How to make shell continually ask for input after execv in C++

Question

I'm attempting to implement a shell in c++, using fork() and exec(). My code is as follows:

#include <iostream>
#include <string>
#include <unistd.h>
#include <sstream>
#include <vector>

using namespace std;

int main(int argc, char** argv){
    while(true){
        cout << "minish> ";
        string input = "";
        getline(cin, input);

        int count = 1;
        int shouldCount = 1;
        if(input.length() == 0) shouldCount = 0;

        if(shouldCount == 1){
            int didFindLetter = 0;
            for(int i = 1; i < input.length()-1;i++){
                if(input[i] ==' ' && didFindLetter == 1){
                    count++;
                    didFindLetter = 0;
                }
                else if(input[i]!=' '){
                    didFindLetter = 1;
                }
            }
        }
    
        //need to create a wordArray here...
        vector<string> wordArray;
        std::istringstream iss (input);
        string tempWord;
        for(int i = 0; i < count; i++){
            iss >> tempWord;
            wordArray.push_back(tempWord);
        }
        
        char* argsArray[1024];
        count = 0;
        for (int i = 0; i < wordArray.size(); i++) {
            //strdup returns pointer to a char array copy of its parameter
            argsArray[count++] = strdup(wordArray[i].c_str());
        }
        argsArray[count++] = (char *)NULL;


        pid_t pid = fork();

        if(pid == 0){
            //child
            execvp(argsArray[0], argsArray);
            fprintf(stderr, "exec: %s\n", strerror(errno));
            exit(errno);

        }
        else if(pid == 1){
            //int waitStatus;
            //pid_t terminated_child_pid = wait(&waitStatus);
            wait(NULL);
        }
    }

    return 0;
}

When I run this code and try to execute a single command, it seems to work properly

Elliots-MacBook-Pro:minish Elliot$ ./minish

minish> ls

minish> makefile minish minish.cpp test

After running ls via execv, the code does not print "minish> " to prompt me for more input, however, is able to continually execute if i continue to type commands i.e "ls".

What is the cause of this problem, and how might i be able to fix it?

Answer 1

I'm not quite sure I follow your example, but you have the semantics of fork() wrong:

RETURN VALUE
       On success, the PID of the child process is returned 
       in the parent, and 0 is returned in the child.  On
       failure, -1 is returned in the  parent, no child 
       process is created, and errno is set appropriately.

Your code has the parent checking for a return value of 1 - meaning it skips waiting for the child; it immediately continues with another iteration.

Answer 2

Your problem is you're getting the minish prompt before the child process manages to finish its own output.

You can prevent this by waiting on the child process. The child process will never have pid == 1 or 0, so your else clause should be

else if(pid >= 0)  /*instead of `if pid==1`*/ {
    wait(NULL);
}else { /*report fork failure */; }