用于Shell的C程序：Execvp打印输出，然后在执行后台进程时发生分段错误

Question

I am writing a simple C program to create my own shell. It takes in input as commands and executes them. But when I try to execute a process in background( ie I fork a process from parent. The parent won't wait for the child process to finish, it just goes on to take more input commands while the child process runs in the background.) The execvp does execute the command but then gives a segmentation fault immediately. Can you help me? I'll post my part of the code which I think is relevant. Let me know if you need to know anything more, i'll edit my question accordingly.

while(1){

pid = fork();
        if(pid == 0)
            executeCommand(info);
        else
            {
                if(info->boolBackground ==1)
                    {
                        waitpid(pid , status , WNOHANG);
                    }
                else
                    wait(NULL);                     
            }

} //Info contains the command to be executed and it's arguments.

Here is me executeCommand function:

void executeCommand(parseInfo * info)
{
    FILE *infile, *outfile;
    struct commandType * com;
    char * cmd;
    int i , status;
    cmd = (char*)malloc(1024);
    strcpy(cmd , info->CommArray[0].command);
    if(info->boolOutfile == 1)
        {
            outfile = fopen(info->outFile, "w");
            dup2(fileno(outfile), 1);
        }
    if(info->boolInfile == 1)
        {
            infile = fopen(info->inFile, "r");
            dup2(fileno(infile), 0);
        }
    status = execvp(cmd , info->CommArray[0].VarList); //VarList contains the arguments
    if(status == -1){
        printf("%s\n",strerror(errno));}

    exit(0);
}

When I give an input command: ls & (& means that ls should be executed in background.) It forks a child process which executes ls and prints the list of files/directories in the directory and then gives segmentation fault. Can you spot the error? I tried running execvp in background with simply ls command. It also lead to a segmentation fault.

Answer 1

Yes, as Mark Plotnick pointed out in a comment, you'll need probably need &status. I'd use &info->status. Also, if you do detached jobs, you need to maintain a list of their info objects and do a waitpid loop on them:

forall (info in detached_detach_job_list) {
    pid = waitpid(info->pid,&info->status,WNOHANG);
    if (pid > 0) {
        report_status(info);
        remove_job_from_list(info);
    }
}

Hopefully, the code frag you gave for your outer loop does something like this.

Also, I might not do "wait(NULL)" for a foreground. I'd treat it similarly to a detached job. Consider a case where a user does:

det1 &
det2 &
...
det9000 &
run_long_30_minute_job

Because your shell is doing a hard wait on the foreground, it can't reap the detached jobs as they finish and you'll end up with zombie processes. Do the list/loop approach, just don't give user a prompt until the foreground completes (eg it's in the list, it's just the one with the background flag cleared). In other words, call the list something like child_list to denote all child processes, not just background. Put a sleep in the outer loop. Or, attach to SIGCHLD and do a single long sleep