 0 poll([{fd=4, events=POLLIN}], 1, 1000) = ? ERESTART_RESTARTBLOCK (Interrupted by signal) --- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} --- +++ killed by SIGINT +++ As you can see, I do close(3) and then inotify_rm_watch, and it returns EINVAL. If I do the inotify_rm_watch first and then the close, then both returns 0. What is going on here?! Also, I was thrown off a bit because the next inotify_add_watch returns 2. I somehow expected it to work like open, i.e. when you close fd 1 and then call inotify_add_watch, you get 1 again as wd. But that is apparently not how it works. This could maybe be clarified in the man page. Regards, Felix