Nonlocal Gotos

GCC provides the built-in functions __builtin_setjmp and __builtin_longjmp which are similar to, but not interchangeable with, the C library functions setjmp and longjmp. The built-in versions are used internally by GCC’s libraries to implement exception handling on some targets. You should use the standard C library functions declared in <setjmp.h> in user code instead of the builtins.

The built-in versions of these functions use GCC’s normal mechanisms to save and restore registers using the stack on function entry and exit. The jump buffer argument buf holds only the information needed to restore the stack frame, rather than the entire set of saved register values.

An important caveat is that GCC arranges to save and restore only those registers known to the specific architecture variant being compiled for. This can make __builtin_setjmp and __builtin_longjmp more efficient than their library counterparts in some cases, but it can also cause incorrect and mysterious behavior when mixing with code that uses the full register set.

You should declare the jump buffer argument buf to the built-in functions as:

#include <stdint.h>
intptr_t buf[5];

int __builtin_setjmp(intptr_t *buf)

This function saves the current stack context in buf. __builtin_setjmp returns 0 when returning directly, and 1 when returning from __builtin_longjmp using the same buf.

void __builtin_longjmp(intptr_t *buf, int val)

This function restores the stack context in buf, saved by a previous call to __builtin_setjmp. After __builtin_longjmp is finished, the program resumes execution as if the matching __builtin_setjmp returns the value val, which must be 1.

Because __builtin_longjmp depends on the function return mechanism to restore the stack context, it cannot be called from the same function calling __builtin_setjmp to initialize buf. It can only be called from a function called (directly or indirectly) from the function calling __builtin_setjmp.