On Mon, Oct 23, 2023 at 10:29:45AM -0400, tom kronmiller wrote: > On Mon, Oct 23, 2023 at 10:16 AM Jon Leonard <jleon...@slimy.com> wrote: > > > More specifically, fork() does not play nicely with stdio buffering. > > > > But the fork() should not be changing the address space of the calling > process. The duplicated buffers in the child process might be an issue in > general (they aren't in this case), but the buffers in the parent process > should not be disturbed. That seems like a bug in fork() to me.
There are multiple moving parts here. You've got the memory buffers allocated by stdio, which are indeed private to each process. But you've also got the underlying file descriptors, which point to structures in the kernel corresponding to each open file/stream. unicorn:~$ seq 1 2 > 12 unicorn:~$ strace -f ./foo < 12 [...] write(1, "1\n", 21 ) = 2 write(1, "num_read: 2 at line 1\n", 22num_read: 2 at line 1 ) = 22 clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLDstrace: Process 471118 attached , child_tidptr=0x7f861ca2aa10) = 471118 [pid 471118] set_robust_list(0x7f861ca2aa20, 24 <unfinished ...> [pid 471117] wait4(471118, <unfinished ...> [pid 471118] <... set_robust_list resumed>) = 0 [pid 471118] lseek(0, -2, SEEK_CUR) = 2 [pid 471118] exit_group(0) = ? [pid 471118] +++ exited with 0 +++ [...] Here you can see that stdin (which points to a regular file) is rewound by 2 bytes in the child process. But since the child and the parent are both sharing the same open file descriptor (per fork(2)), this means stdin is rewound for the parent as well. Now, I have a couple questions at this point: 1) Why did the child process rewind stdin? 2) Why did only *one* of the child processes do this, and not the rest? But since this is not my project, I don't require answers to these questions. I can just live in my confused state, and accept Jon's maxim that "fork() does not play nicely with stdio buffering". We're not quite up to the "nasal demons" level yet, but I feel like we're approaching it. http://www.catb.org/jargon/html/N/nasal-demons.html
