execvp not giving output

Question

I have to program a little shell for school but I am stuck at even executing a command. execvp worked when I executed it in the wait for input function, but in the execute command function it doesnt e.g I don't get any output to stdout for commands like ls.

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <errno.h>

int executecommand(char commandarray[]){
    char * command = &commandarray[0];
    pid_t pid = fork();
    if (pid < 0){
        perror("failed");
        return errno;
    } else if (0 == pid) {
        execvp(command, &commandarray);
        return 0;
    } else {
        int waitstatus;
        waitpid(pid, &waitstatus, 0);
        return 0;
    }
}

int waitforinput(){
    printf("$ ");
    char cmd[256];
    fgets(cmd, 256, stdin);
    executecommand(cmd);
    return 0;
}

int main(int argc, char **argv) {
    waitforinput();
    return 0;
}

Answer 1

The argument to execvp() must be an array of strings ending with NULL, not a pointer to a single string. Normally you would parse the input line into an array containing the command followed by arguments, but if you don't care about arguments you can just create an array with 2 elements, the command followed by NULL.

You also need to remove the newline from the end of the input line returned by fgets(). See Removing trailing newline character from fgets() input for many ways to do this.

If you want to return errno, you need to save it before calling perror(), because that may change errno.

int executecommand(char *commandarray[]){
    char * command = commandarray[0];
    pid_t pid = fork();
    if (pid < 0){
        int saved_errno = errno;
        perror("fork");
        return saved_errno;
    } else if (0 == pid) {
        execvp(command, commandarray);
        int saved_errno = errno;
        perror("execvp");
        return saved_errno;
    } else {
        int waitstatus;
        waitpid(pid, &waitstatus, 0);
        return 0;
    }
}

int waitforinput(){
    printf("$ ");
    char cmd[256];
    char *cmd_array[2];
    fgets(cmd, 256, stdin);
    cmd[strcspn(cmd, "\n")] = 0; // remove trailing newline
    cmd_array[0] = cmd;
    cmd_array[1] = NULL;
    return executecommand(cmd_array);
}