C program to open binary elf files, read from them, and print them out (like objcopy)

Question

I am trying to implement a functionality similar to objcopy where bytes of a binary file (specifically the .text section) will be printed out using open() and read(). How would I set the buffer sizes and iterate till the end of a .text section so that I don't read more bytes than I have to in order to avoid errors?

Answer 1

Here is how you read a file using open() and read().

P.S I used fopen() and fread() instead of open() and read() because I am currently working with a Windows machine. However, the results will be the same for either.

int main()
{
    FILE *file = fopen("input.txt", "r");
    char buffer[2048];

    if (file)
    {
        /* Loop will continue until an end of file is reached i.e. fread returns 0 elements read */
        while (fread(buffer, 4, 1, file) == 1)
        {
            printf("%s", buffer);
        }
        fclose(file);
    }
}

Update: For interpreting ELF files specifically, I would recommend taking a look at the following resources:

Check out the following code snippet. It shows how you can interpret an ELF file.

#include <stdio.h>
#include <libelf.h>
#include <stdlib.h>
#include <string.h>
static void failure(void);
void main(int argc, char **argv)
{
    Elf32_Shdr *shdr;
    Elf32_Ehdr *ehdr;
    Elf *elf;
    Elf_Scn *scn;
    Elf_Data *data;
    int fd;
    unsigned int cnt;

    /* Open the input file */
    if ((fd = open(argv[1], O_RDONLY)) == -1)
        exit(1);

    /* Obtain the ELF descriptor */
    (void)elf_version(EV_CURRENT);
    if ((elf = elf_begin(fd, ELF_C_READ, NULL)) == NULL)
        failure();

    /* Obtain the .shstrtab data buffer */
    if (((ehdr = elf32_getehdr(elf)) == NULL) ||
        ((scn = elf_getscn(elf, ehdr->e_shstrndx)) == NULL) ||
        ((data = elf_getdata(scn, NULL)) == NULL))
        failure();

    /* Traverse input filename, printing each section */
    for (cnt = 1, scn = NULL; scn = elf_nextscn(elf, scn); cnt++)
    {
        if ((shdr = elf32_getshdr(scn)) == NULL)
            failure();
        (void)printf("[%d]    %s\n", cnt,
                     (char *)data->d_buf + shdr->sh_name);
    }
} /* end main */

static void
failure()
{
    (void)fprintf(stderr, "%s\n", elf_errmsg(elf_errno()));
    exit(1);
}

I would also recommend checking out the elfutils library, which can be found here.