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copied!<p><em>Forgive me for diverting from the matter at hand in my previous answer (by suggesting the use of threads). Since I'm going in a completely new direction here, I feel compelled to add this as a separate answer.</em></p> <p><strong>Short version:</strong></p> <p>Please make the following changes in your program:</p> <pre><code>1. length = argc; // in place of length = sizeof(argv); 2. execl(argv[i],argv[i],0); // in place of execvp(argv[i],0); 3. #include <unistd.h> // if you haven't already </code></pre> <p><strong>Long version:</strong></p> <p><strong>(1)</strong> By the variable <code>length</code>, I presume you want to get the total number of arguments. <code>argv</code> is a <code>pointer-to-char-pointer</code>, and as such is simply a memory address. If you print out the length in your program, you will notice it is always 4 (or whatever is the size of a memory address in your system).</p> <p>So this:</p> <pre><code>length = sizeof(argv); </code></pre> <p>Should really be this:</p> <pre><code>length = argc; </code></pre> <p><code>argc</code> holds the total number of arguments passed when executing the process. For example,</p> <pre><code>./a.out /bin/ps /bin/ls </code></pre> <p>gives: argc = <strong>3</strong> (and not <strong>2</strong>, a very common pitfall)</p> <p><strong>(2)</strong> Another issue with your program, is the <code>execvp</code> call.</p> <p>The prototpye for the execvp is as follows:</p> <pre><code>int execvp(const char *file, char *const argv[]); </code></pre> <p>where, argv is the list of arguments passed to the new command, very similar to the argv in your own program.</p> <p>What you use in your program is:</p> <pre><code>execvp(argv[i],0); </code></pre> <p>Suppose <code>i=1</code> and <code>argv[1] = "/bin/ls"</code>. What this command does is look for the <code>/bin/ls</code> executable & pass a NULL pointer (<code>0</code>) to it. This may lead to the following runtime error:</p> <pre><code>A NULL argv[0] was passed through an exec system call. </code></pre> <p>Referring to the exec man page,</p> <blockquote> <p>The first argument, by convention, should point to the filename associated with the file being executed.</p> </blockquote> <p>Though it is not mandatory to pass the filename again, you certainly shouldn't pass a NULL pointer. Since you don't want to pass any arguments, I suggest you use the following <code>execl</code> call, instead:</p> <pre><code>execl(argv[i],argv[i],0); </code></pre> <p>Remember that all such calls are finally converted to <code>execve()</code> finally & then executed, making them equivalent eventually. </p> <p>I encourage you to read more about the <a href="http://linux.die.net/man/3/exec" rel="nofollow">exec</a> family of functions using <code>man</code>.</p>

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