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POProgram strangely returning to the top of the loop
 

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  1. POProgram strangely returning to the top of the loop
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    <p>I would like to preface this question by saying I'm new to C and therefore terrible at it, so I apologize in advance for any blatant errors or bad style. Also, I'm not sure how to introduce the problem before showing you my code, so here it is:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;string.h&gt; int main() { int MAX_INPUT_SIZE = 200; volatile int running = 1; while (running) { char input[MAX_INPUT_SIZE]; char *tokens[100]; const char *cmds[] = { "wait", "pwd", "cd", "exit" }; char *cmdargs[100]; printf("shell&gt; "); fgets(input, MAX_INPUT_SIZE, stdin); // remove newline character at end int nl = strlen(input) - 1; if (input[nl] == '\n') { input[nl] = '\0'; } // tokenize input string, put each token into an array char *space; space = strtok(input, " "); tokens[0] = space; int i = 1; while (space != NULL) { space = strtok(NULL, " "); tokens[i] = space; ++i; } // copy tokens after first one into string int noargscheck; if (tokens[1] != NULL) { noargscheck = 0; strcpy((char *)cmdargs, tokens[1]); for (i = 2; tokens[i] != NULL; i++) { strcat((char *)cmdargs, " "); strcat((char *)cmdargs, tokens[i]); } } else { noargscheck = 1; } // compare tokens[0] to list of internal commands int isInternal = -1; for (i = 0; i &lt; 4; i++) { if (strcmp(tokens[0], cmds[i]) == 0) { isInternal = i; } } // internal commands char wd[200]; if (isInternal != -1) { switch (isInternal) { case 0: // wait break; case 1: // pwd if (getcwd(wd, sizeof(wd)) == NULL) { perror("getcwd() error!"); } else { printf("%s\n", wd); } break; case 2: // cd if (noargscheck) { chdir("/home"); } else if (chdir((const char *)cmdargs) != 0) { perror("cd failed"); } break; case 3: // exit exit(1); break; } } else { // external commands pid_t child_pid; switch (child_pid = fork()) { case -1: perror("Fork failed"); return 1; case 0: // child printf("\nHERE\n"); // for debugging execvp(tokens[0], cmdargs); break; } } } } </code></pre> <p>When I run this code with the input <code>echo hello world</code>, the program successfully enters the <code>case 0</code> case in the second switch statement that starts with <code>switch (child_pid=fork())</code> but the output, which is unexpected, is as follows:</p> <p>OUTPUT: (including one line that shows my input at the prompt)</p> <p><code>shell&gt; echo hello world</code> (my input)</p> <p><code>shell&gt;</code> (this is the part I don't understand)</p> <p><code>HERE</code></p> <p><code>shell&gt;</code> (the program now waits here at the prompt for the next user input)</p> <p>I cannot figure out why the extra <code>shell&gt;</code> prompt is printing. Can anybody see the problem?</p> <p>EDIT: fixed the first parameter of execvp. Changed from <code>"echo"</code> (which was there because I'm silly) to <code>tokens[0]</code>.</p>
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    1. USflexcalibur6
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    1. COCan you explain why you *don't* expect to see it? In other words, describe how you think your code should not be resulting in this behavior.
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      1. USPaul Griffiths
    2. COI don't want the extra `shell> ` printed, meaning the one in between the first `shell> ` and `HERE`. Dan has pointed out below that the problem is that my child process is reporting an error from execvp, which I am not checking, and then returning to the top of the loop.
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      1. USflexcalibur6
 

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