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# setjmp

From cppreference.com
< c‎ | program

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 a selection or iteration statement (if, switch, for, while, do-while)

    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 a selection or iteration statement

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

- the operand of a unary ! operator with the resulting expression being the entire controlling expression of a selection or iteration statement

    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 setjmp's scope, whose values are indeterminate if they have been changed since the setjmp invocation.

### [edit] Parameters

env | - | variable to save the execution state of the program to.

### [edit] 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 in the same as passed to longjmp.

### [edit] 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

### [edit] 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

### [edit] See also

longjmp | jumps to specified location (function) [edit]
C++ documentation for setjmp

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