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longjmp

Defined in header `<setjmp.h>`
void longjmp( jmp_buf env, int status );		(until C11)
_Noreturn void longjmp( jmp_buf env, int status );		(since C11)

Loads the execution context env saved by a previous call to setjmp. This function does not return. Control is transferred to the call site of the macro setjmp that set up env. That setjmp then returns the value, passed as the status.

If the function that called setjmp has exited (whether by return or by a different longjmp higher up the stack), the behavior is undefined. In other words, only long jumps up the call stack are allowed.

Jumping across threads (if the function that called setjmp was executed by another thread) is also undefined behavior.

(since C11)

If when setjmp was called, a VLA or another variably-modified type variable was in scope and control left that scope, longjmp to that setjmp invokes undefined behavior even if control remained within the function.

On the way up the stack, longjmp does not deallocate any VLAs, memory leaks may occur if their lifetimes are terminated in this way:

void g(int n)
{
    int a[n]; // a may remain allocated
    h(n); // does not return
}
void h(int n)
{
    int b[n]; // b may remain allocated
    longjmp(buf, 2); // might cause a memory leak for h's b and g's a
}

(since C99)

Parameters

env	-	variable referring to the execution state of the program saved by `setjmp`
status	-	the value to return from `setjmp`. If it is equal to `0`, `1` is used instead

Return value

(none).

Notes

longjmp is intended for handling unexpected error conditions where the function cannot return meaningfully. This is similar to exception handling in other programming languages.

Example

#include <stdio.h>
#include <setjmp.h>
#include <stdnoreturn.h>
 
jmp_buf my_jump_buffer;
 
noreturn void foo(int count) 
{
    printf("foo(%d) called\n", count);
    longjmp(my_jump_buffer, count+1); // will return count+1 out of setjmp
}
 
int main(void)
{
    volatile int count = 0; // modified local vars in setjmp scope must be volatile
    if (setjmp(my_jump_buffer) != 5) // compare against constant in an if
        foo(++count);
}

Output:

foo(1) called
foo(2) called
foo(3) called
foo(4) called

References

C17 standard (ISO/IEC 9899:2018):

7.13.2.1 The longjmp macro (p: 191-192)

C11 standard (ISO/IEC 9899:2011):

7.13.2.1 The longjmp macro (p: 263-264)

C99 standard (ISO/IEC 9899:1999):

7.13.2.1 The longjmp macro (p: 244-245)

C89/C90 standard (ISO/IEC 9899:1990):

4.6.2.1 The longjmp function