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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ALLOCA(3) Linux Programmer's Manual ALLOCA(3)
NAME
alloca - allocate memory that is automatically freed
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 allocated space. If the allocation causes stack overflow, program behavior
is undefined.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
+----------+---------------+---------+
|Interface | Attribute | Value |
+----------+---------------+---------+
|alloca() | Thread safety | MT-Safe |
+----------+---------------+---------+
CONFORMING TO
This function is not in POSIX.1.
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.
NOTES
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
-std=c11 option is given and the header <alloca.h> is not included. Otherwise, (without an -ansi or -std=c* option) the glibc version of
<stdlib.h> includes <alloca.h> and that contains the lines:
#ifdef __GNUC__
#define alloca(size) __builtin_alloca (size)
#endif
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.
BUGS
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 arguments.
SEE ALSO
brk(2), longjmp(3), malloc(3)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the
latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
GNU
2017-09-15 ALLOCA(3)