std::longjmp

 

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abort exit quick_exit (C++11) _Exit (C++11)	atexit at_quick_exit (C++11) EXIT_SUCCESSEXIT_FAILURE
Unreachable control flow
unreachable (C++23)
Communicating with the environment
system	getenv
Signals
signal raise sig_atomic_t	SIG_DFLSIG_IGN SIG_ERR
Signal types
SIGABRTSIGFPESIGILL	SIGINTSIGSEGVSIGTERM
Non-local jumps
setjmp	longjmp
Types
jmp_buf

 

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 = count + 1; // ++count, count += 1, etc on 'volatile'-qualified
                           // left operand are deprecated since C++20 (P1152),
                           // but they are de-deprecated since C++23 (P2327).
        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

See also

setjmp	saves the context (function macro)
C documentation for longjmp