Setjmp and Longjmp(3)

NAME
Setjmp and Longjmp -

Detailed Description

While the C language has the dreaded goto statement, it can only be used to jump to a label in the same (local) function. In order to jump directly to another (non-local) function, the C library provides the setjmp() and longjmp() functions. setjmp() and longjmp() are useful for dealing with errors and interrupts encountered in a low-level subroutine of a program.

Note:
    setjmp() and longjmp() make programs hard to understand and maintain. If possible, an alternative should be used.

    longjmp() can destroy changes made to global register variables (see How to permanently bind a variable to a register?).

For a very detailed discussion of setjmp()/longjmp(), see Chapter 7 of Advanced Programming in the UNIX Environment, by W. Richard Stevens.

Example:

    #include <setjmp.h>
    jmp_buf env;
    int main (void)
    {
        if (setjmp (env))
        {
            ... handle error ...
        }
        while (1)
        {
           ... main processing loop which calls foo() some where ...
        }
    }
    ...
    void foo (void)
    {
        ... blah, blah, blah ...
        if (err)
        {
            longjmp (env, 1);
        }
    }

  Functions
int setjmp (jmp_buf __jmpb)
void longjmp (jmp_buf __jmpb, int __ret) __ATTR_NORETURN__

Function Documentation

  void longjmp (jmp_buf __jmpb, int __ret)

Non-local jump to a saved stack context.

 #include <setjmp.h>

longjmp() restores the environment saved by the last call of setjmp() with the corresponding __jmpb argument. After longjmp() is completed, program execution continues as if the corresponding call of setjmp() had just returned the value __ret.

Note:
    longjmp() cannot cause 0 to be returned. If longjmp() is invoked with a second argument of 0, 1 will be returned instead.

Parameters:
    __jmpb Information saved by a previous call to setjmp().
    __ret Value to return to the caller of setjmp().

Returns:
    This function never returns.

  int setjmp (jmp_buf __jmpb)

Save stack context for non-local goto.

 #include <setjmp.h>

setjmp() saves the stack context/environment in __jmpb for later use by longjmp(). The stack context will be invalidated if the function which called setjmp() returns.

Parameters:
    __jmpb Variable of type jmp_buf which holds the stack information such that the environment can be restored.

Returns:
    setjmp() returns 0 if returning directly, and non-zero when returning from longjmp() using the saved context.