std::longjmp

Since C++17

Header: <csetjmp>

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.

# Declarations

void longjmp( std::jmp_buf env, int status );

(until C++17)

[[noreturn]] void longjmp( std::jmp_buf env, int status );

(since C++17)

# 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

#include <array>
#include <cmath>
#include <csetjmp>
#include <cstdlib>
#include <format>
#include <iostream>
 
std::jmp_buf solver_error_handler;
 
std::array<double, 2> solve_quadratic_equation(double a, double b, double c)
{
    const double discriminant = b * b - 4.0 * a * c;
    if (discriminant < 0)
        std::longjmp(solver_error_handler, true); // Go to error handler
 
    const double delta = std::sqrt(discriminant) / (2.0 * a);
    const double argmin = -b / (2.0 * a);
    return {argmin - delta, argmin + delta};
}
 
void show_quadratic_equation_solution(double a, double b, double c)
{
    std::cout << std::format("Solving {}x² + {}x + {} = 0...\n", a, b, c);
    auto [x_0, x_1] = solve_quadratic_equation(a, b, c);
    std::cout << std::format("x₁ = {}, x₂ = {}\n\n", x_0, x_1);
}
 
int main()
{
    if (setjmp(solver_error_handler))
    {
        // Error handler for solver
        std::cout << "No real solution\n";
        return EXIT_FAILURE;
    }
 
    for (auto [a, b, c] : {std::array{1, -3, 2}, {2, -3, -2}, {1, 2, 3}})
        show_quadratic_equation_solution(a, b, c);
 
    return EXIT_SUCCESS;
}

# Defect reports

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 replacingstd::longjmp with throw and setjmp withcatch would destroy any automatic object	the behavior is only undefinedif a non-trivial destructor forany automatic object is invoked