Question

I’m trying to implement auto completion and history in my shell using the GNU Readline library. I was using fgets() to retrieve user and after reading on how the readline function works, I decide on using it so as to support auto completion etc. But when I execute my program, the readline function outputs weird characters on the shell before I even type any input. Weird results like P�6, PJ� `,P�#,P�s`. For some reason it always start with P. Here’s my code:

int main(int argc, char* argv[]) {

char *historic, userInput[1000];
static char **cmdArgv;/**The argv of the main*/

sa.sa_handler = handle_signal;
sigaction(SIGINT, &sa, NULL);
sa.sa_flags = SA_RESTART; /** Restart function incase it's interrupted by handler */
cmdArgv = malloc(sizeof (*cmdArgv));
welcomeScreen();//just printf's nothing special
while(TRUE)
{
    shellPrompt();// getcwd function
    historic = readline(userInput);
    if (!historic)
        break;
     //path autocompletion when tabulation hit
    rl_bind_key('\t', rl_complete);
     //adding the previous input into history
    add_history(historic);    
    if( check_syntax(userInput) == 0 ){
        createVariable(userInput);
    }

    else{
        tokenize(cmdArgv, userInput);
        special_chars(cmdArgv);
        executeCommands(cmdArgv, userInput);
    }
}

Any ideas as to what the problem is? Thanks.

Answer 1

Initialise userInput before passing it to readLine():

memset(userInput, 0, sizeof(userInput));

This is the description for argument passed to the readLine() function (which I found here man readline):

If the argument is NULL or the empty string, no prompt is issued.

As you had not initialised userInput it was displaying whatever happened to be in there.