Thread: How to check if a file is an object file?

Originally Posted by rempas

What's the type of an object file? Is it an executable or something else?

It's very nearly an executable file. It's compiled machine code with labels attached. So you can't actually run it. However it's relatively simple to replace the labels with addresses (linking) and create an executable from several object files.

stat will not tell you if it's an executable or object file. If the binaries are in ELF format then you can check byte 16 to tell the type.

Executable and Linkable Format - Wikipedia

Also take note of the file and nm commands.

Code:

#include <stdio.h>
#include <stdlib.h>
 
int main(int argc, char **argv) {
    if (argc != 2) {
        printf("Usage: elftype FILENAME\n");
        exit(EXIT_FAILURE);
    }
 
    FILE *fin = fopen(argv[1], "rb");
    if (!fin) {
        printf("Cannot open %s\n", argv[1]);
        exit(EXIT_FAILURE);
    }
 
    unsigned char buf[20];
    fread(buf, 20, 1, fin);
 
    if (buf[0] != 0x7F || buf[1] != 0x45 || buf[2] != 0x4c || buf[3] != 0x46)
        printf("Not an ELF file\n");
    else if (buf[16] == 1)
        printf("ELF object file\n");
    else if (buf[16] == 2)
        printf("ELF executable file\n");
    else if (buf[16] == 3)
        printf("ELF shared library file\n");
    else if (buf[16] == 4)
        printf("ELF core dump file\n");
    else
        printf("Unknown ELF file\n");
 
    fclose(fin);
 
    return 0;
}