246 lines
5.6 KiB
HTML
246 lines
5.6 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of ALLOCA</TITLE>
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</HEAD><BODY>
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<H1>ALLOCA</H1>
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Section: Linux Programmer's Manual (3)<BR>Updated: 2019-03-06<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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alloca - allocate memory that is automatically freed
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>#include <<A HREF="file:///usr/include/alloca.h">alloca.h</A>></B>
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<P>
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<B>void *alloca(size_t </B><I>size</I><B>);</B>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The
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<B>alloca</B>()
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function allocates
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<I>size</I>
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bytes of space in the stack frame of the caller.
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This temporary space is
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automatically freed when the function that called
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<B>alloca</B>()
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returns to its caller.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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The
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<B>alloca</B>()
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function returns a pointer to the beginning of the allocated space.
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If the allocation causes stack overflow, program behavior is undefined.
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<A NAME="lbAF"> </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>alloca</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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<A NAME="lbAG"> </A>
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<H2>CONFORMING TO</H2>
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This function is not in POSIX.1.
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<P>
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There is evidence that the
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<B>alloca</B>()
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function appeared in 32V, PWB, PWB.2, 3BSD, and 4BSD.
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There is a man page for it in 4.3BSD.
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Linux uses the GNU version.
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<A NAME="lbAH"> </A>
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<H2>NOTES</H2>
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The
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<B>alloca</B>()
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function is machine- and compiler-dependent.
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For certain applications,
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its use can improve efficiency compared to the use of
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<B><A HREF="/cgi-bin/man/man2html?3+malloc">malloc</A></B>(3)
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plus
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<B><A HREF="/cgi-bin/man/man2html?3+free">free</A></B>(3).
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In certain cases,
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it can also simplify memory deallocation in applications that use
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<B><A HREF="/cgi-bin/man/man2html?3+longjmp">longjmp</A></B>(3)
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or
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<B><A HREF="/cgi-bin/man/man2html?3+siglongjmp">siglongjmp</A></B>(3).
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Otherwise, its use is discouraged.
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<P>
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Because the space allocated by
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<B>alloca</B>()
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is allocated within the stack frame,
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that space is automatically freed if the function return
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is jumped over by a call to
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<B><A HREF="/cgi-bin/man/man2html?3+longjmp">longjmp</A></B>(3)
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or
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<B><A HREF="/cgi-bin/man/man2html?3+siglongjmp">siglongjmp</A></B>(3).
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<P>
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The space allocated by
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<B>alloca</B>()
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is
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<I>not</I>
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automatically deallocated if the pointer that refers to it
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simply goes out of scope.
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<P>
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Do not attempt to
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<B><A HREF="/cgi-bin/man/man2html?3+free">free</A></B>(3)
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space allocated by
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<B>alloca</B>()!
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<A NAME="lbAI"> </A>
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<H3>Notes on the GNU version</H3>
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Normally,
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<B><A HREF="/cgi-bin/man/man2html?1+gcc">gcc</A></B>(1)
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translates calls to
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<B>alloca</B>()
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with inlined code.
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This is not done when either the
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<I>-ansi</I>,
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<I>-std=c89</I>,
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<I>-std=c99</I>,
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or the
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<I>-std=c11</I>
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option is given
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<B>and</B>
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the header
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<I><<A HREF="file:///usr/include/alloca.h">alloca.h</A>></I>
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is not included.
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Otherwise, (without an -ansi or -std=c* option) the glibc version of
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<I><<A HREF="file:///usr/include/stdlib.h">stdlib.h</A>></I>
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includes
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<I><<A HREF="file:///usr/include/alloca.h">alloca.h</A>></I>
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and that contains the lines:
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<P>
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#ifdef __GNUC__
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#define alloca(size) __builtin_alloca (size)
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#endif
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<P>
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with messy consequences if one has a private version of this function.
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<P>
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The fact that the code is inlined means that it is impossible
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to take the address of this function, or to change its behavior
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by linking with a different library.
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<P>
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The inlined code often consists of a single instruction adjusting
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the stack pointer, and does not check for stack overflow.
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Thus, there is no NULL error return.
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<A NAME="lbAJ"> </A>
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<H2>BUGS</H2>
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There is no error indication if the stack frame cannot be extended.
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(However, after a failed allocation, the program is likely to receive a
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<B>SIGSEGV</B>
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signal if it attempts to access the unallocated space.)
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<P>
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On many systems
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<B>alloca</B>()
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cannot be used inside the list of arguments of a function call, because
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the stack space reserved by
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<B>alloca</B>()
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would appear on the stack in the middle of the space for the
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function arguments.
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<A NAME="lbAK"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+brk">brk</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?3+longjmp">longjmp</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?3+malloc">malloc</A></B>(3)
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<A NAME="lbAL"> </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="1"><A HREF="#lbAB">NAME</A><DD>
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<DT id="2"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="3"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="4"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="5"><A HREF="#lbAF">ATTRIBUTES</A><DD>
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<DT id="6"><A HREF="#lbAG">CONFORMING TO</A><DD>
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<DT id="7"><A HREF="#lbAH">NOTES</A><DD>
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<DL>
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<DT id="8"><A HREF="#lbAI">Notes on the GNU version</A><DD>
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</DL>
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<DT id="9"><A HREF="#lbAJ">BUGS</A><DD>
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<DT id="10"><A HREF="#lbAK">SEE ALSO</A><DD>
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<DT id="11"><A HREF="#lbAL">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: 00:05:35 GMT, March 31, 2021
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</BODY>
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</HTML>
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