c - Syscall implementation of exit()

Question

I wrote a simple C program which just calls the exit() function, however strace says that the binary is actually calling exit_group, is exit() a exit_group() wrapper? Are these two functions equivalent? If so why would the compiler choose exit_group() over exit()?

Answer 1

The Linux and glibc man pages document all of this (See especially the "C library/kernel differences" in the NOTES section).

• _exit(2): In glibc 2.3 and later, this wrapper function actually uses the Linux SYS_exit_group system call to exit all threads. Before glibc2.3, it was a wrapper for SYS_exit to exit just the current thread.

• exit_group(2): glibc wrapper for SYS_exit_group, which exits all threads.

• exit(3): The ISO C89 function which flushes buffers and then exits the whole process. (It always uses exit_group() because there's no benefit to checking if the process was single-threaded and deciding to use SYS_exit vs. SYS_exit_group). As @Matteo points out, recent ISO C / POSIX standards are thread-aware and one or both probably require this behaviour.

  But apparently exit(3) itself is not thread-safe (in the C library cleanup parts), so I guess don't call it from multiple threads at once.

• syscall / int 0x80 with SYS_exit: terminates just the current thread, leaving others running. AFAIK, modern glibc has no thin wrapper function for this Linux system call, but I think pthread_exit() uses it if this isn't the last thread. (Otherwise exit(3) -> exit_group(2).)

Only exit(), not _exit() or exit_group(), flushes stdout, leading to "printf doesn't print anything" problems in newbie asm programs if writing to a pipe (which makes stdout full-buffered instead of line-buffered), or if you forgot the in the format string. For example, How come _exit(0) (exiting by syscall) prevents me from receiving any stdout content?. If you use any buffered I/O functions, or at_exit, or anything like that, it's usually a good idea to call the libc exit(3) function instead of the system call directly. But of course you can call fflush before SYS_exit_group.

(Also related: On x64 Linux, what is the difference between syscall, int 0x80 and ret to exit a program? - ret from main is equivalent to calling exit(3))

---

It's not of course the compiler that chose anything, it's libc. When you include headers and write read(fd, buf, 123) or exit(1), the C compiler just sees an ordinary function call.

Some C libraries (e.g. musl, but not glibc) may use inline asm to inline a syscall instruction into your binary, but still the headers are part of the C library, not the compiler.