I am trying to map a .img file, and I am not sure why my code is not working.
Here is my code, and when I run the code I keep getting my error, that p is equal to MAP_FAILED
int diskinfo(int argc, char* argv[]){
void *p;
char *size
if (argc < 2) {
printf("Please put ./diskinfo <file> \n");
exit(1);
}
int fp = open(argv[1],"rb+");
if(fp == NULL) {
printf("Error opening file");
exit(1);
}
struct stat buf;
fstat(fp, &buf);
p = mmap(NULL,buf.st_size, PROT_READ, MAP_PRIVATE, fp, 0);
if(p == MAP_FAILED){
printf("Error mapping memory\n");
exit(1);
}}
If anyone has any suggestions on where my code is wrong or if I am missing a piece of information I would be very grateful.
Changing to perror() does not work. Also changing this function doesn't change the fact that p is still equal to MAP_FAILED
if(p == MAP_FAILED){
return;
}
I changed the below what is the solution:
int fp = open(argv[1],O_RDWR);
if(fp < 0){
. . .
But I am still returning
It is unclear from the state of your question whether you were ever able to get mmap to work. Your last edit added:
int fp = open(argv[1],O_RDWR);
Which is fine if you are writing back to the file you have opened, but if not, you should open using O_RDONLY to prevent inadvertent modification of your original file.
While not an error, fp is generally used as a file pointer associated with file stream operations when the file is opened with fopen. Here you are using low-level I/O with read/write which uses a file descriptor instead of a stream pointer. When referencing a descriptor, the general vernacular uses fd as short-hand for file descriptor. (personally, it was awkward to see the two used in a interchanged manner -- which I suspect is the case for others as well)
Your remaining use of fstat, the resulting buf.st_size and your call to mmap are not the problem. Your problem lies elsewhere -- which is one of the primary reasons you should post A Minimal, Complete, and Verifiable Example (MCVE).
That said, to insure you have incorporated your changes in the proper manner, I'll leave you with a simple example that mmaps a file and simply dumps the file to stdout (so limit your input filenames to a reasonably short text file to work with the example -- otherwise you will see all sorts of strange characters). Work through the following:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
int diskinfo (int argc, char *argv[]) {
char *p = NULL; /* pointer to mmapped file */
int fd = 0; /* file descriptor */
struct stat buf = {0}; /* struct stat */
ssize_t size = 0; /* file size (typed for write return) */
if (argc < 2) { /* validate at least 2 arguments */
printf ("Please put %s <file> \n", argv[0]);
exit (EXIT_FAILURE);
}
if ((fd = open (argv[1], O_RDONLY)) == -1) { /* open/validate file */
perror ("Error opening file");
exit (EXIT_FAILURE);
}
if (fstat (fd, &buf) == -1) { /* stat file for size */
perror ("error: fstat buf");
exit (EXIT_FAILURE);
}
size = buf.st_size; /* get file size */
/* mmap file and validate return */
if ((p = mmap (NULL, buf.st_size, PROT_READ, MAP_PRIVATE, fd, 0))
== (void *) -1) {
perror ("mmap failed");
exit (EXIT_FAILURE);
}
/* simple example, output mmapped file to stdout */
if (write (STDOUT_FILENO, p, size) != size) {
perror ("error on write");
exit (EXIT_FAILURE);
}
munmap (p, size); /* unmap file */
return 1; /* return success (fn could be void due to exit) */
}
int main (int argc, char **argv) {
diskinfo (argc, argv); /* call diskinfo function */
return 0;
}
(note: your check of if (argc < 2) should really be done in the calling function, main() here. There is no reason to call diskinfo until you have validated you have a filename to open. You could actually refactor your code to check the arguments and open the file in main() and simply pass an open file-descriptor to diskinfo as a parameter)
Example Use/Output
$ ./bin/mmapdiskinfo dat/captnjack.txt
This is a tale
Of Captain Jack Sparrow
A Pirate So Brave
On the Seven Seas.
Look things over and let me know if you have any questions. If you still cannot get your function to work, then post a MCVE so that we can help further.