alloca(3) - Linux manual page (original) (raw)

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)](../man7/attributes.7.html).
   ┌──────────────────────────────────────┬───────────────┬─────────┐
   │ **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)](../man3/malloc.3.html) and
   [free(3)](../man3/free.3.html).  In certain cases, it can also simplify memory
   deallocation in applications that use [longjmp(3)](../man3/longjmp.3.html) or [siglongjmp(3)](../man3/siglongjmp.3.html).
   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)](../man3/longjmp.3.html) or [siglongjmp(3)](../man3/siglongjmp.3.html).

   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)](../man3/free.3.html) 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)](../man3/strdupa.3.html).

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)](../man2/brk.2.html), [longjmp(3)](../man3/longjmp.3.html), [malloc(3)](../man3/malloc.3.html)

COLOPHON top

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Linux man-pages 6.10 2024-11-19 alloca(3)

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