SETJMP(3) | Library Functions Manual | SETJMP(3)

# NAME

`sigsetjmp`, `siglongjmp`, `setjmp`, `longjmp`, `_setjmp`, `_longjmp` — non-local jumps

# 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);

# 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 value specified by val must be non-zero; a 0 value is treated as 1 to allow the programmer to differentiate between a direct invocation of `setjmp`() and a return via `longjmp`().

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 the `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 the `_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.

# SEE ALSO

sigprocmask(2)

# STANDARDS

The `setjmp`() and `longjmp`() functions conform to ANSI X3.159-1989 (“ANSI C89”). The `sigsetjmp`() and `siglongjmp`() functions conform to IEEE Std 1003.1-1990 (“POSIX.1”).

# HISTORY

The `setjmp`() and `longjmp`() functions first appeared in the Programmer's Workbench (PWB/UNIX).

# CAVEATS

Historically, on AT&T System V 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, as discussed in signal(3). 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.

September 11, 2022 | Debian