17 Language support library [support]

17.14 Other runtime support [support.runtime]

17.14.1 General [support.runtime.general]

Headers <csetjmp> (nonlocal jumps), <csignal> (signal handling), <cstdarg> (variable arguments), and <cstdlib> (runtime environment getenv, system), provide further compatibility with C code.

Calls to the function getenv ([cstdlib.syn]) shall not introduce a data race ([res.on.data.races]) provided that nothing modifies the environment.

[Note 1:

Calls to the POSIX functions setenv and putenv modify the environment.

— end note]

A call to the setlocale function may introduce a data race with other calls to the setlocale function or with calls to functions that are affected by the current C locale.

The implementation shall behave as if no library function other than locale::global calls the setlocale function.

17.14.2 Header <cstdarg> synopsis [cstdarg.syn]

The contents of the header are the same as the C standard library header <stdarg.h>, with the following changes:

(1.1)
In lieu of the default argument promotions specified in ISO/IEC 9899:2024 6.5.2.2, the definition in [expr.call] applies.
(1.2)
The preprocessing tokens comprising the second and subsequent arguments to va_start (if any) are discarded.

[Note 1:
va_start accepts a second argument for compatibility with prior revisions of C++.
— end note]

17.14.3 Header <csetjmp> synopsis [csetjmp.syn]

🔗

#define __STDC_VERSION_SETJMP_H__ 202311L namespace std { using jmp_buf = see below; [[noreturn]] void longjmp(jmp_buf env, int val); } #define setjmp(env) see below

The contents of the header are the same as the C standard library header <setjmp.h>.

The function signature longjmp(jmp_buf jbuf, int val) has more restricted behavior in this document.

A setjmp/longjmp call pair has undefined behavior if replacing the setjmp and longjmp by catch and throw would invoke any non-trivial destructors for any objects with automatic storage duration.

A call to setjmp or longjmp has undefined behavior if invoked in a suspension context of a coroutine ([expr.await]).

17.14.4 Header <csignal> synopsis [csignal.syn]

🔗

namespace std { using sig_atomic_t = see below; // [support.signal], signal handlers extern "C" using signal-handler = void(int); // exposition only signal-handler* signal(int sig, signal-handler* func); int raise(int sig); } #define SIG_DFL see below #define SIG_ERR see below #define SIG_IGN see below #define SIGABRT see below #define SIGFPE see below #define SIGILL see below #define SIGINT see below #define SIGSEGV see below #define SIGTERM see below

The contents of the header are the same as the C standard library header <signal.h>.

17.14.5 Signal handlers [support.signal]

A call to the function signal synchronizes with any resulting invocation of the signal handler so installed.

A plain lock-free atomic operation is an invocation of a function f from [atomics], such that:

(2.1)
f is the function atomic_is_lock_free(), or
(2.2)
f is the member function is_lock_free(), or
(2.3)
f is a non-static member function of class atomic_flag, or
(2.4)
f is a non-member function, and the first parameter of f has type cv atomic_flag*, or
(2.5)
f is a non-static member function invoked on an object A, such that A.is_lock_free() yields true, or
(2.6)
f is a non-member function, and for every pointer-to-atomic argument A passed to f, atomic_is_lock_free(A) yields true.

An evaluation is signal-safe unless it includes one of the following:

(3.1)
a call to any standard library function, except for plain lock-free atomic operations and functions explicitly identified as signal-safe;
[Note 1:
This implicitly excludes the use of new and delete expressions that rely on a library-provided memory allocator.
— end note]
(3.2)
an access to an object with thread storage duration;
(3.3)
a dynamic_cast expression;
(3.4)
throwing of an exception;
(3.5)
control entering a try-block or function-try-block;
(3.6)
initialization of a variable with static storage duration requiring dynamic initialization ([basic.start.dynamic], [stmt.dcl])191 ; or
(3.7)
waiting for the completion of the initialization of a variable with static storage duration ([stmt.dcl]).

A signal handler invocation has undefined behavior if it includes an evaluation that is not signal-safe.

The function signal is signal-safe if it is invoked with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler.