Reading and printing an entire file in binary mode using C++

Question

a follow up to my previous question (Reading an entire file in binary mode using C++)

After reading a jpg file in binary mode, the result of the read operation is always 4 bytes. The code is:

FILE *fd = fopen("c:\\Temp\\img.jpg", "rb");
if(fd == NULL) {
    cerr << "Error opening file\n";
    return;
}
fseek(fd, 0, SEEK_END);
long fileSize = ftell(fd);
int *stream = (int *)malloc(fileSize);
fseek(fd, 0, SEEK_SET);
int bytes_read = fread(stream, fileSize, 1, fd);
printf("%x\n", *stream);
fclose(fd);

The second last printf statement is always printing the first 4 bytes and not the entire file contents. How can I print the entire content of the jpg file?

Thanks.

How do you want it formatted? You asked `printf` to display a single integer in hex, which is what it did. You could just do that in a `for` loop, but it'll be unreadable, so what's the output for? — Useless, Feb 09 '12 at 18:12
well, i just want to dump the entire content of the file in hex... — pokiman, Feb 09 '12 at 18:14
This is C, you should type `man fread` somewhere. You already got an answer for this question. — sinsedrix, Feb 09 '12 at 18:16
C is a subset of C++. As such, this is just as much C++ as anything more object-oriented and modern. However, it makes sense to use simpler and safer techniques if you have the opportunity. — Amish Programmer, Feb 09 '12 at 18:25
@JoshG: C is *no longer* a subset of C++. C99 is no longer compatible with C++. — Griwes, Feb 09 '12 at 18:27
@JoshG C and C++ have a common subset. C is not a subset of C++ (not even C89). — R. Martinho Fernandes, Feb 09 '12 at 18:27
@Griwes: C89 wasn't either. See that `(int*)` cast on the result of malloc? You don't need it in C. C++ code doesn't compile without it. (This code however, won't compile as C because it uses `cerr`) — R. Martinho Fernandes, Feb 09 '12 at 18:29
@ Griwes & R. Martinho Fernandes - Thanks, I learned something today. At the same time, this is still valid C++ — Amish Programmer, Feb 09 '12 at 18:31
@R.MartinhoFernandes, hmm, right. Forgot about how such casts are unnecessary in C... — Griwes, Feb 09 '12 at 19:28

score 3 · Answer 1 · edited Mar 19 '21 at 04:46

You want it in C++? This opens a file, reads the entire contents into an array and prints the output to the screen:

#include <fstream>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;

void hexdump(void *ptr, int buflen)
{
   unsigned char *buf = (unsigned char*)ptr;
   int i, j;
   for (i=0; i<buflen; i+=16) {
      printf("%06x: ", i);
      for (j=0; j<16; j++) { 
         if (i+j < buflen)
            printf("%02x ", buf[i+j]);
         else
            printf("   ");
      }
      printf(" ");
      for (j=0; j<16; j++) {
         if (i+j < buflen)
            printf("%c", isprint(buf[i+j]) ? buf[i+j] : '.');
      }
      printf("\n");
   }
}

int main()
{
   ifstream in;

   in.open("C:\\ISO\\ITCHOUT.txt", ios::in | ios::binary);

   if(in.is_open())
   {
      // get the starting position
      streampos start = in.tellg();

      // go to the end
      in.seekg(0, std::ios::end);

      // get the ending position
      streampos end = in.tellg();

      // go back to the start
      in.seekg(0, std::ios::beg);

      // create a vector to hold the data that
      // is resized to the total size of the file    
      std::vector<char> contents;
      contents.resize(static_cast<size_t>(end - start));

      // read it in
      in.read(&contents[0], contents.size());

      // print it out (for clarity)
      hexdump(contents.data(), contents.size());
   }
}

Since you were using C++11 anyway (lambdas), I changed it to use a range-based for loop. Hope you don't mind :) — R. Martinho Fernandes, Feb 09 '12 at 18:24
If you're just going to print out the contents of the file, `std::cout << in.rdbuf()` is a lot easier (and probably faster) than copying to memory and then printing. — André Caron, Feb 09 '12 at 18:32
@AndréCaron Sure, but the printing was just for brevity, because obviously the submitter is not looking to print the contents (of a JPEG) to the standard output. — Chad, Feb 09 '12 at 18:49

score 0 · Answer 2 · answered Feb 09 '12 at 18:26

stream is a pointer to an int (the first element of the array you allocated¹). *stream dereferences that pointer and gives you the first int.

A pointer is not an array. A pointer is not a buffer. Therefore, it carries no information about the size of the array it points to. There is no way you can print the entire array by providing only a pointer to the first element.

Whatever method you use to print that out, you'll need to provide the size information along with the pointer.

C++ happens to have a pointer + size package in its standard library: std::vector. I would recommend using that. Alternatively, you can just loop through the array yourself (which means using the size information) and print all its elements.

¹_{Make sure the size of the file is a multiple of sizeof(int)!}

score 0 · Answer 3 · answered Feb 09 '12 at 18:36

Something like the following should do it. bytes_read() gives you the number of blocks read, in your case the block size is the file size so only one block can be read.

You should use a for loop to print the whole file. You're only printing one pointer address.

            char *stream = (char *)malloc(fileSize);
            fseek(fd, 0, SEEK_SET);
            int bytes_read = fread(stream, fileSize, 1, fd);
            for(int i=0; i<fileSize; i++){
                    printf("%d ", stream[i]);
            }

I print the chars as numbers as binary data is not readable in the console. I don't know how you wanted the data to be formatted.

This is just meant as reference to your sample. You should really consider using Chad's sample. This is a far worse solution (as mixing C/C++ far too much) just for sake of completeness.

Reading and printing an entire file in binary mode using C++

3 Answers3