stdout thread-safe in C on Linux?

Question

Is writing to stdout using printf thread-safe on Linux? What about using the lower-level write command?

Answer 1

It's thread-safe; printf should be reentrant, and you won't cause any strangeness or corruption in your program.

You can't guarantee that your output from one thread won't start half way through the output from another thread. If you care about that you need to develop your own locked output code to prevent multiple access.

Answer 2

They are both thread-safe to the point that your application won't crash if multiple threads call them on the same file descriptor. However, without some application-level locking, whatever is written could be interleaved.

Answer 3

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.

Answer 4

Whether you'd call it "thread-safe" depends on your definition of thread-safe. POSIX requires stdio functions to use locking, so your program will not crash, corrupt the FILE object states, etc. if you use printf simultaneously from multiple threads. However, all stdio operations are formally specified in terms of repeated calls to fgetc and fputc, so there is no larger-scale atomicity guaranteed. That is to say, if threads 1 and 2 try to print "Hello\n" and "Goodbye\n" at the same time, there's no guarantee that the output will be either "Hello\nGoodbye\n" or "Goodbye\nHello\n". It could just as well be "HGelolodboy\ne\n". In practice, most implementations will acquire a single lock for the entire higher-level write call simply because it's more efficient, but your program should not assume so. There may be corner cases where this is not done; for instance an implementation could probably entirely omit locking on unbuffered streams.

One way around this problem is the flockfile, ftrylockfile, and funlockfile functions POSIX provides. These allow one particular thread to lock a FILE for exclusive use, making it possible to achieve larger-than-single-char atomic writes.