Can't create float array with a variable name as size parameter?

Question

Hi,
When trying to compile code that users a variable in the array subscript to set the size, CC gives the following error:
Error: An integer constant expression is required within the array subscript operator.
1 Error(s) detected.The code is as follows:
int main()
//blah blah
const int arrSize = numberVariables;
    float tempArr[arrSize];
//blah blah
}Output from CC -V:
unknown% CC -V
CC: Sun C++ 5.9 SunOS_i386 Patch 124864-01 2007/07/25Output from uname -a:
unknown% uname -a
SunOS unknown 5.10 Generic_137138-09 i86pc i386 i86pcAny ideas on why CC is giving that error?
~Slow
Edited by: SlowToady on Nov 15, 2008 8:28 PM
Edited by: SlowToady on Nov 15, 2008 8:36 PM

Answer 1

Marc_Glisse wrote:
Last time I checked, I did not see VLAs in the draft of the next C++ standard, which I believe is now feature complete. So either I missed it (quite possible) or VLAs were considered a bad idea (for C as well many people consider alloca and VLA bad ideas). It is however a very reasonable extension to have (care to file a RFE?).The following code performs like this on an IBM x366 with 4 single-core 3.16 GHz Xeons, running OpenSolaris build 96:
-bash-3.2$ ./thrtest
malloc() took 440895924 ns
alloca() took 2122522 ns
-bash-3.2$Using alloca is two hundred times faster.
Two hundred times faster.
Think about that next time some pedant says alloca "sucks".
Try for yourself. Here's the code:
#include <stdlib.h>
#include <pthread.h>
#include <alloca.h>
#include <strings.h>
#include <stdio.h>
#define NUM_THREADS 8
#define ITERS 10000
#define BYTES 1024
typedef void ( *mem_func_t )( void );
void alloca_iter()
    char *ptr;
    ptr = ( char * ) alloca( BYTES );
    memset( ptr, 0, BYTES );
    return;
void malloc_iter()
    char *ptr;
    ptr = ( char * ) malloc( BYTES );
    memset( ptr, 0, BYTES );
    free( ptr );
    return;
void *thread_proc( void *arg )
    mem_func_t mem_iter;
    mem_iter = ( mem_func_t ) arg;
    for ( int ii = 0; ii < ITERS; ii++ )
        mem_iter();
    return( NULL );
int main( int argc, char **argv )
    pthread_t tids[ NUM_THREADS ];
    void *results[ NUM_THREADS ];
    hrtime_t start;
    hrtime_t end;
    int ii;
    start = gethrtime();
    for ( ii = 0; ii < NUM_THREADS; ii++ )
        pthread_create( &tids[ ii ], NULL, thread_proc, malloc_iter );
    for ( ii = 0; ii < NUM_THREADS; ii++ )
        pthread_join( tids[ ii ], &results[ ii ] );
    end = gethrtime();
    printf( "malloc() took %ld ns\n", end - start );
    start = gethrtime();
    for ( ii = 0; ii < NUM_THREADS; ii++ )
        pthread_create( &tids[ ii ], NULL, thread_proc, alloca_iter );
    for ( ii = 0; ii < NUM_THREADS; ii++ )
        pthread_join( tids[ ii ], &results[ ii ] );
    end = gethrtime();
    printf( "alloca() took %ld ns\n", end - start );
    return( 0 );
}