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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO |
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alloca(3) Library Functions Manual alloca(3)
alloca - allocate memory that is automatically freed
Standard C library (libc, -lc)
#include <alloca.h>
void *alloca(size_t size);
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.
The alloca() function returns a pointer to the beginning of the
allocated space. If the allocation causes stack overflow,
program behavior is undefined.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│alloca() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
None.
PWB, 32V.
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
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).
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.
brk(2), longjmp(3), malloc(3)
Linux man-pages 6.04 2023-03-30 alloca(3)
Pages that refer to this page: malloc(3), strdup(3)
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HTML rendering created 2023-06-24 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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