Pretty recently I learned about setjmp() and longjmp(). They’re a neat pair of libc functions which allow you to save your program’s current execution context and resume it at an arbitrary point in the future (with some caveats¹). If you’re wondering why this is particularly useful, to quote the manpage, one of their main use cases is “…for dealing with errors and interrupts encountered in a low-level subroutine of a program.” These functions can be used for more sophisticated error handling than simple error code return values.

I was curious how these functions worked, so I decided to take a look at musl libc’s implementation for x86. First, I’ll explain their interfaces and show an example usage program. Next, since this post isn’t aimed at the assembly wizard, I’ll cover some basics of x86 and Linux calling convention to provide some required background knowledge. Lastly, I’ll walk through the source, line by line.