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setjmp

Section: C Library Functions (3)
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NAME

setjmp, _setjmp, longjmp, _longjmp - Saves and restores the current execution context  

LIBRARY

Standard C Library (libc.a, libc.so) System V Library (libsys5.a, libsys5.so)  

SYNOPSIS

#include <setjmp.h>

int setjmp(
        jmp_buf environment);

void longjmp(
        jmp_buf environment,
        int value);

int _setjmp (
        jmp_buf environment);

void _longjmp(
        jmp_buf environment,
        int value);  

STANDARDS

Interfaces documented on this reference page conform to industry standards as follows:

setjmp(), longjmp(): XPG4, XPG4-UNIX

_setjmp(), _longjmp(): XPG4-UNIX

Refer to the standards(5) reference page for more information about industry standards and associated tags.  

PARAMETERS

Specifies an address for a jmp_buf structure. Specifies the value you want written to the execution context as the return value of the setjmp() or _setjmp() function. If you specify 0 (zero) in this parameter, the execution context contains a value of 1 as the setjmp() or _setjmp() return value. See the RETURN VALUES section for more information.  

DESCRIPTION

The setjmp() and longjmp() functions are useful when handling errors and interrupts encountered in low-level functions of a program.

The setjmp() function saves the current stack context and signal mask in the buffer specified by the environment parameter. You then use the buffer in a later call to the longjmp() function. The longjmp() function restores the stack context and signal mask that were saved by the setjmp() function.

After the longjmp() function runs, program execution continues as though the corresponding call to the setjmp() function had just returned the value of the value parameter. The function that called the setjmp() function must not have returned before the completion of the longjmp() function.

The _setjmp() and _longjmp() functions operate identically to the setjmp() and longjmp() functions, respectively, except that _setjmp() and _longjmp() manipulate only the stack context. These functions do not restore the signal mask.

All accessible objects have values at the time longjmp() is called, except for some objects of automatic storage duration. Objects of automatic storage duration will have indeterminant values if they meet all of the following conditions: They are local to the function containing the corresponding setjmp() invocation. They do not have volatile-qualified type. They are changed between the setjmp() and the longjmp() call.

Because it bypasses the usual function call and return mechanisms, the longjmp() function executes correctly in contexts of interrupts, signals, and any of their associated functions. However, if the longjmp() function is invoked from a nested signal handler (that is, from a function invoked as a result of a signal raised during the handling of another signal), the behavior is undefined.  

NOTES

[Digital]  For compatibility, the System V versions of the setjmp() and longjmp() functions, which are equivalent to _setjmp() and _longjmp(), respectively, are also supported. To use the System V versions of setjmp() and longjmp(), you must link with the libsys5 library before you link with libc.  

CAUTION

The results of the longjmp() function are undefined in the following situations: The longjmp() function is called with an environment parameter that was not previously set by the setjmp() function. The function that made the corresponding call to the setjmp() function has already returned.

If the longjmp() function detects one of these conditions, it calls the longjmperror() function. If longjmperror() returns, the program is aborted. The default version of longjmperror() displays an error message to standard error and returns. If you want your program to exit more gracefully, you can write your own version of the longjmperror() program.  

RETURN VALUES

After the longjmp() function is finished executing, program execution continues as though the corresponding call of the setjmp() function just returned. In other words, the execution context saved by the corresponding setjmp() function is in place and execution continues at the statement immediately following the call to the setjmp() function.

Part of that execution context is the return value from the setjmp() function. When the setjmp() function actually returns (before the call to the longjmp() function), that return value is 0 (zero). When the longjmp() function returns, the execution context contains a non-zero value as the return value from the setjmp() function.

The value you specify in the value parameter to the longjmp() function is written to the execution context as the return value for the setjmp() function. You cannot cause the execution context to contain a 0 (zero) value for the setjmp() return value. If you specify 0 in the value parameter, the execution context contains a 1 as the setjmp() return value.  

RELATED INFORMATION

Routines: siglongjmp(3), sigsetjmp(3)

Standards: standards(5) delim off


 

Index

NAME
LIBRARY
SYNOPSIS
STANDARDS
PARAMETERS
DESCRIPTION
NOTES
CAUTION
RETURN VALUES
RELATED INFORMATION

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Time: 02:42:11 GMT, October 02, 2010