Magic number in files Linux. C programming

Question

I want to get the Magic Number from a binary file (for example file ELF hava 7f 45 4c 46). I wrote a program to print out the magic number of the file but i get the error zsh: segmentation fault./magic. How should I fix this?

int main() 
{ 
    //setlocale(LC_ALL, "Russian"); 
    //FILE *fp; 
    //fopen (&fp, "/Documents/OCP/lab1test/lab1call", "rb"); 
    FILE *fp; 
    long fSize; 
    fp = fopen("/Documents/OCP/lab1test/lab1call", "rb"); 
    fseek (fp , 0 , SEEK_END); 
    fSize = ftell (fp);  
    rewind (fp);  
    char *magic_number; 
    magic_number=(char *)malloc(fSize+1); 
    //unsigned char magic_number[4]; 
    fread(magic_number, sizeof(char), 4, fp); 
    printf ("A magic number of your file is:\n");
    //magic_number[4] = '\0'; 
    //for (int i = 0; i < 4; i++) 
    printf ("%02hhx%02hhx%02hhx%02hhx\n ", magic_number[0],magic_number[1], magic_number[2], magic_number[3]);  
    printf("\n"); 
}   

Answer 1

A few things first:

1. I doubt that /Documents/OCP/lab1test/lab1call is a valid path on your system. It's usually /home/[USER]/Documents/... , you should double check that. Even better, pass it as an argument to your program so you can do fopen(argv[1], ...) after checking that argc == 2 .

2. You definitely do not need to seek all the way to the end of the file, and you definitely do not need to allocate a buffer the entire size of the file only to read four bytes at the beginning of the file. Simply fread 4 bytes. You can safely declare a 4-byte array on the stack of main .

3. You must do error checking. Almost every library function in C can fail and return an error. Check the manual page (usually simply type man 3 function_name in your terminal) and see which errors can happen for each function and which is a "good" return value.

With this said, what you're trying to do can be simplified down to:

int main(int argc, char **argv) {
    FILE *fp;
    unsigned char magic[4];

    if (argc != 2) {
        fprintf(stderr, "Usage: %s FILENAME\n", argv[0]);
        return 1;
    }

    fp = fopen(argv[1], "rb");
    if (fp == NULL) {
        perror("fopen failed");
        return 1;
    }

    if (fread(magic, 1, 4, fp) != 4) {
        if (feof(fp)) 
            fputs("File is less than 4 bytes\n", stderr);
        else
            fputs("Error reading the file\n", stderr);
        return 1;
    }

    fclose(fp);
    
    puts("The magic number of your file is:");
    printf("%02hhx%02hhx%02hhx%02hhx\n", magic[0], magic[1], magic[2], magic[3]);

    return 0;
}

You can compile and then run your program like this:

./magic path/to/the/file

Beware that if the path contains spaces you will have to enclose it in quotes otherwise it will be interpreted as two different arguments:

./magic 'path/with some/spaces'