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setjmp(3) Library Functions Manual setjmp(3) NAME setjmp, _setjmp, longjmp, _longjmp - Saves and restores the current ex- ecution context LIBRARY Standard C Library (libc.a, libc.so) System V Library (libsys5.a, lib- sys5.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 stan- dards as follows: setjmp(), longjmp(): XPG4, XPG4-UNIX _setjmp(), _longjmp(): XPG4-UNIX Refer to the standards(5) reference page for more information about in- dustry 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 para- meter, the execution context contains a value of 1 as the setjmp() or _setjmp() return value. See the RETURN VALUES section for more infor- mation. 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() func- tion 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 re- turned 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, ex- cept for some objects of automatic storage duration. Objects of auto- matic 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-qual- ified 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, sig- nals, and any of their associated functions. However, if the longjmp() function is invoked from a nested signal handler (that is, from a func- tion invoked as a result of a signal raised during the handling of an- other 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 para- meter that was not previously set by the setjmp() function. The func- tion that made the corresponding call to the setjmp() function has al- ready 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 mes- sage 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 cor- responding 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() re- turn value. RELATED INFORMATION Routines: siglongjmp(3), sigsetjmp(3) Standards: standards(5) delim off setjmp(3)