Using fwrite to store an ar_hdr struct to a file

Question

I'm working on a problem to create my own C version of unix ar. Right now I'm trying to create the header and write it to the file.

Initially, I used a stat struct and printed the contents using an fprintf. However, when I try to use the ar command on a file printed with such a method, it does not work. I was told that the better solution would be to use an fwrite to write the struct directly to the file. So I'm now trying to implement that.

I've tried to populate then fwrite my ar_hdr struct with the stat info, however when I write to the file, I get garbage.

Update 2: using just an fprintf based upon @fvu's recommendations. Last item on fprintf is the constant ARFMAG from ar.h which I believe is the same as printing the two characters.

void header(char *archive, char *read_file){
    struct stat sb;
    FILE *fp;

    if (stat(read_file, &sb) == -1)
        fail('s');

    fp = fopen(archive, "a");
    if (!fp)
        fail('f');

    fprintf(fp, "%-16s%-12ld%-6ld%-6ld%-8d%-10lld%s", read_file, (long)sb.st_mtimespec.tv_sec,
            (long)sb.st_uid, (long)sb.st_gid, sb.st_mode, (long long)sb.st_size, ARFMAG);

    fclose(fp);
}

The test output of my program is now something like this:

!<arch>
b.txt           1359332639  502   20    33188   28        `
Appending B. shortb long b

d.txt           1359332655  502   20    33188   28        `
Appending D. shortb long b

c.txt           1359332646  502   20    33188   17        `
COpy this.

When I try the unix command: ar -tv myfile.a

result is: Inappropriate file type or format

If I use nano to look at test.a this is the result

!<arch>
^@b.txt           1359332639  502   20    100644  28        `
Appending B. shortb long b

d.txt           1359332655  502   20    100644  28        `
Appending D. shortb long b

c.txt           1359332646  502   20    100644  17        `
COpy this shit.

There's a weird shift@ symbol before the first header. Here's my code to write the overall file header, any hints would be appreciated.

char title[] = ARMAG; //constant defined in ar.h


    //open or create the output file
    sf = open(argv[2], O_WRONLY | O_CREAT | O_APPEND, perms);
    if (sf == -1)
        fail('o');  //open fail

    title_num = write(sf, title, sizeof(title));

Adding results from od -x file | head -n 2:

0000000      3c21    7261    6863    0a3e    2e62    7874    2074    2020
0000020      2020    2020    2020    2020    3331    3935    3333    3632

Answer 1

Not the cause of your error (see below) but very odd nevertheless

struct ar_hdr d;
struct ar_hdr* bob = &d;
...
bob = malloc(sizeof(struct ar_hdr));
...
fwrite(bob, sizeof(d),1, fp);

is quite convoluted, I'd just drop everything into d and that's it.

Now, as your ar_hdr looks like this:

 struct  ar_hdr                  /* file member header */
 {
     char    ar_name[16];        /* '/' terminated file member name */
     char    ar_date[12];        /* file member date */
     char    ar_uid[6]           /* file member user identification */
     char    ar_gid[6]           /* file member group identification */
     char    ar_mode[8]          /* file member mode (octal) */
     char    ar_size[10];        /* file member size */
     char    ar_fmag[2];         /* header trailer string */
 };

The Wikipedia article mentions that a textual header is the most common format being used:

Offset  Length  Name                            Format
0           16  File name                       ASCII
16          12  File modification timestamp     Decimal
28           6  Owner ID                        Decimal
34           6  Group ID                        Decimal
40           8  File mode                       Octal
48          10  File size in bytes              Decimal
58           2  File magic                      0x60 0x0A

which means that fwrite'ing the struct directly to the file is not what you need to do: C strings as produced by individual sprintf's are null terminated, and what's behind the null (up to the length of the array) is garbage. The file format wants space characters instead of that garbage, and also no null's....

Actually I think your initial fprintf was quite close:

fprintf(fp, "%-16s%-12ld%-6ld%-6ld%-8o%-10lld%c%c", 
    fname, (long)sb.st_mtimespec.tv_sec,
    (long)sb.st_uid, (long)sb.st_gid, sb.st_mode, (long long)sb.st_size,0x60,0xa);

• pad the ar_name field to 16
• ar_mode to octal
• added the file magic

As I'm unable to quickly test that here, there may still be some minor issues but it should be pretty close.

Addendum: for this kind of work a tool to look at the file content byte per byte (like od that comes with all linux distros) and a tool to bytewise compare two files (like dhex) are really a must, because just testing what ar is willing to open is just too hard and time-consuming.