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ALLOCA(3) Linux Programmer's Manual ALLOCA(3)
alloca - memory allocator
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
The alloca function returns a pointer to the beginning of the allocated space. If the
allocation causes stack overflow, program behaviour is undefined.
There is evidence that the alloca function appeared in 32v, pwb, pwb.2, 3bsd, and 4bsd.
There is a man page for it in BSD 4.3. Linux uses the GNU version. This function is not
in POSIX or SUSv3.
NOTES ON THE GNU VERSION
Normally, gcc translates calls to alloca by inlined code. This is not done when either the
-ansi or the -fno-builtin option is given. 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 behaviour 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.
The alloca function is machine and compiler dependent. On many systems its implementation
is buggy. Its use is discouraged.
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), pagesize(2), calloc(3), malloc(3), realloc(3)
GNU 2002-07-17 ALLOCA(3)
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