17 Language support library [support]

17.13 Other runtime support [support.runtime]

17.13.1 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.

17.13.2 Header <cstdarg> synopsis [cstdarg.syn]

// all freestanding namespace std { using va_list = see below; } #define va_arg(V, P) see below #define va_copy(VDST, VSRC) see below #define va_end(V) see below #define va_start(V, P) see below
The contents of the header <cstdarg> are the same as the C standard library header <stdarg.h>, with the following changes:
  • In lieu of the default argument promotions specified in ISO C, the definition in [expr.call] applies.
  • The restrictions that ISO C places on the second parameter to the va_start macro in header <stdarg.h> are different in this document.
    The parameter parmN is the rightmost parameter in the variable parameter list of the function definition (the one just before the ...).196
    If the parameter parmN is a pack expansion ([temp.variadic]) or an entity resulting from a lambda capture ([expr.prim.lambda]), the program is ill-formed, no diagnostic required.
    If the parameter parmN is of a reference type, or of a type that is not compatible with the type that results when passing an argument for which there is no parameter, the behavior is undefined.
See also: ISO/IEC 9899:2018,
Note that va_start is required to work as specified even if unary operator& is overloaded for the type of parmN.

17.13.3 Header <csetjmp> synopsis [csetjmp.syn]

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 <csetjmp> 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]).
See also: ISO/IEC 9899:2018, 7.13

17.13.4 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 <csignal> are the same as the C standard library header <signal.h>.

17.13.5 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:
  • f is the function atomic_is_lock_free(), or
  • f is the member function is_lock_free(), or
  • f is a non-static member function of class atomic_flag, or
  • f is a non-member function, and the first parameter of f has type cv atomic_flag*, or
  • f is a non-static member function invoked on an object A, such that A.is_lock_free() yields true, or
  • 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:
  • 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]
  • an access to an object with thread storage duration;
  • a dynamic_cast expression;
  • throwing of an exception;
  • control entering a try-block or function-try-block;
  • initialization of a variable with static storage duration requiring dynamic initialization ([basic.start.dynamic], [stmt.dcl])197 ; or
  • 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.
See also: ISO/IEC 9899:2018, 7.14
Such initialization can occur because it is the first odr-use ([basic.def.odr]) of that variable.