std::longjmp
| Defined in header <csetjmp>
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| 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).
Contents |
[edit] 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.
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The behavior is undefined if |
(since C++20) |
[edit] 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 |
[edit] Return value
(none)
[edit] 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.
[edit] 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; }
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
[edit] 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 replacingstd::longjmp with throw and setjmp withcatch would destroy any automatic object |
the behavior is only undefined if a non-trivial destructor for any automatic object is invoked |
[edit] See also
| saves the context (function macro) | |
| C documentation for longjmp
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