CODEWITHSUNDEEP
c++
Marco Vinicio
Marco Vinicio

Reputation: 73

Problems interpreting ar file within code

I've been trying to interpret an ar(the libglib-2.0.a) file using this struct here declared in ar.h. Acording to the wiki the ending characters shoud be 0x60 and 0x0A, but what I got is 0x35 and 0x34, in fact the ending characters are actually 8 bytes ahead in the stream! Here's the code:

#include <iostream>
#include <unistd.h>
#include <fcntl.h>
#include <elf.h>
#include <ar.h>


int main(){
    int fd = open("libglib-2.0.a", O_RDONLY);

    char b[1000];

    read(fd, b, 1000);

    ar_hdr *arS = (ar_hdr*) b;

    int dummy = 0;
}

Am I missing something?

Upvotes: 0

Views: 77

Answers (1)

vtronko
vtronko

Reputation: 488

First of all, you miss the 8 bytes offset at the top.

#define ARMAG   "!<arch>\n" /* String that begins an archive file.  */
#define SARMAG  8       /* Size of that string.  */

Then, you create a buffer of a bizarre size — 1000. That value makes absolutely no sense, we have a correct buffer size for it, which is the size of header itself — we know it statically, it's 60 bytes. Not to mention that to interpret the buffer as a correct struct, memory representation should be properly aligned.

Here's a working example, for the sake of brevity, error-checking is omitted.

#include <stdio.h>
#include "unistd.h"
#include <fcntl.h>
#include <string.h>

#include "ar.h"

int main() {
    int fd = open("/usr/lib/libc.a", O_RDONLY);

    lseek(fd, SARMAG, SEEK_SET);

    ssize_t bufSize = sizeof(struct ar_hdr);
    char buf[bufSize];
    read(fd, buf, bufSize);

    struct ar_hdr header;
    memcpy(&header, buf, bufSize);

    printf("\%02hhx, \%02hhx\n", header.ar_fmag[0], header.ar_fmag[1]);

    return 0;
}

$ ./read
60, 0a

Upvotes: 3

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