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Difference between revisions of "cpp/utility/program/longjmp"

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< cpp‎ | utility‎ | program
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(Added LWG issues #619 and #894 DR.)
 
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{{cpp/title| longjmp}}
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{{cpp/title|longjmp}}
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{{cpp/utility/program/sidebar}}
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{{cpp/utility/program/navbar}}
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{{ddcl | header=csetjmp |
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{{dcl begin}}
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void longjmp( jmp_buf env, int status );
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{{dcl header|csetjmp}}
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{{dcl rev begin}}
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{{dcl|until=c++17|
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             void longjmp( std::jmp_buf env, int status );
 
}}
 
}}
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{{dcl|since=c++17|
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[[noreturn]] void longjmp( std::jmp_buf env, int status );
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}}
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{{dcl rev end}}
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{{dcl end}}
  
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Loads the execution context {{tt|env}} saved by a previous call to {{rlpf|setjmp}}. This function does not ever return. Control is transferred to the particular call site of {{rlpf|setjmp}} that set up {{tt|env}}. That {{rlpf|setjmp}} function then returns the value, passed as the {{tt|status}}.
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Loads the execution context {{c|env}} saved by a previous call to {{lc|setjmp}}. This function does not return. Control is transferred to the call site of the macro {{lc|setjmp}} that set up {{c|env}}. That {{lc|setjmp}} then returns the value, passed as the {{c|status}}.
  
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No cleanup such as calling destructors is done during transfer of control.  
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If the function that called {{lc|setjmp}} has exited, the behavior is undefined (in other words, only long jumps up the call stack are allowed).
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===Extra restrictions in C++===
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On top of C {{ltt|c/program/longjmp}}, C++ {{tt|std::longjmp}} has more restricted behavior.
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If replacing {{tt|std::longjmp}} with {{c/core|throw}} and {{lc|setjmp}} with {{c/core|catch}} would invoke a [[cpp/language/destructor#Trivial destructor|non-trivial destructor]] for any automatic object, the behavior of such {{tt|std::longjmp}} is undefined.
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{{rev begin}}
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{{rev|since=c++20|
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The behavior is undefined if {{tt|std::longjmp}} is called in a [[cpp/language/coroutines|coroutine]] in a place where the {{c/core|co_await}} operator may be used.
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}}
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{{rev end}}
  
 
===Parameters===
 
===Parameters===
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{{param list begin}}
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{{par begin}}
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{{param list item | env | variable referring to the execution state of the program saved by {{rlpf|setjmp}}}}
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{{par|env|variable referring to the execution state of the program saved by {{lc|setjmp}}}}
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{{param list item | status | the value to return from {{rlpf|setjmp}}. If it is equal to {{cpp|0}}, {{cpp|1}} is used instead.
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{{par|status|the value to return from {{lc|setjmp}}. If it is equal to {{c|0}}, {{c|1}} is used instead}}
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{{param list end}}
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{{par end}}
  
 
===Return value===
 
===Return value===
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{{return none}}
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(none)
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===Notes===
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{{tt|std::longjmp}} is the mechanism used in C to handle unexpected error conditions where the function cannot return meaningfully. C++ generally uses [[cpp/language/exceptions|exception handling]] for this purpose.
  
 
===Example===
 
===Example===
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{{example cpp
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{{cpp/utility/program/example setjmp}}
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|
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| code=
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===Defect reports===
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| output=
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{{dr list begin}}
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}}
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{{dr list item|wg=lwg|dr=619|std=C++98|before=the wording of the extra restrictions in C++ was vague|after=improved the wording}}
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{{dr list item|wg=lwg|dr=894|std=C++98|before=the behavior was undefined if replacing<br>{{tt|std::longjmp}} with {{c/core|throw}} and {{lc|setjmp}} with<br>{{c/core|catch}} would destroy any automatic object|after=the behavior is only undefined<br>if a non-trivial destructor for<br>any automatic object is invoked}}
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{{dr list end}}
  
 
===See also===
 
===See also===
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{{dcl list begin}}
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{{dsc begin}}
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{{dcl list template | cpp/utility/program/dcl list setjmp}}
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{{dsc inc|cpp/utility/program/dsc setjmp}}
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{{dcl list end}}
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{{dsc see c|c/program/longjmp}}
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{{dsc end}}
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{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 19:25, 2 March 2023

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Program support utilities
 
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).

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.

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)

[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

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

[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 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

[edit] See also

 saves the context
(function macro) [edit]
C documentation for longjmp
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