What Are Some Conditions That May Cause Fork() or System() Calls to Fail on Linux

fork and waitpid fail on linux. Without hitting the hard or soft limits

I finally found the problem, it is because of a memory leak in the Perl's thread module that causes the memory to grow after a long time. Then open2 can not allocate memory and fails.

Linux x86-64 fork syscall strange behavior against C standard libc FILE I/O (keywords: fork, fclose, linux)

Thanks to @Jonathan Leffler!

This problem is a duplicate of Why does forking my process cause the file to be read infinitely

The missing knowledge, why does the bug not occur on Linux 5.10.0-051000-generic turned out that it is not related to the kernel.

It turned out that the parent process competes with the child processes (not related to kernel).

• Note: change the offset of file handle from child process will also change the offset in parent process if the handle is created by the parent.
• If there is no fclose(3) in the childs, the child processes will call lseek(2) as soon as they call exit(3). This will cause the parent re-read the same offset, because the childs call lseek(2) with negative offset + SEEK_CUR.

(I don't know why it is necessary to call lseek(2) before exit, it might have been explained in @Jonathan Leffler's answer, I did not read the whole answer carefully).

• If the parent finishes to read the entire file before the childs call lseek(2). Then there is no problem at all.

Also, as @iBug has mentioned But keep in mind that process scheduling may make the result non-predictable, unless you implement some kind of "syncing".

The parent process on Linux 5.10.0-051000-generic machine I used was just a lucky process that always won to read the entire file first before the childs call lseek(2).

I tried to add more lines to the file (to be 150 lines), so the parent will mostly be slower than reading 6 lines, and the undefined behavior happens.

Test result: https://gist.githubusercontent.com/ammarfaizi2/b72bd03fcc13779f96b8bbeef9253e66/raw/da1eff4ed5434aa51929e5c810d54de8ffe15548/test2_fix.txt

Related Topics