ADC sample code MapLargeFile.c returns an error

Question

I need some help running sample code from the ADC web site. I'm running 10.5.6 on a Mac Pro with 6 GB of RAM.
The example in question "Shows a technique for mapping files greater than 4GB into memory for faster access."
I compile it like this:
gcc MapLargeFile.c -o MapLargeFile
Then I make a 6GB file like this:
dd if=/dev/zero of=foobar.txt bs=1024 count=6291456
Finally I run it like this:
./MapLargeFile foobar.txt
The program fails with this warning:
./MapLargeFile: failed to mmap file at offset 0x0
I get the same error message if I give it a nonexistent file name. Looking at the code I would think it would say it couldn't open the nonexistent file.
What am I doing wrong?
Here is the sample code from the Apple ADC site:
File: MapLargeFile.c
Description: Shows a technique for accessing files > 4GB in size.
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Change History (most recent first):
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/time.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/mman.h>
struct timeval lastTime, thisTime;
This sample is set up to map a 6GB file into memory in 256MB chunks.
#define kFileSize (0x180000000ULL) // size of file we expect to map = 6GB
#define kBufferSize (0x10000000UL) // we will map it in 256MB chunks
int main( int argc, const char *argv[] )
int fd, cnt, loops;
unsigned long long *buffer, sum;
off_t offset;
if ( argc != 2 )
fprintf( stderr, "usage: %s filename
", argv[0] );
return -1;
// open the file
fd = open( argv[1], O_RDONLY, 0 );
if ( fd == 0 )
fprintf( stderr, "%s: failed to open %s for read
", argv[0], argv[1] );
return -1;
sum = 0;
offset = 0;
loops = 9; // loop through the file 10 times
gettimeofday( &lastTime, NULL );
while ( 1 ) // loop for ever, loop exit is handled below
// map in a kBufferSize segment of the file
buffer = mmap( 0, kBufferSize, PROT_READ, MAP_FILE, fd, offset );
if ( buffer == MAP_FAILED )
fprintf( stderr, "%s: failed to mmap file at offset 0x%llx
", argv[0], offset );
break;
// touch every byte in this segment of the file.
// this 'warms up' the pages in memory so that subsequent accesses will be much faster
for ( cnt = 0; cnt < kBufferSize / sizeof(sum); cnt += 16 )
sum += buffer[cnt + 0];
sum += buffer[cnt + 1];
sum += buffer[cnt + 2];
sum += buffer[cnt + 3];
sum += buffer[cnt + 4];
sum += buffer[cnt + 5];
sum += buffer[cnt + 6];
sum += buffer[cnt + 7];
sum += buffer[cnt + 8];
sum += buffer[cnt + 9];
sum += buffer[cnt + 10];
sum += buffer[cnt + 11];
sum += buffer[cnt + 12];
sum += buffer[cnt + 13];
sum += buffer[cnt + 14];
sum += buffer[cnt + 15];
// unmap the current segment of the file
munmap( buffer, kBufferSize );
// increment the offset into the file
offset += kBufferSize;
if ( offset >= kFileSize )
offset = 0; // reset the offset if we're at the end of the file
gettimeofday( &thisTime, NULL );
printf( "Time = %u seconds, sum = %llx
", thisTime.tv_sec - lastTime.tv_sec, sum );
if ( loops-- == 0 ) break; // exit if we've done all loops.
lastTime = thisTime;
close( fd ); // all done, close the file
return 0;

Answer 1

The short comment explains it:
// touch every byte in this segment of the file.
// this 'warms up' the pages in memory so that subsequent accesses will be much faster
This is basically just buffered I/O. If you know you are going to need to read 10000 bytes, there is no need to read each byte one at a time. You read all 10000 bytes at once, then access each byte in memory instead of going to disk for it.