alloca(3) Library Functions Manual alloca(3)
NAME
alloca - allocate memory that is automatically freed
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <alloca.h>
void *alloca(size_t size);
DESCRIPTION
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
The alloca() function returns a pointer to the beginning of the allo-
cated space. If the allocation causes stack overflow, program behavior
is undefined.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│alloca() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
This function is not in POSIX.1.
alloca() originates from PWB and 32V, and appears in all their deriva-
tives.
NOTES
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 discour-
aged.
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 trans-
late all uses of alloca() into the built-in, but this is forbidden if
standards conformance is requested (-ansi, -std=c*), in which case <al-
loca.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 im-
plementing functionality like strdupa(3).
BUGS
Due to the nature of the stack, it is impossible to check if the allo-
cation would overflow the space available, and, hence, neither is indi-
cating 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 ap-
pear on the stack in the middle of the space for the function argu-
ments.
SEE ALSO
brk(2), longjmp(3), malloc(3)
Linux man-pages 6.03 2022-12-15 alloca(3)
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