在C中打开二进制文件

Question

I'm trying to open a binary file and read the contents for a class assignment. Even after doing research, I'm having trouble getting anything to appear while attempting open and prints contents of a binary file. I'm not even sure what I should be getting, how to check that it's right but I know that nothing (which is what I'm currently getting) is bad. Here's the code I got from searching on this site

#include<stdio.h>

int main()
{
    FILE *ptr_myfile;
    char buf[8];

    ptr_myfile = fopen("packets.1","rb");
    if (!ptr_myfile)
    {
        printf("Unable to open file!");
        return 1;
    }

    fread(buf, 1, 8, ptr_myfile);

    printf("First Character: %c", buf[0]);

    fclose(ptr_myfile);
    return 0;
}

When this prints, I get "First Character: " with nothing else printed. Maybe it doesn't print normally in terminal? I'm not sure, any help would be greatly appreciated, thanks

Answer 1

First, you need to check how much data you have in the buffer. fread returns length; if it is zero, accessing buf[0] is not legal.

Not all characters are printable You can see what data you are getting if you print the character code of c , rather than c itself. Use %d for that.

size_t len = fread(buf, 1, 8, ptr_myfile);
if (len != 0) {
    printf("First Character: '%c', code %d", buf[0], buf[0]);
} else {
    printf("The file has no data\n");
}

Answer 2

If it's a binary file, it's very likely that its contents don't print particularly well as text (that's what makes a binary a binary file). Instead of printing as characters try printing as hexadecimal numbers:

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

int main()
{
    FILE *ptr_myfile;
    char buf[8];

    ptr_myfile = fopen("packets.1","rb");
    if (!ptr_myfile)
    {
        printf("Unable to open file!");
        return 1;
    }

    size_t rb;
    do {
        rb = fread(buf, 1, 8, ptr_myfile);
        if( rb ) {
            size_t i;
            for(i = 0; i < rb; ++i) {
                    printf("%02x", (unsigned int)buf[i]);
            }
            printf("\n");
        }
     } while( rb );

    fclose(ptr_myfile);
    return 0;
}