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
       │ Interface                           Attribute     Value   │
       │ alloca()                            │ Thread safety │ MT-Safe │

STANDARDS         top


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.

       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

       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

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

       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)

Linux man-pages (unreleased)     (date)                        alloca(3)

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