mmap invalid argument error

Question

This is the first time I use mmap system call. I am getting invalid argument error and I do not understand why , obviously I missing something Please help me, thank you

#include <stdio.h>
#include <sys/mman.h>


int main() {

    long pageSize = getpagesize () ; 

    size_t length = 4096 ;


    int * map = (int * ) mmap ( 0 , length , PROT_READ | PROT_WRITE , MAP_ANONYMOUS , 0 , 0 ) ; 
        if ( map == MAP_FAILED ) {

            perror ( " error mapping " ) ;

        }

    return 0 ;
}

Answer 1

You need to specify at least one of MAP_PRIVATE or MAP_SHARED in the flags. Also, as the other answer says, you should have -1 as the file descriptor for portability, but that's not where your problem is (since you tagged this question with linux and linux ignores the file descriptor for anon mappings).

Answer 2

You are passing 0 as the file descriptor. Anonymous mappings should always use -1 as the file descriptor, since they are not backed by a file. Also, as the other answer says, MAP_ANONYMOUS should be complemented by either MAP_PRIVATE or MAP_SHARED.

The correct way to call it would be:

int *map = mmap(0, length, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);

Note that the cast is not necessary since mmap returns void *.