c - segmentation fault when accessing virtual memory

Question

I am trying to write a program that takes a command argument required_address prints the byte of memory located at this address if it exists.

This is my code thus far:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

/*
  - Takes a single arg: required_address
  - if the address is in virtual memory:
  -   print to stdout the value of the single byte of memory located at address
  -   return with exit code 1
  - else:
  -   print nothing
  -   return with exit code 0

  - 00405000-00426000 [heap]

 */

int main(int argc, char const *argv[]) {
    unsigned long ret_adr = strtoul(argv[1], NULL, 10);
    int pid = getpid();
    FILE *file;
    char c[1000];

    char file_addr[20];
    sprintf(file_addr, "/proc/%d/maps", pid);
    puts(file_addr);

    if ((file = fopen(file_addr,"r")) == NULL){
       printf("Error! opening file\n");

       // Program exits if the file pointer returns NULL.
       exit(1);
    }

    while (fgets(c, sizeof(c), file) != NULL) {
        printf("%s\n", c);
        sleep(1);
    }

    printf("pid: %d\n", pid);

    unsigned long* p = (unsigned long *)ret_adr;
    unsigned long first_byte = p[0];
    printf("%p\n", p);
    return 0;
}

This is my terminal:

[awhite4@fastx07 task1]$ gcc -ggdb -O0 -o task1 task1.c
[awhite4@fastx07 task1]$ setarch x86_64 -R ./task1 400000
/proc/32687/maps
00400000-00401000 r--p 00000000 00:37 3014860647                         /mnt/nfs/clasnetappvm/homedirs/awhite4/Downloads/task1/task1

00401000-00402000 r-xp 00001000 00:37 3014860647                         /mnt/nfs/clasnetappvm/homedirs/awhite4/Downloads/task1/task1

00402000-00403000 r--p 00002000 00:37 3014860647                         /mnt/nfs/clasnetappvm/homedirs/awhite4/Downloads/task1/task1

00403000-00404000 r--p 00002000 00:37 3014860647                         /mnt/nfs/clasnetappvm/homedirs/awhite4/Downloads/task1/task1

00404000-00405000 rw-p 00003000 00:37 3014860647                         /mnt/nfs/clasnetappvm/homedirs/awhite4/Downloads/task1/task1

00405000-00426000 rw-p 00000000 00:00 0                                  [heap]

7ffff7db7000-7ffff7dd9000 r--p 00000000 fd:00 5597327                    /usr/lib64/libc-2.29.so

7ffff7dd9000-7ffff7f26000 r-xp 00022000 fd:00 5597327                    /usr/lib64/libc-2.29.so

7ffff7f26000-7ffff7f72000 r--p 0016f000 fd:00 5597327                    /usr/lib64/libc-2.29.so

7ffff7f72000-7ffff7f73000 ---p 001bb000 fd:00 5597327                    /usr/lib64/libc-2.29.so

7ffff7f73000-7ffff7f77000 r--p 001bb000 fd:00 5597327                    /usr/lib64/libc-2.29.so

7ffff7f77000-7ffff7f79000 rw-p 001bf000 fd:00 5597327                    /usr/lib64/libc-2.29.so

7ffff7f79000-7ffff7f7f000 rw-p 00000000 00:00 0 

7ffff7fce000-7ffff7fd1000 r--p 00000000 00:00 0                          [vvar]

7ffff7fd1000-7ffff7fd2000 r-xp 00000000 00:00 0                          [vdso]

7ffff7fd2000-7ffff7fd3000 r--p 00000000 fd:00 5515071                    /usr/lib64/ld-2.29.so

7ffff7fd3000-7ffff7ff3000 r-xp 00001000 fd:00 5515071                    /usr/lib64/ld-2.29.so

7ffff7ff3000-7ffff7ffb000 r--p 00021000 fd:00 5515071                    /usr/lib64/ld-2.29.so

7ffff7ffc000-7ffff7ffd000 r--p 00029000 fd:00 5515071                    /usr/lib64/ld-2.29.so

7ffff7ffd000-7ffff7ffe000 rw-p 0002a000 fd:00 5515071                    /usr/lib64/ld-2.29.so

7ffff7ffe000-7ffff7fff000 rw-p 00000000 00:00 0 

7ffffffdd000-7ffffffff000 rw-p 00000000 00:00 0                          [stack]

ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]

pid: 32687
Segmentation fault (core dumped)

How do I use the 400000 address to access the value that is stored at this address? I tried to use a pointer and got an address but then when I try to get the value I get a Segmentation fault error.

How do I directly access the memory since I know it is valid (it is in the heap shown in /proc//maps)?

Am I just not understanding the correct way to access this memory?

Answer 1

The addresses shown in /proc/.../maps are shown in hexadecimal , ie base 16.

You are reading the 400000 address in base 10 ( 0x61a80 ), which means you are trying to access unmapped memory below 0x400000 which is the start of the first map.

Check your strtoul() parameters!