longjmp() and setjmp() are useful for dealing with errors and interrupts encountered in a low-level subroutine of a program. longjmp() restores the environment saved by the last call of setjmp() with the corresponding env argument. After longjmp() is completed, program execution continues as if the corresponding call of setjmp() had just returned the value val. longjmp() cannot cause 0 to be returned. If longjmp is invoked with a second argument of 0, 1 will be returned instead.
siglongjmp() is similar to longjmp() except for the type of its env argument. If the sigsetjmp() call that set this env used a nonzero savesigs flag, siglongjmp() also restores the set of blocked signals.
These functions never return.
POSIX does not specify whether longjmp will restore the signal context. If you want to save and restore signal masks, use siglongjmp.
longjmp() and siglongjmp() make programs hard to understand and maintain. If possible an alternative should be used.