a program that allocates huge chunks of memory using mmap(say 1GB) [duplicate] - mmap

Question

I am writing a program that allocates huge chunks of memory using mmap and then accesses random memory locations to read and write into it.
I just tried out the following code:
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
int main() {
int fd,len=1024*1024;
fd=open("hello",O_READ);
char*addr=mmap(0,len,PROT_READ+PROT_WRITE,MAP_SHARED,fd,0);
for(fd=0;fd<len;fd++)
putchar(addr[fd]);
if (addr==MAP_FAILED) {perror("mmap"); exit(1);}
printf("mmap returned %p, which seems readable and writable\n",addr);
munmap(addr,len);
return 0;
}
But I cannot execute this program, is there anything wrong with my code?

Answer 1

First of all, the code won't even compile on my debian box. O_READ isn't a correct flag for open() as far as I know.
Then, you first use fd as a file descriptor and the you use it as a counter in your for loop.
I don't understand what you're trying to do, but I think you misunderstood something about mmap.
mmap is used to map a file into the memory, this way you can read / write to the created memory mapping instead of using functions to access the file.
Here's a short program that open a file, map it the the memory and print the returner pointer :
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int main() {
int fd;
int result;
int len = 1024 * 1024;
fd = open("hello",O_RDWR | O_CREAT | O_TRUNC, (mode_t) 0600);
// stretch the file to the wanted length, writting something at the end is mandatory
result = lseek(fd, len - 1, SEEK_SET);
if(result == -1) { perror("lseek"); exit(1); }
result = write(fd, "", 1);
if(result == -1) { perror("write"); exit(1); }
char*addr = mmap(0, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (addr==MAP_FAILED) { perror("mmap"); exit(1); }
printf("mmap returned %p, which seems readable and writable\n",addr);
result = munmap(addr, len);
if (result == -1) { perror("munmap"); exit(1); }
close(fd);
return 0;
}
I left out the for loop, since I didn't understood its purpose. Since you create a file and you want to map it on a given length, we have to "stretch" the file to the given length too.
Hope this helps.