/**
 * gcc -Wall -g -o dt_needed dt_needed.c
 */

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <elf.h>
#include <errno.h>
#include <unistd.h>

int main( int argc, char **argv )
{
    Elf32_Ehdr hdr;
    char *strdata = 0;
    int fd;
    int i;

    if( argc < 2 ) {
        fprintf( stderr, "Usage %s <binary>\n", argv[ 0 ] );
        return 1;
    }

    fd = open( argv[ 1 ], O_RDONLY );
    if( fd < 0 ) {
        fprintf( stderr, "%s: Cannot open %s: %s\n",
                 argv[ 0 ], argv[ 1 ], strerror( errno ) );
        return 1;
    }

    fprintf( stdout, "Libraries required by %s:\n", argv[ 1 ] );

    if( read( fd, &hdr, sizeof( Elf32_Ehdr ) ) < 0 ) {
        fprintf( stderr, "%s: Cannot read elf header: %s\n",
                 argv[ 0 ], strerror( errno ) );
        return 1;
    }

    /* Jump to the section header table. */
    lseek( fd, hdr.e_shoff, SEEK_SET );

    /* Find and load the dynamic symbol strings. */
    for( i = 0; i < hdr.e_shnum; i++ ) {
        Elf32_Shdr shdr;

        lseek( fd, hdr.e_shoff + (i * hdr.e_shentsize), SEEK_SET );
        read( fd, &shdr, sizeof( Elf32_Shdr ) );

        if( shdr.sh_type == SHT_DYNSYM ) {
            Elf32_Shdr symstr;

            lseek( fd, hdr.e_shoff + (shdr.sh_link * hdr.e_shentsize),
                   SEEK_SET );
            read( fd, &symstr, sizeof( Elf32_Shdr ) );
            lseek( fd, symstr.sh_offset, SEEK_SET );
            strdata = malloc( symstr.sh_size );
            read( fd, strdata, symstr.sh_size );
            break;
        }
    }

    /* Quit if we couldn't find any. */
    if( strdata == 0 ) {
        fprintf( stderr, "%s: No symbol strings found.\n", argv[ 0 ] );
        return 1;
    }

    /* Now find the .dynamic section. */
    for( i = 0; i < hdr.e_shnum; i++ ) {
        Elf32_Shdr shdr;

        lseek( fd, hdr.e_shoff + (i * hdr.e_shentsize), SEEK_SET );
        read( fd, &shdr, sizeof( Elf32_Shdr ) );

        if( shdr.sh_type == SHT_DYNAMIC ) {
            int j;

            /* Load its data and print all DT_NEEDED strings. */
            lseek( fd, shdr.sh_offset, SEEK_SET );
            for( j = 0; j < shdr.sh_size / sizeof( Elf32_Dyn ); j++ ) {
                Elf32_Dyn cur;
                read( fd, &cur, sizeof( Elf32_Dyn ) );
                if( cur.d_tag == DT_NEEDED ) {
                    fprintf( stdout, "\t%s\n",
                             strdata + cur.d_un.d_val );
                }
            }
        }
    }

    return 0;
}