Is glibc's implementation of fprintf() thread-safe?

Question

Is fprintf thread-safe? The glibc manual seems to say it is, but my application, which writes to a file using single call to fprintf() seems to be intermingling partial writes from different processes.

edit: To clarify, the program in question is a lighttpd plugin, and the server is running with multiple worker threads.

Looking at the file, some of the writes are intermingled.

edit 2: It seems the problem I'm seeing might be due to lighttpd's "worker threads" actually being separate processes: http://redmine.lighttpd.net/wiki/lighttpd/Docs:MultiProcessor

Problems

By running 2 or more processes on the same socket you will have a better concurrency, but will have a few drawbacks that you have to be aware of:

• mod_accesslog might create broken access logs, as the same file is opened twice and is NOT synchronized.
• mod_status will have n separate counters, one set for each process.
• mod_rrdtool will fail as it receives the same timestamp twice.
• mod_uploadprogress will not show correct status.

Answer 1

You're confusing two concepts - writing from multiple threads and writing from multiple processes.

Inside a process its possible to ensure that one invocation of fprintf is completed before the next is allowed access to the output buffer, but once your app pumps that output to a file you're at the mercy of the OS. Without some kind of OS based locking mechanism you cant ensure that an entirely different application doesnt write to your log file.