10-28-2009
Quote:
Originally Posted by
Corona688
Any time you want a local buffer without predefining its size, for one thing.
I get that but my question was more about how are you going to use a dynamically allocated stack buffer.
Quote:
Originally Posted by
Corona688
achenle also had a good point about it being threadsafe hence much faster than trying to synchronize malloc for multithreaded things.
It's just as easy to do with malloc.
Quote:
Originally Posted by
Corona688
It's interesting how each platform discourages the use of alloca because of how it might behave on some other unmentionable platform without confessing to being guilty themselves. This could stand some testing, I think.
I would be leary of using a C lib function that is non-portable and strongly discouraged by the vendors who make it as it goes against the very purpose for which C was created i.e. portability. Otherwise we would all be coding in assembly which would be very cumbersome given the number of different mpus in the market today.
Quote:
Originally Posted by
Corona688
alloca's almost as old as UNIX itself, circa AT&T V32 1979, making me wonder how historical this complaint is and how relevant it actually remains.
I have a hard copy of the UNIX Programmer's Manual (7th ed. Vol. 1 Jan. 1979) and alloca is nowhere in it.
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LEARN ABOUT NETBSD
alloca
ALLOCA(3) BSD Library Functions Manual ALLOCA(3)
NAME
alloca -- memory allocator
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdlib.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 on return.
RETURN VALUES
The alloca() function returns a pointer to the beginning of the allocated space. If the allocation failed, a NULL pointer is returned.
SEE ALSO
brk(2), calloc(3), getpagesize(3), malloc(3), realloc(3)
CAVEATS
Few limitations can be mentioned:
o The alloca() function is machine dependent; its use is discouraged.
o The alloca() function is slightly unsafe because it cannot ensure that the pointer returned points to a valid and usable block of memory.
The allocation made may exceed the bounds of the stack, or even go further into other objects in memory, and alloca() cannot determine
such an error. Avoid alloca() with large unbounded allocations.
o Since alloca() modifies the stack at runtime, it causes problems to certain security features. See security(7) for a discussion.
BSD
March 21, 2011 BSD