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487 lines
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of SETJMP</TITLE>
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</HEAD><BODY>
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<H1>SETJMP</H1>
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Section: Linux Programmer's Manual (3)<BR>Updated: 2017-03-13<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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setjmp, sigsetjmp, longjmp, siglongjmp - performing a nonlocal goto
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/setjmp.h">setjmp.h</A>></B>
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<B>int setjmp(jmp_buf </B><I>env</I><B>);</B>
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<B>int sigsetjmp(sigjmp_buf </B><I>env</I><B>, int </B><I>savesigs</I><B>);</B>
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<B>void longjmp(jmp_buf </B><I>env</I><B>, int </B><I>val</I><B>);</B>
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<B>void siglongjmp(sigjmp_buf </B><I>env</I><B>, int </B><I>val</I><B>);</B>
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</PRE>
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<P>
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Feature Test Macro Requirements for glibc (see
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<B><A HREF="/cgi-bin/man/man2html?7+feature_test_macros">feature_test_macros</A></B>(7)):
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<P>
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<B>setjmp</B>():
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see NOTES.
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<P>
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<B>sigsetjmp</B>():
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_POSIX_C_SOURCE
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The functions described on this page are used for performing "nonlocal gotos":
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transferring execution from one function to a predetermined location
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in another function.
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The
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<B>setjmp</B>()
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function dynamically establishes the target to which control
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will later be transferred, and
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<B>longjmp</B>()
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performs the transfer of execution.
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<P>
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The
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<B>setjmp</B>()
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function saves various information about the calling environment
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(typically, the stack pointer, the instruction pointer,
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possibly the values of other registers and the signal mask)
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in the buffer
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<I>env</I>
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for later use by
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<B>longjmp</B>().
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In this case,
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<B>setjmp</B>()
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returns 0.
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<P>
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The
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<B>longjmp</B>()
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function uses the information saved in
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<I>env</I>
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to transfer control back to the point where
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<B>setjmp</B>()
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was called and to restore ("rewind") the stack to its state at the time of the
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<B>setjmp</B>()
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call.
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In addition, and depending on the implementation (see NOTES),
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the values of some other registers and the process signal mask
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may be restored to their state at the time of the
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<B>setjmp</B>()
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call.
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<P>
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Following a successful
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<B>longjmp</B>(),
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execution continues as if
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<B>setjmp</B>()
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had returned for a second time.
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This "fake" return can be distinguished from a true
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<B>setjmp</B>()
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call because the "fake" return returns the value provided in
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<I>val</I>.
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If the programmer mistakenly passes the value 0 in
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<I>val</I>,
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the "fake" return will instead return 1.
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<P>
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<A NAME="lbAE"> </A>
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<H3>sigsetjmp() and siglongjmp()</H3>
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<B>sigsetjmp</B>()
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and
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<B>siglongjmp</B>()
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also perform nonlocal gotos, but provide predictable handling of
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the process signal mask.
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<P>
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If, and only if, the
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<I>savesigs</I>
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argument provided to
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<B>sigsetjmp</B>()
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is nonzero, the process's current signal mask is saved in
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<I>env</I>
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and will be restored if a
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<B>siglongjmp</B>()
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is later performed with this
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<I>env</I>.
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<A NAME="lbAF"> </A>
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<H2>RETURN VALUE</H2>
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<B>setjmp</B>()
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and
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<B>sigsetjmp</B>()
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return 0 when called directly;
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on the "fake" return that occurs after
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<B>longjmp</B>()
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or
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<B>siglongjmp</B>(),
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the nonzero value specified in
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<I>val</I>
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is returned.
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<P>
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The
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<B>longjmp</B>()
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or
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<B>siglongjmp</B>()
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functions do not return.
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<A NAME="lbAG"> </A>
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<H2>ATTRIBUTES</H2>
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For an explanation of the terms used in this section, see
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<B><A HREF="/cgi-bin/man/man2html?7+attributes">attributes</A></B>(7).
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<TABLE BORDER>
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<TR VALIGN=top><TD><B>Interface</B></TD><TD><B>Attribute</B></TD><TD><B>Value</B><BR></TD></TR>
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<TR VALIGN=top><TD>
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<B>setjmp</B>(),
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<B>sigsetjmp</B>()
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</TD><TD>Thread safety</TD><TD>MT-Safe<BR></TD></TR>
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<TR VALIGN=top><TD>
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<B>longjmp</B>(),
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<B>siglongjmp</B>()
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</TD><TD>Thread safety</TD><TD>MT-Safe<BR></TD></TR>
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</TABLE>
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<P>
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<A NAME="lbAH"> </A>
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<H2>CONFORMING TO</H2>
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<B>setjmp</B>(),
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<B>longjmp</B>():
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POSIX.1-2001, POSIX.1-2008, C89, C99.
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<P>
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<B>sigsetjmp</B>(),
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<B>siglongjmp</B>():
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POSIX.1-2001, POSIX.1-2008.
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<A NAME="lbAI"> </A>
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<H2>NOTES</H2>
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POSIX does not specify whether
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<B>setjmp</B>()
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will save the signal mask
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(to be later restored during
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<B>longjmp</B>()).
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In System V it will not.
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In 4.3BSD it will, and there
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is a function
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<B>_setjmp</B>()
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that will not.
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The behavior under Linux depends on the glibc version
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and the setting of feature test macros.
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On Linux with glibc versions before 2.19,
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<B>setjmp</B>()
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follows the System V behavior by default,
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but the BSD behavior is provided if the
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<B>_BSD_SOURCE</B>
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feature test macro is explicitly defined
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and none of
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<B>_POSIX_SOURCE</B>,
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<B>_POSIX_C_SOURCE</B>,
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<B>_XOPEN_SOURCE</B>,
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<B>_GNU_SOURCE</B>,
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or
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<B>_SVID_SOURCE</B>
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is defined.
