setjmp

Saves the current execution context into a variable env of type std::jmp_buf . This variable can later be used to restore the current execution context by std::longjmp function. That is, when a call to std::longjmp function is made, the execution continues at the particular call site that constructed the std::jmp_buf variable passed to std::longjmp . In that case setjmp returns the value passed to std::longjmp .

The invocation of setjmp must appear only in one of the following contexts:

1. the entire controlling expression of if , switch , while , do-while , for .

   switch (setjmp(env)) { // ...

2. one operand of a relational or equality operator with the other operand an integer constant expression, with the resulting expression being the entire controlling expression of if , switch , while , do-while , for .

   if (setjmp(env) > 0) { // ...

3. the operand of a unary ! operator with the resulting expression being the entire controlling expression of if , switch , while , do-while , for .

   while (!setjmp(env)) { // ...

4. the entire expression of an expression statement (possibly cast to void ).

   setjmp(env);

If setjmp appears in any other context, the behavior is undefined.

Upon return to the scope of setjmp :

• all accessible objects, floating-point status flags, and other components of the abstract machine have the same values as they had when std::longjmp was executed,
• except for the non- volatile local variables in the function containing the invocation of setjmp , whose values are indeterminate if they have been changed since the setjmp invocation.

Parameters

Return value

​ 0 ​ if the macro was called by the original code and the execution context was saved to env .

Non-zero value if a non-local jump was just performed. The return value is the same as passed to std::longjmp .

Notes

Above requirements forbid using return value of setjmp in data flow (e.g. to initialize or assign an object with it). The return value can only be either used in control flow or discarded.

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;
}

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

See also