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?
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.
