Headers <csetjmp> (nonlocal jumps), <csignal> (signal handling), <cstdalign> (alignment), <cstdarg> (variable arguments), <cstdbool> (__bool_true_false_are_defined), 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: Calls to the POSIX functions setenv and putenv modify the environment. — end note ]
A call to the setlocale function ([c.locales]) 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.
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: The restrictions that ISO C places on the second parameter to the va_start() macro in header <stdarg.h> are different in this International Standard. The parameter parmN is the identifier of the rightmost parameter in the variable parameter list of the function definition (the one just before the ...).220 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 C 7.16.1.1.
Note that va_start is required to work as specified even if unary operator& is overloaded for the type of parmN.
namespace std {
using jmp_buf = see below;
[[noreturn]] void longjmp(jmp_buf env, int val);
}
#define setjmp(env) see below
The function signature longjmp(jmp_buf jbuf, int val) has more restricted behavior in this International Standard. 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 automatic objects.
See also: ISO C 7.13.
#define __bool_true_false_are_defined 1
#define __alignas_is_defined 1
namespace std {
using sig_atomic_t = see below;
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
A call to the function signal synchronizes with any resulting invocation of the signal handler so installed.
The common subset of the C and C++ languages consists of all declarations, definitions, and expressions that may appear in a well formed C++ program and also in a conforming C program. A POF (“plain old function”) is a function that uses only features from this common subset, and that does not directly or indirectly use any function that is not a POF, except that it may use plain lock-free atomic operations. A plain lock-free atomic operation is an invocation of a function f from Clause [atomics], such that f is not a member function, and either f is the function atomic_is_lock_free, or for every atomic argument A passed to f, atomic_is_lock_free(A) yields true. All signal handlers shall have C linkage. The behavior of any function other than a POF used as a signal handler in a C++ program is implementation-defined.221
See also: ISO C 7.14.
In particular, a signal handler using exception handling is very likely to have problems. Also, invoking std::exit may cause destruction of objects, including those of the standard library implementation, which, in general, yields undefined behavior in a signal handler (see [intro.execution]).