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alloca - allocate memory that is automatically freed
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.
This function is not in POSIX.1-2001.
There is evidence that the alloca() function appeared in 32V, PWB,
PWB.2, 3BSD, and 4BSD. There is a man page for it in 4.3BSD. Linux
uses the GNU version.
The alloca() function is machine- and compiler-dependent. For certain
applications, its use can improve efficiency compared to the use of
malloc(3) plus 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).
Do not attempt to free(3) space allocated by alloca()!
Notes on the GNU Version
Normally, gcc(1) translates calls to alloca() with inlined code. This
is not done when either the -ansi, -std=c89, -std=c99, or the
-fno-builtin option is given (and the header <alloca.h> is not
included). But beware! By default the glibc version of <stdlib.h>
includes <alloca.h> and that contains the line:
#define alloca(size) __builtin_alloca (size)
with messy consequences if one has a private version of this function.
The fact that the code is inlined means that it is impossible to take
the address of this function, or to change its behavior by linking with
a different library.
The inlined code often consists of a single instruction adjusting the
stack pointer, and does not check for stack overflow. Thus, there is
no NULL error return.
There is no error indication if the stack frame cannot be extended.
(However, after a failed allocation, the program is likely to receive a
SIGSEGV signal if it attempts to access the unallocated 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
brk(2), longjmp(3), malloc(3)
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