Trying to read from a file using file descriptor prints numbers and slashes to console

Question

I am trying to write a simple program that reads a file by encapsulating functions like open, lseek, pread.

My file for test contains:

first second third forth fifth sixth
seventh eighth

my main function that tries to read 20 bytes with offset 10 from the file:

#include <iostream>
#include "CacheFS.h"
using namespace std;
int main(int argc, const char * argv[]) {
    char * filename1 = "/Users/Desktop/File";
    int fd1 = CacheFS_open(filename1);
    //read from file and print it
    void* buf[20];
    CacheFS_pread(fd1, &buf, 20, 10);
    cout << (char*)buf << endl;
}

implementation of the functions that the main is using:

int CacheFS_open(const char *pathname)
{
    mode_t modes = O_SYNC | 0 | O_RDONLY;
    int fd = open(pathname, modes);
    return fd;
}

int CacheFS_pread(int file_id, void *buf, size_t count, off_t offset)
{
    off_t seek = lseek(file_id, offset, SEEK_SET);
    off_t fileLength = lseek(file_id, 0, SEEK_END);
    if (count + seek <= fileLength) //this case we are not getting to the file end when readin this chunk
    {
        pread(file_id, &buf, count, seek);
    } else { //count is too big so we can only read a part of the chunk
        off_t size = fileLength - seek;
        pread(file_id, &buf, size, seek);
    }
    return 0;
}

My main function prints this to the console:

\350\366\277_\377

I would expect it to print some values from the file itself, and not some numbers and slashes that represenet something I do not really understand. Why does this happen?

Answer 1

The following changes will make your program work:

1. Your buffer has to be an existent char array and your CacheFS_pread function is called without the address operator & then. Also use the buffer size minus 1 because the pread function will override the terminating \0 because it's just read n bytes of the file. I use a zero initialized char array here so that there will be a null terminating \0 at least at the end.

   char buf[20] = { '\0' }; // declare and initialize with zeros
   CacheFS_pread(fd1, buf, sizeof(buf) - 1, 10);
   

2. Your function header should accept only a char pointer for typesafety reasons.

   int CacheFS_pread(int file_id, char* buf, size_t count, off_t offset)
   

3. Your pread call is then without the address operator &:

   pread(file_id, buf, count, seek);
   

Output: nd third forth fift because buffer is just 20!

Also I would check if your calculations and your if statements are right. I have the feeling that it's not exactly right. I would also recomment to use the return value of pread.