Hi all,
Occasionally, I get a weird behavior where sprintf replaces a comma with a null character. It doesn't happen all the time, but maybe one out of a hundred? The comma is in the middle of the string, and it's always the comma. I'm guessing that the sprintf is being interrupted by a task change, and is not handling it properly.

Thoughts?

That is completely dependent on the library you are using. Each compiler has their own, and there are also some specialist libraries around too.

Generally speaking, if you were output to a device, such as printf'ing to a UART or a console, then the code is almost for sure not going to like a multi tasking environment and you will need to use a mutex to guard the peripheral. If you are just sprintf'ing to a buffer, and the buffer is not global or static, then most of the time mult tasking will not upset it. That is just the general case though. Complex libraries can perform malloc()s and all sorts of weird stuff that will quite likely get upset.