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alloca(3) — Linux manual page

alloca(3) Library Functions Manual alloca(3)

NAME top

 alloca - allocate memory that is automatically freed

LIBRARY top

 Standard C library (libc, -lc)

SYNOPSIS top

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

DESCRIPTION top

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

RETURN VALUE top

 The alloca() function returns a pointer to the beginning of the
 allocated space. If the allocation causes stack overflow, program
 behavior is undefined.

ATTRIBUTES top

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

STANDARDS top

 None.

HISTORY top

 PWB, 32V.

NOTES top

 The alloca() function is machine- and compiler-dependent. Because
 it allocates from the stack, it's faster than malloc(3) and
 free(3). In certain cases, it can also simplify memory
 deallocation in applications that use longjmp(3) or siglongjmp(3).
 Otherwise, its use is discouraged.
 Because the space allocated by alloca() is allocated within the
 stack frame, that space is automatically freed if the function
 return is jumped over by a call to longjmp(3) or siglongjmp(3).
 The space allocated by alloca() is not automatically deallocated
 if the pointer that refers to it simply goes out of scope; it is
 automatically deallocated when the caller function returns.
 Do not attempt to free(3) space allocated by alloca()!
 By necessity, alloca() is a compiler built-in, also known as
 __builtin_alloca(). By default, modern compilers automatically
 translate all uses of alloca() into the built-in, but this is
 forbidden if standards conformance is requested (-ansi, -std=c*),
 in which case <alloca.h> is required, lest a symbol dependency be
 emitted.
 The fact that alloca() is a built-in means it is impossible to
 take its address or to change its behavior by linking with a
 different library.
 Variable length arrays (VLAs) are part of the C99 standard,
 optional since C11, and can be used for a similar purpose.
 However, they do not port to standard C++, and, being variables,
 live in their block scope and don't have an allocator-like
 interface, making them unfit for implementing functionality like
 strdupa(3).

BUGS top

 Due to the nature of the stack, it is impossible to check if the
 allocation would overflow the space available, and, hence, neither
 is indicating an error. (However, the program is likely to
 receive a SIGSEGV signal if it attempts to access unavailable
 space.)
 On many systems alloca() cannot be used inside the list of
 arguments of a function call, because the stack space reserved by
 alloca() would appear on the stack in the middle of the space for
 the function arguments.

SEE ALSO top

 brk(2), longjmp(3), malloc(3)

COLOPHON top

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 user-space interface documentation) project. Information about
 the project can be found at 
 ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
 for this manual page, see
 ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
 This page was obtained from the tarball man-pages-6.15.tar.gz
 fetched from
 ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
 2025年08月11日. If you discover any rendering problems in this HTML
 version of the page, or you believe there is a better or more up-
 to-date source for the page, or you have corrections or
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 part of the original manual page), send a mail to
 man-pages@man7.org
Linux man-pages 6.15 2025年05月17日 alloca(3)

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Pages that refer to this page: malloc(3), strdup(3)

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