alloca(3)

1ALLOCA(3)                  Linux Programmer's Manual                 ALLOCA(3)
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NAME

6       alloca - allocate memory that is automatically freed
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SYNOPSIS

9       #include <alloca.h>
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11       void *alloca(size_t size);
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DESCRIPTION

14       The  alloca() function allocates size bytes of space in the stack frame
15       of the caller.  This temporary space is automatically  freed  when  the
16       function that called alloca() returns to its caller.
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RETURN VALUE

19       The  alloca()  function returns a pointer to the beginning of the allo‐
20       cated space.  If the allocation causes stack overflow, program behavior
21       is undefined.
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ATTRIBUTES

24       For   an   explanation   of   the  terms  used  in  this  section,  see
25       attributes(7).
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27       ┌──────────┬───────────────┬─────────┐
28       │Interface │ Attribute     │ Value   │
29       ├──────────┼───────────────┼─────────┤
30       │alloca()  │ Thread safety │ MT-Safe │
31       └──────────┴───────────────┴─────────┘

CONFORMING TO

33       This function is not in POSIX.1.
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35       There is evidence that the alloca()  function  appeared  in  32V,  PWB,
36       PWB.2,  3BSD,  and  4BSD.  There is a man page for it in 4.3BSD.  Linux
37       uses the GNU version.
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NOTES

40       The alloca() function is machine- and compiler-dependent.  For  certain
41       applications,  its  use  can  improve efficiency compared to the use of
42       malloc(3) plus free(3).  In certain cases, it can also simplify  memory
43       deallocation  in  applications  that  use  longjmp(3) or siglongjmp(3).
44       Otherwise, its use is discouraged.
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46       Because the space allocated by alloca() is allocated within  the  stack
47       frame,  that  space  is  automatically  freed if the function return is
48       jumped over by a call to longjmp(3) or siglongjmp(3).
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50       Do not attempt to free(3) space allocated by alloca()!
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52   Notes on the GNU version
53       Normally, gcc(1) translates calls to alloca() with inlined code.   This
54       is  not done when either the -ansi, -std=c89, -std=c99, or the -std=c11
55       option is given and the header <alloca.h> is not included.   Otherwise,
56       (without  an  -ansi  or -std=c* option) the glibc version of <stdlib.h>
57       includes <alloca.h> and that contains the lines:
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59           #ifdef  __GNUC__
60           #define alloca(size)   __builtin_alloca (size)
61           #endif
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63       with messy consequences if one has a private version of this function.
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65       The fact that the code is inlined means that it is impossible  to  take
66       the address of this function, or to change its behavior by linking with
67       a different library.
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69       The inlined code often consists of a single instruction  adjusting  the
70       stack  pointer,  and does not check for stack overflow.  Thus, there is
71       no NULL error return.
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BUGS

74       There is no error indication if the stack  frame  cannot  be  extended.
75       (However, after a failed allocation, the program is likely to receive a
76       SIGSEGV signal if it attempts to access the unallocated space.)
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78       On many systems alloca() cannot be used inside the list of arguments of
79       a  function  call,  because  the stack space reserved by alloca() would
80       appear on the stack in the middle of the space for the  function  argu‐
81       ments.
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COLOPHON

87       This  page  is  part of release 4.16 of the Linux man-pages project.  A
88       description of the project, information about reporting bugs,  and  the
89       latest     version     of     this    page,    can    be    found    at
90       https://www.kernel.org/doc/man-pages/.
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94GNU                               2017-09-15                         ALLOCA(3)