SETJMP(3)                   Linux Programmer's Manual                  SETJMP(3)

       setjmp, sigsetjmp, longjmp, siglongjmp  - performing a nonlocal goto

       #include <setjmp.h>

       int setjmp(jmp_buf env);
       int sigsetjmp(sigjmp_buf env, int savesigs);

       noreturn void longjmp(jmp_buf env, int val);
       noreturn void siglongjmp(sigjmp_buf env, int val);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       setjmp(): see NOTES.


       The functions described on this page are used for performing "nonlocal
       gotos": transferring execution from one function to a predetermined
       location in another function.  The setjmp() function dynamically
       establishes the target to which control will later be transferred, and
       longjmp() performs the transfer of execution.

       The setjmp() function saves various information about the calling
       environment (typically, the stack pointer, the instruction pointer,
       possibly the values of other registers and the signal mask) in the buffer
       env for later use by longjmp().  In this case, setjmp() returns 0.

       The longjmp() function uses the information saved in env to transfer
       control back to the point where setjmp() was called and to restore
       ("rewind") the stack to its state at the time of the setjmp() call.  In
       addition, and depending on the implementation (see NOTES), the values of
       some other registers and the process signal mask may be restored to their
       state at the time of the setjmp() call.

       Following a successful longjmp(), execution continues as if setjmp() had
       returned for a second time.  This "fake" return can be distinguished from
       a true setjmp() call because the "fake" return returns the value provided
       in val.  If the programmer mistakenly passes the value 0 in val, the
       "fake" return will instead return 1.

   sigsetjmp() and siglongjmp()
       sigsetjmp() and siglongjmp() also perform nonlocal gotos, but provide
       predictable handling of the process signal mask.

       If, and only if, the savesigs argument provided to sigsetjmp() is
       nonzero, the process's current signal mask is saved in env and will be
       restored if a siglongjmp() is later performed with this env.

       setjmp() and sigsetjmp() return 0 when called directly; on the "fake"
       return that occurs after longjmp() or siglongjmp(), the nonzero value
       specified in val is returned.

       The longjmp() or siglongjmp() functions do not return.

       For an explanation of the terms used in this section, see attributes(7).

       │Interface                                     Attribute     Value   │
       │setjmp(), sigsetjmp()                         │ Thread safety │ MT-Safe │
       │longjmp(), siglongjmp()                       │ Thread safety │ MT-Safe │

       setjmp(), longjmp(): POSIX.1-2001, POSIX.1-2008, C89, C99.

       sigsetjmp(), siglongjmp(): POSIX.1-2001, POSIX.1-2008.

       POSIX does not specify whether setjmp() will save the signal mask (to be
       later restored during longjmp()).  In System V it will not.  In 4.3BSD it
       will, and there is a function _setjmp() that will not.  The behavior
       under Linux depends on the glibc version and the setting of feature test
       macros.  On Linux with glibc versions before 2.19, setjmp() follows the
       System V behavior by default, but the BSD behavior is provided if the
       _BSD_SOURCE feature test macro is explicitly defined and none of
       _SVID_SOURCE is defined.  Since glibc 2.19, <setjmp.h> exposes only the
       System V version of setjmp().  Programs that need the BSD semantics
       should replace calls to setjmp() with calls to sigsetjmp() with a nonzero
       savesigs argument.

       setjmp() and longjmp() can be useful for dealing with errors inside
       deeply nested function calls or to allow a signal handler to pass control
       to a specific point in the program, rather than returning to the point
       where the handler interrupted the main program.  In the latter case, if
       you want to portably save and restore signal masks, use sigsetjmp() and
       siglongjmp().  See also the discussion of program readability below.

       The compiler may optimize variables into registers, and longjmp() may
       restore the values of other registers in addition to the stack pointer
       and program counter.  Consequently, the values of automatic variables are
       unspecified after a call to longjmp() if they meet all the following

       •  they are local to the function that made the corresponding setjmp()

       •  their values are changed between the calls to setjmp() and longjmp();

       •  they are not declared as volatile.

       Analogous remarks apply for siglongjmp().

   Nonlocal gotos and program readability
       While it can be abused, the traditional C "goto" statement at least has
       the benefit that lexical cues (the goto statement and the target label)
       allow the programmer to easily perceive the flow of control.  Nonlocal
       gotos provide no such cues: multiple setjmp() calls might employ the same
       jmp_buf variable so that the content of the variable may change over the
       lifetime of the application.  Consequently, the programmer may be forced
       to perform detailed reading of the code to determine the dynamic target
       of a particular longjmp() call.  (To make the programmer's life easier,
       each setjmp() call should employ a unique jmp_buf variable.)

       Adding further difficulty, the setjmp() and longjmp() calls may not even
       be in the same source code module.

       In summary, nonlocal gotos can make programs harder to understand and
       maintain, and an alternative should be used if possible.

       If the function which called setjmp() returns before longjmp() is called,
       the behavior is undefined.  Some kind of subtle or unsubtle chaos is sure
       to result.

       If, in a multithreaded program, a longjmp() call employs an env buffer
       that was initialized by a call to setjmp() in a different thread, the
       behavior is undefined.

       POSIX.1-2008 Technical Corrigendum 2 adds longjmp() and siglongjmp() to
       the list of async-signal-safe functions.  However, the standard
       recommends avoiding the use of these functions from signal handlers and
       goes on to point out that if these functions are called from a signal
       handler that interrupted a call to a non-async-signal-safe function (or
       some equivalent, such as the steps equivalent to exit(3) that occur upon
       a return from the initial call to main()), the behavior is undefined if
       the program subsequently makes a call to a non-async-signal-safe
       function.  The only way of avoiding undefined behavior is to ensure one
       of the following:

       *  After long jumping from the signal handler, the program does not call
          any non-async-signal-safe functions and does not return from the
          initial call to main().

       *  Any signal whose handler performs a long jump must be blocked during
          every call to a non-async-signal-safe function and no non-async-
          signal-safe functions are called after returning from the initial call
          to main().

       signal(7), signal-safety(7)

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       latest version of this page, can be found at

                                   2021-03-22                          SETJMP(3)