SETJMP(3)

NAME
sigsetjmp, siglongjmp, setjmp, longjmp, _setjmp, _longjmp, longjmperror - non-local jumps

LIBRARY
libc

SYNOPSIS
#include <setjmp.h>
int
sigsetjmp(sigjmp_buf env, int savemask);
void
siglongjmp(sigjmp_buf env, int val);
int
setjmp(jmp_buf env);
void
longjmp(jmp_buf env, int val);
int
_setjmp(jmp_buf env);
void
_longjmp(jmp_buf env, int val);
void
longjmperror(void);

DESCRIPTION
The sigsetjmp(), setjmp(), and _setjmp() functions save their calling environment in env. Each of these functions returns 0.

The corresponding longjmp() functions restore the environment saved by the most recent invocation of the respective setjmp() function. They then return so that program execution continues as if the corresponding invocation of the setjmp() call had just returned the value specified by val, instead of 0. The longjmp() functions cannot cause setjmp() to return 0; if val is 0, setjmp() returns 1 instead.

Pairs of calls may be intermixed, i.e., both sigsetjmp() and siglongjmp() as well as setjmp() and longjmp() combinations may be used in the same program. However, individual calls may not, e.g., the env argument to setjmp() may not be passed to siglongjmp().

The longjmp() routines may not be called after the routine which called the setjmp() routines returns.

All accessible objects have values as of the time longjmp() routine was called, except that the values of objects of automatic storage invocation duration that do not have the volatile type and have been changed between the setjmp() invocation and longjmp() call are indeterminate.

The setjmp()/longjmp() function pairs save and restore the signal mask while _setjmp()/_longjmp() function pairs save and restore only the register set and the stack. (See sigprocmask(2).)

The sigsetjmp()/siglongjmp() function pairs save and restore the signal mask if the argument savemask is non-zero. Otherwise, only the register set and the stack are saved.

In other words, setjmp()/longjmp() are functionally equivalent to sigsetjmp()/siglongjmp() when sigsetjmp() is called with a non-zero savemask argument. Conversely, _setjmp()/_longjmp() are functionally equivalent to sigsetjmp()/siglongjmp() when sigsetjmp() is called with a zero-value savemask.

The sigsetjmp()/siglongjmp() interfaces are preferred for maximum portability.

ERRORS
If the contents of the env are corrupted or correspond to an environment that has already returned, the longjmp() routine calls the routine longjmperror(3). If longjmperror() returns, the program is aborted (see abort(3)). The default version of longjmperror() prints the message “longjmp botch” to standard error and returns. User programs wishing to exit more gracefully should write their own versions of longjmperror().

SEE ALSO
sigaction(2), sigaltstack(2), sigprocmask(2), pthread_sigmask(3), signal(3)

STANDARDS
The setjmp() and longjmp() functions conform to ansiC. The sigsetjmp() and siglongjmp() functions conform to p1003.1-90.

CAVEATS
Historically, on AT&T UNIX, the setjmp()/longjmp() functions have been equivalent to the BSD _setjmp()/_longjmp() functions and do not restore the signal mask. Because of this discrepancy, the sigsetjmp()/siglongjmp() interfaces should be used if portability is desired.

Use of longjmp() or siglongjmp() from inside a signal handler is not as easy as it might seem. Generally speaking, all possible code paths between the setjmp() and longjmp() must be signal race safe. Furthermore, the code paths must not do resource management (such as open(2) or close(2)) without blocking the signal in question, or resources might be mismanaged. Obviously this makes longjmp() much less useful than previously thought.