使用 mmap 读/写文件时如何解决 ENODEV

Question

I'm trying to mmap a file for reading and writing, and currently I am receiving errcode 19: ENODEV. I'm using a blank file with 2MB of space.

According to the man pages, this error occurs when the file is not compatible for mapping. If this is true, I don't understand why; It's just a regular empty file. Additionally, it was provided by my professor, so presumably, it should work fine. Is anything else going on?

Here is my code:

#include <stdio.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <stdlib.h>
#include <errno.h>

int main(int argc, const char * argv[]) {
    int fd = open("Drive2MB", O_RDWR, S_IRUSR | S_IWUSR | S_IXUSR);
    if(fd < 0) {
        printf("open failure\n");
        exit(1);
    }

    char *drive = mmap(NULL, 1048576, PROT_WRITE, fd, MAP_SHARED, 0); 
    if(drive == MAP_FAILED) {
        printf("map failure\n");
        fprintf(stderr, "Value of errno: %d\n", errno);
        exit(1);
    }

    //test
    drive[513] = 'p';
    printf("%c", drive[513]);
}

If it is indeed the file, how would I go about creating a file that is compatible with mmap().

Answer 1

Two mistakes:

char *drive = mmap(NULL, 1048576, PROT_WRITE, fd, MAP_SHARED, 0); 

should be

char *drive = mmap(NULL, 1048576, PROT_WRITE, MAP_SHARED, fd, 0); 

and you can't map an empty file but only a file that has the length already.

To populate Drive2MB do dd if=/dev/zero of=Drive2MB bs=1048576 count=1

MAP_SHARED happens to have the value of 1 so it tried to mmap stdout which is typically a terminal, which cannot be mapped.