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Use of alloca function


 
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# 15  
Old 10-29-2009
Quote:
Originally Posted by shamrock
I get that but my question was more about how are you going to use a dynamically allocated stack buffer.
Code:
int main(void)
{
        void *buffer=alloca(512);
        while(fread(stdin, 512, buffer) != NULL)
        { /* do stuff */ }
}

I already get there are ways you can't use it. That doesn't mean its useless. Nobody's forcing you to use it either.
Quote:
It's just as easy to do with malloc.
Many implementations of malloc aren't threadsafe. But we're beginning to repeat ourselves here.
Quote:
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.
It's a stretch to call something that exists on any UNIX you can name, and beyond into the non-UNIX world, nonportable. But don't forget I'm trying to prove you right here. You're assuming it's problematic, I'm hunting for better reasons instead of assuming you're wrong.

In that respect I've found two items of interest; a makefile that suggests some historical System V's didn't have it, and an SCO compiler bug that happens under extremely specific circumstances. The latter is very interesting, I'll have to modify my tests to include it.

Quote:
I have a hard copy of the UNIX Programmer's Manual (7th ed. Vol. 1 Jan. 1979) and alloca is nowhere in it.
I never said it was in the standard or documentation -- I'd be shocked if it was. I didn't just make it up, though -- some sources think v32 has it.

Last edited by Corona688; 10-29-2009 at 04:34 AM..
# 16  
Old 10-29-2009
Yes 32V had it. It was implemented as an assember routine in libc (/usr/src/libc/sys/alloca.s)
Code:
# like alloc, but automatic
# automatic free in return

.globl	_alloca
_alloca:
	.word	0x0000
	subl2	4(ap),sp            # crude allocation
	movl	16(fp),r1	                 # pc
	movq	8(fp),ap	         # new (old) ap and fp
	bicl2	$3,sp		         # 4-byte align
	addl2	$7*4,sp            # reuse space of mscp
	movl	sp,r0		         # return value
	jmp 	(r1)                           # funny return


Last edited by fpmurphy; 10-30-2009 at 10:25 AM.. Reason: Fix typo
# 17  
Old 10-29-2009
Why use alloca()? Oh, how about a few orders of magnitude faster performance under multithreaded heap contention?

This

Code:
malloc() took 103497088262 ns
alloca() took 135368983 ns

came from this code:

Code:
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <pthread.h>
#include <alloca.h>

#define BUFSIZE 8192
#define NUM_THR 8

static volatile int flag = 0;

typedef void ( *func_ptr )( void );

void test_malloc( void )
{
    void *ptr;
    ptr = malloc( BUFSIZE );
    free( ptr );
    return;
}

void test_alloca( void )
{
    void *ptr;
    ptr = alloca( BUFSIZE );
    return;
}

void *run( void *arg )
{
    func_ptr f;
    int ii;

    if ( flag )
        f = test_malloc;
    else
        f = test_alloca;
    for ( ii = 0; ii < ( 10 * 1024 * 1024 ); ii++ )
        f();
    return( NULL );
}

int main( int argc, char **argv )
{
    pthread_t tids[ NUM_THR ];
    hrtime_t start;
    hrtime_t end;
    void *status;
    int ii;

    flag = 1;
    start = gethrtime();
    for ( ii = 0; ii < NUM_THR; ii++ )
        pthread_create( &tids[ ii ], NULL, run, NULL );
    for ( ii = 0; ii < NUM_THR; ii++ )
        pthread_join( tids[ ii ], &status );
    end = gethrtime();
    printf( "malloc() took %lld ns\n", end - start );

    flag = 0;
    start = gethrtime();
    for ( ii = 0; ii < NUM_THR; ii++ )
        pthread_create( &tids[ ii ], NULL, run, NULL );
    for ( ii = 0; ii < NUM_THR; ii++ )
        pthread_join( tids[ ii ], &status );
    end = gethrtime();
    printf( "alloca() took %lld ns\n", end - start );
    return( 0 );
}

run under Solaris 10 running on an IBM e326m with 2 dual-core 2.4 GHz Opterons:
# 18  
Old 10-29-2009
Considering how minimal alloca really is, I'd be worried about the compiler optimizing it away completely in your example...
# 19  
Old 10-30-2009
Quote:
Originally Posted by Corona688
Code:
int main(void)
{
        void *buffer=alloca(512);
        while(fread(stdin, 512, buffer) != NULL)
        { /* do stuff */ }
}

I already get there are ways you can't use it. That doesn't mean its useless. Nobody's forcing you to use it either.
All I m trying to say is that the value from alloca cant be returned from the function that called it. I most likely wont be using it since I have to deal with multiple platforms.
Quote:
Originally Posted by Corona688
Many implementations of malloc aren't threadsafe. But we're beginning to repeat ourselves here.
Care to list some platforms where the malloc implementation isnt threadsafe...just curious btw.
Quote:
Originally Posted by Corona688
It's a stretch to call something that exists on any UNIX you can name, and beyond into the non-UNIX world, nonportable. But don't forget I'm trying to prove you right here. You're assuming it's problematic, I'm hunting for better reasons instead of assuming you're wrong.
Not concerned about the right or wrong aspect of it. I probably am totally wrong...but as we deal with multiple platforms I am leary of something that's billed non-portable and strongly discouraged i.e. "dont want to rush in like a fool where vendors fear to tread"
Quote:
Originally Posted by Corona688
In that respect I've found two items of interest; a makefile that suggests some historical System V's didn't have it, and an SCO compiler bug that happens under extremely specific circumstances. The latter is very interesting, I'll have to modify my tests to include it.

I never said it was in the standard or documentation -- I'd be shocked if it was. I didn't just make it up, though -- some sources think v32 has it.
Not a big deal again I was just curious why my copy didnt have it but as fpmurphy clarified it was an assembly routine.
# 20  
Old 11-02-2009
i just compiled the program but i am getting the following errors

alloca.c: In function `main':
alloca.c:46: `hrtime_t' undeclared (first use in this function)
alloca.c:46: (Each undeclared identifier is reported only once
alloca.c:46: for each function it appears in.)
alloca.c:46: parse error before "start"
alloca.c:52: `start' undeclared (first use in this function)
alloca.c:57: `end' undeclared (first use in this function)

i think we are missing some header file.
# 21  
Old 11-02-2009
The library call "gethrtime()" and the type "hrtime_t" are Solaris features. IIRC they're in "time.h". I used them because I was on Solaris at the time and they provide nanosecond resolution. The drawback of gethrtime() is that it has no reference frame so it's only a relative time - the value returned by gethrtime() is only meaningful when compared to the value from another call to gethrtime(), and then probably only from within the same process.

It's great for timing durations within a process but that's about it.

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