NAME¶

std::longjmp - std::longjmp

Synopsis¶

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).

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 improved the wording
in C++ was vague
the behavior was undefined if the behavior is only
replacing undefined
LWG 894 C++98 std::longjmp with throw and setjmp if a non-trivial destructor
with for
catch would destroy any automatic any automatic object is
object invoked

See also¶

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