I have Inline::C working on my system now. Before proceeding further, I decided to make a little test to see what malloc() is doing. Often more memory is allocated than requested and I wanted to see if I could get an idea of "how much more?". Sometimes there is a lucky accident where the code works although it is not guaranteed to work.

I have 2 programs.
1) 32 bit gcc independent of Perl
2) 64 bit gcc that is part of my 64 bit Perl 5.24 installation

Code for both is shown below.
For 32 bit version, I am not surprised to see 64 bit instead of 32 bit alignment (lower 3 address bits always zero). 64 bit alignment also appears to be the case for the 64 bit gcc code as well. Anyway the alignment drives the absolute minimum memory allocation unit. In both cases, 8 bytes (64 bit alignment). The question is then whether there is additional space due to the quanta that malloc() uses to track allocation?

The 32 bit standalone version does the exact same thing every run. I don't know why there is the difference between 2nd and 1st allocation, but after that we see the pattern of 16 bytes given for a single byte requested.

The 64 bit inline C does something different every run! For all I know this could be some kind of Perl security feature?

Anyway, I thought the results relevant to our discussion about allocation of this weird type. 

#include <stdio.h>
#include <stdlib.h>

//testing malloc() this is 32 bit gcc - separate from Perl
void testMalloc(void)
{
    char* x = (char *) malloc(1);
    printf ("     Byte Starting Memory Addr is %p\n",x);
    char* y = (char *) malloc(1);
    printf ("Next Byte Starting Memory Addr is %p\n",y);
    printf ("difference in bytes between byte2 and byte1 = %d\n",y-x);
    char* z = (char *) malloc(1);
    printf ("Next Byte Starting Memory Addr is %p\n",z);
    printf ("difference in bytes between byte3 and byte2 = %d\n",z-y);
    char* alpha = (char *) malloc(1);
    printf ("Next Byte Starting Memory Addr is %p\n",alpha);
    printf ("difference in bytes between byte4 and byte3 = %d\n",z-y);
    free(x);
    free(y);
    free(z);
    free(alpha);

    return;
}

int main(int argc, char *argv[])
{

   testMalloc();
   exit(0);
}

/*
     Byte Starting Memory Addr is 00C92FD8
Next Byte Starting Memory Addr is 00C90CC8
difference in bytes between byte2 and byte1 = -8976
Next Byte Starting Memory Addr is 00C90CD8
difference in bytes between byte3 and byte2 = 16
Next Byte Starting Memory Addr is 00C90CE8
difference in bytes between byte4 and byte3 = 16
*/

/*
     Byte Starting Memory Addr is 00B22FD8
Next Byte Starting Memory Addr is 00B20CC8
difference in bytes between byte2 and byte1 = -8976
Next Byte Starting Memory Addr is 00B20CD8
difference in bytes between byte3 and byte2 = 16
Next Byte Starting Memory Addr is 00B20CE8
difference in bytes between byte4 and byte3 = 16
*/

[download]
#################################################
# testing malloc 64 bit Perl
use strict;
use warnings;

use Inline C => Config =>
  BUILD_NOISY => 1,
  CLEAN_AFTER_BUILD => 0,
  USING => 'ParseRegExp',
  ;

use Inline "C";

testMalloc();

=OUTPUT:
     Byte Starting Memory Addr is 000000000308CE68
Next Byte Starting Memory Addr is 000000000308D258
difference in bytes between byte2 and byte1 = 1008
Next Byte Starting Memory Addr is 000000000308D018
difference in bytes between byte3 and byte2 = -576
Next Byte Starting Memory Addr is 000000000308D0A8
difference in bytes between byte4 and byte3 = -576
=cut

=AnotherRun:
     Byte Starting Memory Addr is 00000000031157D8
Next Byte Starting Memory Addr is 0000000003115B98
difference in bytes between byte2 and byte1 = 960
Next Byte Starting Memory Addr is 0000000003115E98
difference in bytes between byte3 and byte2 = 768
Next Byte Starting Memory Addr is 0000000003115F88
difference in bytes between byte4 and byte3 = 768
=cut

=yet Another Run
     Byte Starting Memory Addr is 0000000002EA6CF8
Next Byte Starting Memory Addr is 0000000002EA6EA8
difference in bytes between byte2 and byte1 = 432
Next Byte Starting Memory Addr is 0000000002EA7AA8
difference in bytes between byte3 and byte2 = 3072
Next Byte Starting Memory Addr is 0000000002EA7C58
difference in bytes between byte4 and byte3 = 3072
=cut

=one more time
     Byte Starting Memory Addr is 000000000313CE38
Next Byte Starting Memory Addr is 000000000313C5F8
difference in bytes between byte2 and byte1 = -2112
Next Byte Starting Memory Addr is 000000000313C868
difference in bytes between byte3 and byte2 = 624
Next Byte Starting Memory Addr is 000000000313CA48
difference in bytes between byte4 and byte3 = 624
=cut


__END__
__C__

void testMalloc(void)
{
    char* x = (char *) malloc(1);
    printf ("     Byte Starting Memory Addr is %p\n",x);
    char* y = (char *) malloc(1);
    printf ("Next Byte Starting Memory Addr is %p\n",y);
    printf ("difference in bytes between byte2 and byte1 = %d\n",y-x);
    char* z = (char *) malloc(1);
    printf ("Next Byte Starting Memory Addr is %p\n",z);
    printf ("difference in bytes between byte3 and byte2 = %d\n",z-y);
    char* alpha = (char *) malloc(1);
    printf ("Next Byte Starting Memory Addr is %p\n",alpha);
    printf ("difference in bytes between byte4 and byte3 = %d\n",z-y);
    
    free(x);
    free(y);
    free(z);
    free(alpha);

    return;
}

[download]

In reply to Re^7: Perl XS binding to a struct with an array of chars* by Marshall
in thread Perl XS binding to a struct with an array of chars* by MaxPerl

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