stdout thread-safe in C on Linux?

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It’s not specified by the C standard — it depends on your implementation of the C standard library. In fact, the C standard doesn’t even mention threads at all, since certain systems (e.g. embedded systems) don’t have multithreading.

In the GNU implementation (glibc), most of the higher-level functions in stdio that deal with FILE* objects are thread-safe. The ones that aren’t usually have unlocked in their names (e.g. getc_unlocked(3)). However, the thread safety is at a per-function call level: if you make multiple calls to printf(3), for example, each of those calls is guaranteed to output atomically, but other threads might print things out between your calls to printf(). If you want to ensure that a sequence of I/O calls gets output atomically, you can surround them with a pair of flockfile(3)/funlockfile(3) calls to lock the FILE handle. Note that these functions are reentrant, so you can safely call printf() in between them, and that won’t result in deadlock even thought printf() itself makes a call to flockfile().

The low-level I/O calls such as write(2) should be thread-safe, but I’m not 100% sure of that – write() makes a system call into the kernel to perform I/O. How exactly this happens depends on what kernel you’re using. It might be the sysenter instruction, or the int (interrupt) instruction on older systems. Once inside the kernel, it’s up to the kernel to make sure that the I/O is thread-safe. In a test I just did with the Darwin Kernel Version 8.11.1, write(2) appears to be thread-safe.

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