# std::longjmp

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< cpp‎ | utility‎ | program

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 <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;
    }

Output:

    Solving 1x² + -3x + 2 = 0...
    x₁ = 1, x₂ = 2

    Solving 2x² + -3x + -2 = 0...
    x₁ = -0.5, x₂ = 2

    Solving 1x² + 2x + 3 = 0...
    No real solution

### 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

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