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Since glibc 2.19,
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<I><<A HREF="file:///usr/include/setjmp.h">setjmp.h</A>></I>
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exposes only the System V version of
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<B>setjmp</B>().
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Programs that need the BSD semantics should replace calls to
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<B>setjmp</B>()
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with calls to
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<B>sigsetjmp</B>()
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with a nonzero
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<I>savesigs</I>
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argument.
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<P>
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<B>setjmp</B>()
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and
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<B>longjmp</B>()
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can be useful for dealing with errors inside deeply nested function calls
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or to allow a signal handler to pass control to
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a specific point in the program,
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rather than returning to the point where the handler interrupted
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the main program.
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In the latter case,
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if you want to portably save and restore signal masks, use
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<B>sigsetjmp</B>()
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and
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<B>siglongjmp</B>().
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See also the discussion of program readability below.
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<P>
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The compiler may optimize variables into registers, and
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<B>longjmp</B>()
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may restore the values of other registers in addition to the
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stack pointer and program counter.
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Consequently, the values of automatic variables are unspecified
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after a call to
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<B>longjmp</B>()
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if they meet all the following criteria:
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<DL COMPACT>
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<DT id="1">•<DD>
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they are local to the function that made the corresponding
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<B>setjmp</B>()
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call;
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<DT id="2">•<DD>
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their values are changed between the calls to
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<B>setjmp</B>()
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and
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<B>longjmp</B>();
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and
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<DT id="3">•<DD>
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they are not declared as
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<I>volatile</I>.
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</DL>
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<P>
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Analogous remarks apply for
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<B>siglongjmp</B>().
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<A NAME="lbAJ"> </A>
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<H3>Nonlocal gotos and program readability</H3>
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While it can be abused,
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the traditional C "goto" statement at least has the benefit that lexical cues
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(the goto statement and the target label)
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allow the programmer to easily perceive the flow of control.
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Nonlocal gotos provide no such cues: multiple
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<B>setjmp</B>()
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calls might employ the same
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<I>jmp_buf</I>
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variable so that the content of the variable may change
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over the lifetime of the application.
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Consequently, the programmer may be forced to perform detailed
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reading of the code to determine the dynamic target of a particular
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<B>longjmp</B>()
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call.
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(To make the programmer's life easier, each
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<B>setjmp</B>()
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call should employ a unique
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<I>jmp_buf</I>
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variable.)
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<P>
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Adding further difficulty, the
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<B>setjmp</B>()
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and
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<B>longjmp</B>()
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calls may not even be in the same source code module.
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<P>
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In summary, nonlocal gotos can make programs harder to understand
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and maintain, and an alternative should be used if possible.
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<A NAME="lbAK"> </A>
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<H3>Caveats</H3>
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If the function which called
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<B>setjmp</B>()
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returns before
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<B>longjmp</B>()
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is called, the behavior is undefined.
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Some kind of subtle or unsubtle chaos is sure to result.
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<P>
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If, in a multithreaded program, a
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<B>longjmp</B>()
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call employs an
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<I>env</I>
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buffer that was initialized by a call to
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<B>setjmp</B>()
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in a different thread, the behavior is undefined.
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<P>
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POSIX.1-2008 Technical Corrigendum 2 adds
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<B>longjmp</B>()
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and
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<B>siglongjmp</B>()
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to the list of async-signal-safe functions.
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However, the standard recommends avoiding the use of these functions
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from signal handlers and goes on to point out that
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if these functions are called from a signal handler that interrupted
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a call to a non-async-signal-safe function (or some equivalent,
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such as the steps equivalent to
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<B><A HREF="/cgi-bin/man/man2html?3+exit">exit</A></B>(3)
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that occur upon a return from the initial call to
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<I>main</I>()),
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the behavior is undefined if the program subsequently makes a call to
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a non-async-signal-safe function.
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The only way of avoiding undefined behavior is to ensure one of the following:
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<DL COMPACT>
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<DT id="4">*<DD>
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After long jumping from the signal handler,
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the program does not call any non-async-signal-safe functions
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and does not return from the initial call to
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<I>main</I>().
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<DT id="5">*<DD>
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Any signal whose handler performs a long jump must be blocked during
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<I>every</I>
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call to a non-async-signal-safe function and
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no non-async-signal-safe functions are called after
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returning from the initial call to
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<I>main</I>().
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</DL>
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<A NAME="lbAL"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?7+signal">signal</A></B>(7),
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<B><A HREF="/cgi-bin/man/man2html?7+signal-safety">signal-safety</A></B>(7)
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<A NAME="lbAM"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="6"><A HREF="#lbAB">NAME</A><DD>
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<DT id="7"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="8"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DL>
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<DT id="9"><A HREF="#lbAE">sigsetjmp() and siglongjmp()</A><DD>
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</DL>
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<DT id="10"><A HREF="#lbAF">RETURN VALUE</A><DD>
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<DT id="11"><A HREF="#lbAG">ATTRIBUTES</A><DD>
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<DT id="12"><A HREF="#lbAH">CONFORMING TO</A><DD>
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<DT id="13"><A HREF="#lbAI">NOTES</A><DD>
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<DL>
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<DT id="14"><A HREF="#lbAJ">Nonlocal gotos and program readability</A><DD>
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<DT id="15"><A HREF="#lbAK">Caveats</A><DD>
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</DL>
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<DT id="16"><A HREF="#lbAL">SEE ALSO</A><DD>
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<DT id="17"><A HREF="#lbAM">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:56 GMT, March 31, 2021
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