Why does mmap() fail with ENOMEM on a 1TB sparse file?
- by metadaddy
I've been working with large sparse files on openSUSE 11.2 x86_64. When I try to mmap() a 1TB sparse file, it fails with ENOMEM. I would have thought that the 64 bit address space would be adequate to map in a terabyte, but it seems not. Experimenting further, a 1GB file works fine, but a 2GB file (and anything bigger) fails. I'm guessing there might be a setting somewhere to tweak, but an extensive search turns up nothing.
Here's some sample code that shows the problem - any clues?
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <unistd.h>
int main(int argc, char *argv[]) {
char * filename = argv[1];
int fd;
off_t size = 1UL << 40; // 30 == 1GB, 40 == 1TB
fd = open(filename, O_RDWR | O_CREAT | O_TRUNC, 0666);
ftruncate(fd, size);
printf("Created %ld byte sparse file\n", size);
char * buffer = (char *)mmap(NULL, (size_t)size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if ( buffer == MAP_FAILED ) {
perror("mmap");
exit(1);
}
printf("Done mmap - returned 0x0%lx\n", (unsigned long)buffer);
strcpy( buffer, "cafebabe" );
printf("Wrote to start\n");
strcpy( buffer + (size - 9), "deadbeef" );
printf("Wrote to end\n");
if ( munmap(buffer, (size_t)size) < 0 ) {
perror("munmap");
exit(1);
}
close(fd);
return 0;
}
