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| author | Aki <please@ignore.pl> | 2021-07-26 00:49:06 +0200 |
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| committer | Aki <please@ignore.pl> | 2021-07-26 00:50:07 +0200 |
| commit | 68d5e54b427e3ccd5f1a73dcf8a7936d8b660fb8 (patch) | |
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Published ar guide
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diff --git a/how_to_archive_with_posix_ar.html b/how_to_archive_with_posix_ar.html new file mode 100644 index 0000000..b4dce3b --- /dev/null +++ b/how_to_archive_with_posix_ar.html @@ -0,0 +1,106 @@ +<!doctype html> +<html lang="en"> +<meta charset="utf-8"> +<meta name="viewport" content="width=device-width, initial-scale=1"> +<meta name="author" content="aki"> +<meta name="tags" content="tutorial, posix, ar, archive"> +<link rel="icon" type="image/png" href="cylo.png"> +<link rel="stylesheet" href="style.css"> + +<title>How To Archive With POSIX ar</title> + +<nav><p><a href="https://ignore.pl">ignore.pl</a></p></nav> + +<article> +<h1>How To Archive With POSIX ar</h1> +<p class="subtitle">Published on 2021-07-26 00:47:00+02:00 +<p>Let's continue <a href="how_to_archive_with_posix_tar_cpio_and_pax.html">last POSIX archiving guide</a> from the +very spot we finished. Today, we'll deal with just one tool. Very peculiar one, because it's more of a development +utility than an archiver with portable format. In fact, it doesn't have a defined format at all. More than that, POSIX +acknowledges that several incompatible formats are known and that they don't care. + +<p>It's time to stop pretending that <a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/ar.html">ar</a> +does not exists and learn how to use it. + +<p>Now then, when exactly do we want to use <b>ar</b>? Luckily, that's simple to answer. <b>ar</b> is used to create +libraries of object files that are meant to be later linked against. In other words, it creates <i>static libraries</i>. +In order to do it, <b>ar</b> maintains a symbol table. As long as at least one of the archived files is an object file, +<b>ar</b> is obliged to do it completely automatically. However, usability of the archive for the linker is only +guaranteed if all of the files are objects. + +<p>To create an empty archive: + +<pre> +$ ar -r archive.a +</pre> + +<p>Now, that's not very useful. Luckily for us, option <code>-r</code> actually means "replace or add" and accepts any +number of paths after the archive name: + +<pre> +$ ar -r archive.a file_a.o file_b.o ../../file_c.o +</pre> + +<p>Assuming, that all of these files are present, they'll all be added to a new archive. If the <i>archive.a</i> already +happens to exist, then files that are already occurring in it will get replaced. If not, then files will get appended. + +<p>To list contents of the symbol table, so the contents of the archive, use: + +<pre> +$ ar -t archive.a +file_a.o +file_b.o +file_c.o +</pre> + +<p>Nice! Notice, that the <i>file_c.o</i> was added directly, without any additional path - just basename of the file. +This will happen to every single file added to the archive. While adding files with same names you need to be cautious +about it. Consider modifying our <i>archive.a</i> by: + +<pre> +$ echo "Aloha!" > ../file_c.o +$ ar -r archive.a ../file_c.o +</pre> + +<p>This will overwrite our current <i>file_c.o</i> with one that says "Aloha!". This could be problematic, but it's +generally easy to avoid and there are additional options that allow to do some shenanigans with the archive. Refer to +your friendly manual or standard itself.</p> + +<img src="how_to_archive_with_posix_ar-1.png" alt="palm tree"> + +<p>You probably won't need to extract files from the archive, simply because you put it together so that the linker uses +it and not yourself. Still, you can extract files from it in two ways. First one is <code>-x</code> option: + +<pre> +$ ar -x archive.a +</pre> + +<p>This simply extract all of the files from the archive directly into current directory. Keep in mind that the paths +are not remembered, only the basenames are. Using those basenames you can also extract selected files from the archive: + +<pre> +$ ar -x archive.a file_c.o +$ cat file_c.o +Aloha! +</pre> + +<p>The other way of extracting is closer to <b>cpio</b>. By using <code>-p</code> option content of selected files or +the entire archive will be printed out to standard output: + +<pre> +$ ar -p archive.a file_c.o +Aloha! +</pre> + +<p>Just note that if you specify multiple files to output, they will get concatenated. + +<p>And that's about it regarding basic <b>ar</b> usage. Quite a few sources will probably recommend that you always run +<code>-s</code> option in every single operation you perform, in order to regenerate symbol table. However, that's not +required at all, <b>ar</b> should be able to deal with it without any input from the user. The only case in which it is +actually viable is after the archive was stripped. Otherwise, this suggestion is almost always useless. + +<p>If you are interested in precise management, then there are couple of options that will help i.a., <code>-d</code> +for removal, <code>-u</code> for restricted updates, or <code>-p</code> for unchecked appending. But again, in normal +usage, you probably won't need them as <code>-r</code> will deal with almost anything a <i>Makefile</i> will need. +</article> +<script src="https://stats.ignore.pl/track.js"></script> |