setjmp

Defined in header `<setjmp.h>`
#define setjmp(env) /* implementation-defined */

Saves the current execution context into a variable env of type jmp_buf. This variable can later be used to restore the current execution context by longjmp function. That is, when a call to longjmp function is made, the execution continues at the particular call site that constructed the jmp_buf variable passed to longjmp. In that case setjmp returns the value passed to longjmp.

The invocation of setjmp must appear only in one of the following contexts:

the entire controlling expression of if, switch, while, do-while, for

switch(setjmp(env)) { ..

one operand of a relational or equality operator with the other operand an integer constant expression, with the resulting expression being the entire controlling expression of if, switch, while, do-while, for

if(setjmp(env) > 10) { ...

the operand of a unary ! operator with the resulting expression being the entire controlling expression of if, switch, while, do-while, for

while(!setjmp(env)) { ...

the entire expression of an expression statement (possibly cast to void).

setjmp(env);

If setjmp appears in any other context, the behavior is undefined.

Upon return to the scope of setjmp, all accessible objects, floating-point status flags, and other components of the abstract machine have the same values as they had when longjmp was executed, except for the non-volatile local variables in the function containing the invocation of setjmp, whose values are indeterminate if they have been changed since the setjmp invocation.

Parameters

env

-

variable to save the execution state of the program to.

Return value

0 if the macro was called by the original code and the execution context was saved to env.

Non-zero value if a non-local jump was just performed. The return value is the same as passed to longjmp.

Notes

Above requirements forbid using return value of setjmp in data flow (e.g. to initialize or assign an object with it). The return value can only be either used in control flow or discarded.

Example

Run this code

#include <stdio.h>
#include <setjmp.h>
#include <stdnoreturn.h>
 
jmp_buf jump_buffer;
 
noreturn void a(int count) 
{
    printf("a(%d) called\n", count);
    longjmp(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(jump_buffer) != 9) // compare against constant in an if
        a(++count);
}

Output:

a(1) called
a(2) called
a(3) called
a(4) called
a(5) called
a(6) called
a(7) called
a(8) called

References

C11 standard (ISO/IEC 9899:2011):

7.13.1.1 The setjmp macro (p: 262-263)

C99 standard (ISO/IEC 9899:1999):

7.13.1.1 The setjmp macro (p: 243-244)

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

4.6.1 The setjmp macro

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