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POWhy does `execvp` take a `char *const argv[]`?
 

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  1. POWhy does `execvp` take a `char *const argv[]`?
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    <p>I'm wondering if there is a reason between two exec functions differing in <code>const</code>-ness, of if this is just a bug in the Single Unix Spec:</p> <p>Excerpting from the Linux manpage, which appears to align with the <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/fexecve.html">Single Unix Specification</a>, here are a two versions of <code>exec</code>:</p> <blockquote><code><b>int execlp(const char *</b>file<b>, const char *</b>arg<b>, ...);<br> int execvp(const char *</b>file<b>, char *const </b>argv<b>[]);</b></code></blockquote> <p><code>execlp</code> takes its arguments as <code>const char *</code>, and it takes two or more of them. <code>const</code> in C is a promise that the function will not change the pointed-to data, in this case the actual characters (<code>char</code>) that make up the string.</p> <p><code>execvp</code> instead takes its arguments as an array of pointers. However, instead of an array of pointers to <code>const char *</code> as you'd expect, the <code>const</code> keyword is in a different spot—and this matters quite a bit to C. <code>execvp</code> is saying it may well modify the characters in the strings, but it promises not to modify the array—that is, the pointers to the strings. So, in other words,</p> <pre class="lang-c prettyprint-override"><code>int fake_execvp(const char *file, char *const argv[]) { argv[0] = "some other string"; /* this is an error */ argv[0][0] = 'f'; /* change first letter to 'f': this is perfectly OK! */ /* ⋮ */ } </code></pre> <p>In particular, this makes it hard (technically, prohibited) to call execvp using C++'s <code>std::string</code>'s <code>to_cstr()</code> method, which returns <code>const char *</code>.</p> <p>It seems like <code>execvp</code> really ought to take <code>const char *const argv[]</code>, in other words, it ought to promise not to do either of the above changes.</p>
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    1. COWhy it is migrated if it already has an accepted answer??
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