std::longjmp

Defined in header `<csetjmp>`
void longjmp( std::jmp_buf env, int status );		(until C++17)
[[noreturn]] void longjmp( std::jmp_buf env, int status );		(since C++17)

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, the behavior is undefined (in other words, only long jumps up the call stack are allowed).

Extra restrictions in C++

On top of C longjmp, C++ std::longjmp has more restricted behavior.

If replacing std::longjmp with throw and setjmp with catch would invoke a non-trivial destructor for any automatic object, the behavior of such std::longjmp is undefined.

The behavior is undefined if std::longjmp is called in a coroutine in a place where the co_await operator may be used.

(since C++20)

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

std::longjmp is the mechanism used in C to handle unexpected error conditions where the function cannot return meaningfully. C++ generally uses exception handling for this purpose.

Example

Run this code

#include <csetjmp>
#include <iostream>
 
std::jmp_buf my_jump_buffer;
 
[[noreturn]] void foo(int status) 
{
    std::cout << "foo(" << status << ") called\n";
    std::longjmp(my_jump_buffer, status + 1); // setjmp() will return status + 1
}
 
int main()
{
    volatile int count = 0; // modified locals in setjmp scope must be volatile
    if (setjmp(my_jump_buffer) != 5) // equality against constant expression in an if
    {
        count += 1; // Increment of a volatile variable is deprecated since C++20 (P1152)
        foo(count); // This will cause setjmp() to exit
    }
}

Output:

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 619	C++98	the wording of the extra restrictions in C++ was vague	improved the wording
LWG 894	C++98	the behavior was undefined if replacing `std::longjmp` with throw and setjmp with catch would destroy any automatic object	the behavior is only undefined if a non-trivial destructor for any automatic object is invoked

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries