CODEWITHSUNDEEP
coperating-systemstat
user2203774
user2203774

Reputation: 619

How to print the name of the files inside an archive file?

I'm new to C and system programming. I want to open an archive file and print out the name of the files inside the archive file (e.g., my archive file is weds.a; inside weds.a, I have thurs.txt and fri.txt". I want to create an output that shows thurs.txt fri.txt

EDITED: It should work like the ar -t command.

Can someone give me some tips on how to do it? I've been reading the man page and looking for examples online, but I'm getting no where. I believe I'm missing something. The code I have below only prints the link count. Can someone help? Thanks in advance for your help!!

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

int main (int argc, char **argv)
{
    int in_fd;
    struct stat sb;

    if (argc != 2) {
        printf("Error", argv[0]);
        exit(EXIT_FAILURE);
    }

    if (stat(argv[1], &sb) == -1) {
        perror("stat");
        exit(EXIT_FAILURE);     //change from EXIT_SUCCESS to EXIT_FAILURE
    }

    //open the archive file (e.g., hw.a)
    in_fd = open(argv[1], O_RDONLY);
    if (in_fd == -1)
    {
        perror("Can't open input file\n");
        exit(-1);
    }


    printf("Link Count: %ld\n", (long)sb.st_nlink);

    return 0;
}

Upvotes: 0

Views: 1187

Answers (1)

Jonathan Leffler
Jonathan Leffler

Reputation: 754760

The easiest way is to use the ar program to list the names:

ar -tv weds.a

The - is optional; the v means you'll get size and time information.

If you want to get into reading the archive file itself, you'll have to be aware of the differences in the formats on different systems. The relevant header is (normally) <ar.h>. I have information for a number of systems, many of them obsolete, and there are a variety of different tricks used for handling long file names (and other even more basic file format issues) but you may have a more limited scope in mind. Any such work based on <ar.h> will be non-trivial; you're best off reusing what already exists (the ar program) if at all possible.

This is an archive from a Mac OS X 10.8.4 machine.

$ cat thurs.txt
0123456789:;<=>?@ABCDEFGHIJKLMNO
$ cat fri.txt
PQRSTUVWXYZ[\]^_`abcdefghijklmno
$ odx weds.a
0x0000: 21 3C 61 72 63 68 3E 0A 74 68 75 72 73 2E 74 78   !<arch>.thurs.tx
0x0010: 74 20 20 20 20 20 20 20 31 33 37 34 30 39 36 30   t       13740960
0x0020: 31 32 20 20 32 38 37 36 20 20 35 30 30 30 20 20   12  2876  5000  
0x0030: 31 30 30 36 34 34 20 20 33 33 20 20 20 20 20 20   100644  33      
0x0040: 20 20 60 0A 30 31 32 33 34 35 36 37 38 39 3A 3B     `.0123456789:;
0x0050: 3C 3D 3E 3F 40 41 42 43 44 45 46 47 48 49 4A 4B   <=>?@ABCDEFGHIJK
0x0060: 4C 4D 4E 4F 0A 0A 66 72 69 2E 74 78 74 20 20 20   LMNO..fri.txt   
0x0070: 20 20 20 20 20 20 31 33 37 34 30 39 36 30 30 35         1374096005
0x0080: 20 20 32 38 37 36 20 20 35 30 30 30 20 20 31 30     2876  5000  10
0x0090: 30 36 34 34 20 20 33 33 20 20 20 20 20 20 20 20   0644  33        
0x00A0: 60 0A 50 51 52 53 54 55 56 57 58 59 5A 5B 5C 5D   `.PQRSTUVWXYZ[\]
0x00B0: 5E 5F 60 61 62 63 64 65 66 67 68 69 6A 6B 6C 6D   ^_`abcdefghijklm
0x00C0: 6E 6F 0A 0A                                       no..
0x00C4:
$

Fortunately for you, the same files produce essentially the same archive on Linux too. In the Linux header <ar.h> you find:

/* Archive files start with the ARMAG identifying string.  Then follows a
   `struct ar_hdr', and as many bytes of member file data as its `ar_size'
   member indicates, for each member file.  */

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

#define ARFMAG  "`\n"       /* String in ar_fmag at end of each header.  */

struct ar_hdr
  {
    char ar_name[16];       /* Member file name, sometimes / terminated. */
    char ar_date[12];       /* File date, decimal seconds since Epoch.  */
    char ar_uid[6], ar_gid[6];  /* User and group IDs, in ASCII decimal.  */
    char ar_mode[8];        /* File mode, in ASCII octal.  */
    char ar_size[10];       /* File size, in ASCII decimal.  */
    char ar_fmag[2];        /* Always contains ARFMAG.  */
  };

The Mac OS X header has the same structure and ARMAG and ARFMAG values, but one extra macro:

#define AR_EFMT1        "#1/"           /* extended format #1 */

You can see the ARMAG string at the start of the file. Each file is then preceded by a struct ar_hdr. Note that the example names here are blank terminated, not slash terminated. After that, you find the data for the file. You can read the header in its entirety. Note that if any of the names reaches above 15 characters, or if a name contains spaces, then you get an extra entry at the start of the archive file that contains the file name strings, and you also get a modified name entry in the per-file header that identifies the relevant string in the string table.

Linux archive with long names etc

0x0000: 21 3C 61 72 63 68 3E 0A 2F 2F 20 20 20 20 20 20   !<arch>.//      
0x0010: 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20                   
* (1)
0x0030: 20 20 20 20 20 20 20 20 34 36 20 20 20 20 20 20           46      
0x0040: 20 20 60 0A 66 69 6C 74 65 72 2E 73 74 64 65 72     `.filter.stder
0x0050: 72 2E 73 68 2F 0A 6C 6F 6E 67 20 6E 61 6D 65 20   r.sh/.long name 
0x0060: 77 69 74 68 20 73 70 61 63 65 73 2E 74 78 74 2F   with spaces.txt/
0x0070: 0A 0A 74 68 75 72 73 2E 74 78 74 2F 20 20 20 20   ..thurs.txt/    
0x0080: 20 20 31 33 37 34 30 39 36 32 31 31 20 20 31 39     1374096211  19
0x0090: 39 34 38 34 35 30 30 30 20 20 31 30 30 36 34 30   94845000  100640
0x00A0: 20 20 33 33 20 20 20 20 20 20 20 20 60 0A 30 31     33        `.01
0x00B0: 32 33 34 35 36 37 38 39 3A 3B 3C 3D 3E 3F 40 41   23456789:;<=>?@A
0x00C0: 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F 0A 0A   BCDEFGHIJKLMNO..
0x00D0: 66 72 69 2E 74 78 74 2F 20 20 20 20 20 20 20 20   fri.txt/        
0x00E0: 31 33 37 34 30 39 36 31 39 37 20 20 31 39 39 34   1374096197  1994
0x00F0: 38 34 35 30 30 30 20 20 31 30 30 36 34 30 20 20   845000  100640  
0x0100: 33 33 20 20 20 20 20 20 20 20 60 0A 50 51 52 53   33        `.PQRS
0x0110: 54 55 56 57 58 59 5A 5B 5C 5D 5E 5F 60 61 62 63   TUVWXYZ[\]^_`abc
0x0120: 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F 0A 0A 2F 30   defghijklmno../0
0x0130: 20 20 20 20 20 20 20 20 20 20 20 20 20 20 31 33                 13
0x0140: 37 31 31 34 35 35 38 34 20 20 31 39 39 34 38 34   71145584  199484
0x0150: 35 30 30 30 20 20 31 30 30 36 34 30 20 20 32 33   5000  100640  23
0x0160: 30 20 20 20 20 20 20 20 60 0A 23 21 2F 62 69 6E   0       `.#!/bin
0x0170: 2F 62 61 73 68 0A 73 65 74 20 2D 78 0A 72 6D 20   /bash.set -x.rm 
0x0180: 2D 66 20 6F 75 74 2E 5B 31 32 33 5D 0A 2E 2F 67   -f out.[123]../g
0x0190: 65 6E 6F 75 74 65 72 72 2E 73 68 20 31 3E 2F 64   enouterr.sh 1>/d
0x01A0: 65 76 2F 6E 75 6C 6C 0A 2E 2F 67 65 6E 6F 75 74   ev/null../genout
0x01B0: 65 72 72 2E 73 68 20 32 3E 2F 64 65 76 2F 6E 75   err.sh 2>/dev/nu
0x01C0: 6C 6C 0A 28 20 2E 2F 67 65 6E 6F 75 74 65 72 72   ll.( ./genouterr
0x01D0: 2E 73 68 20 32 3E 26 31 20 31 3E 26 33 20 7C 20   .sh 2>&1 1>&3 | 
0x01E0: 67 72 65 70 20 27 5B 30 2D 39 5D 30 27 20 3E 26   grep '[0-9]0' >&
0x01F0: 32 29 20 33 3E 6F 75 74 2E 33 20 32 3E 6F 75 74   2) 3>out.3 2>out
0x0200: 2E 32 20 31 3E 6F 75 74 2E 31 0A 6C 73 20 2D 6C   .2 1>out.1.ls -l
0x0210: 20 6F 75 74 2E 5B 31 32 33 5D 0A 28 20 2E 2F 67    out.[123].( ./g
0x0220: 65 6E 6F 75 74 65 72 72 2E 73 68 20 32 3E 26 31   enouterr.sh 2>&1
0x0230: 20 31 3E 26 33 20 7C 20 67 72 65 70 20 27 5B 30    1>&3 | grep '[0
0x0240: 2D 39 5D 30 27 20 3E 26 32 29 20 33 3E 26 31 0A   -9]0' >&2) 3>&1.
0x0250: 2F 31 38 20 20 20 20 20 20 20 20 20 20 20 20 20   /18             
0x0260: 31 33 37 34 30 39 36 35 37 37 20 20 31 39 39 34   1374096577  1994
0x0270: 38 34 35 30 30 30 20 20 31 30 30 36 34 30 20 20   845000  100640  
0x0280: 33 33 20 20 20 20 20 20 20 20 60 0A 30 31 32 33   33        `.0123
0x0290: 34 35 36 37 38 39 3A 3B 3C 3D 3E 3F 40 41 42 43   456789:;<=>?@ABC
0x02A0: 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F 0A 0A         DEFGHIJKLMNO..
0x02AE:

Mac OS X archive with long names etc

0x0000: 21 3C 61 72 63 68 3E 0A 74 68 75 72 73 2E 74 78   !<arch>.thurs.tx
0x0010: 74 20 20 20 20 20 20 20 31 33 37 34 30 39 36 30   t       13740960
0x0020: 31 32 20 20 32 38 37 36 20 20 35 30 30 30 20 20   12  2876  5000  
0x0030: 31 30 30 36 34 34 20 20 33 33 20 20 20 20 20 20   100644  33      
0x0040: 20 20 60 0A 30 31 32 33 34 35 36 37 38 39 3A 3B     `.0123456789:;
0x0050: 3C 3D 3E 3F 40 41 42 43 44 45 46 47 48 49 4A 4B   <=>?@ABCDEFGHIJK
0x0060: 4C 4D 4E 4F 0A 0A 66 72 69 2E 74 78 74 20 20 20   LMNO..fri.txt   
0x0070: 20 20 20 20 20 20 31 33 37 34 30 39 36 30 30 35         1374096005
0x0080: 20 20 32 38 37 36 20 20 35 30 30 30 20 20 31 30     2876  5000  10
0x0090: 30 36 34 34 20 20 33 33 20 20 20 20 20 20 20 20   0644  33        
0x00A0: 60 0A 50 51 52 53 54 55 56 57 58 59 5A 5B 5C 5D   `.PQRSTUVWXYZ[\]
0x00B0: 5E 5F 60 61 62 63 64 65 66 67 68 69 6A 6B 6C 6D   ^_`abcdefghijklm
0x00C0: 6E 6F 0A 0A 66 69 6C 74 65 72 2E 73 74 64 65 72   no..filter.stder
0x00D0: 72 2E 73 68 31 33 37 34 30 39 37 37 39 34 20 20   r.sh1374097794  
0x00E0: 32 38 37 36 20 20 35 30 30 30 20 20 31 30 30 36   2876  5000  1006
0x00F0: 34 34 20 20 32 33 30 20 20 20 20 20 20 20 60 0A   44  230       `.
0x0100: 23 21 2F 62 69 6E 2F 62 61 73 68 0A 73 65 74 20   #!/bin/bash.set 
0x0110: 2D 78 0A 72 6D 20 2D 66 20 6F 75 74 2E 5B 31 32   -x.rm -f out.[12
0x0120: 33 5D 0A 2E 2F 67 65 6E 6F 75 74 65 72 72 2E 73   3]../genouterr.s
0x0130: 68 20 31 3E 2F 64 65 76 2F 6E 75 6C 6C 0A 2E 2F   h 1>/dev/null../
0x0140: 67 65 6E 6F 75 74 65 72 72 2E 73 68 20 32 3E 2F   genouterr.sh 2>/
0x0150: 64 65 76 2F 6E 75 6C 6C 0A 28 20 2E 2F 67 65 6E   dev/null.( ./gen
0x0160: 6F 75 74 65 72 72 2E 73 68 20 32 3E 26 31 20 31   outerr.sh 2>&1 1
0x0170: 3E 26 33 20 7C 20 67 72 65 70 20 27 5B 30 2D 39   >&3 | grep '[0-9
0x0180: 5D 30 27 20 3E 26 32 29 20 33 3E 6F 75 74 2E 33   ]0' >&2) 3>out.3
0x0190: 20 32 3E 6F 75 74 2E 32 20 31 3E 6F 75 74 2E 31    2>out.2 1>out.1
0x01A0: 0A 6C 73 20 2D 6C 20 6F 75 74 2E 5B 31 32 33 5D   .ls -l out.[123]
0x01B0: 0A 28 20 2E 2F 67 65 6E 6F 75 74 65 72 72 2E 73   .( ./genouterr.s
0x01C0: 68 20 32 3E 26 31 20 31 3E 26 33 20 7C 20 67 72   h 2>&1 1>&3 | gr
0x01D0: 65 70 20 27 5B 30 2D 39 5D 30 27 20 3E 26 32 29   ep '[0-9]0' >&2)
0x01E0: 20 33 3E 26 31 0A 23 31 2F 32 38 20 20 20 20 20    3>&1.#1/28     
0x01F0: 20 20 20 20 20 20 31 33 37 34 30 39 37 38 32 32         1374097822
0x0200: 20 20 32 38 37 36 20 20 35 30 30 30 20 20 31 30     2876  5000  10
0x0210: 30 36 34 34 20 20 36 31 20 20 20 20 20 20 20 20   0644  61        
0x0220: 60 0A 6C 6F 6E 67 20 6E 61 6D 65 20 77 69 74 68   `.long name with
0x0230: 20 73 70 61 63 65 73 2E 74 78 74 00 00 00 30 31    spaces.txt...01
0x0240: 32 33 34 35 36 37 38 39 3A 3B 3C 3D 3E 3F 40 41   23456789:;<=>?@A
0x0250: 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F 0A 0A   BCDEFGHIJKLMNO..
0x0260:

Differences

The Linux archive has a list of strings at the top of the file that have to be remembered. The Mac OS X archive has the special entry #1/28 which identifies the header as being followed by a 28-byte entry containing the file name (null padded to a multiple of 4 bytes; the length given includes the null padding). The Mac archive has no space after the name when it is exactly 16 characters long; the Linux archive puts the 16-character name into the string table.

Upvotes: 2

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