CODEWITHSUNDEEP
cpipe
S Jiles
S Jiles

Reputation: 53

putting output of execvp into string

I have a shell that has a function which eventually calls execvp(...) which gives an output. For example "echo hello" gives an output of "hello". Everything works, don't worry. I have tested it very much, I just didn't put the full code here because it is 1000 lines of code.

How can i take that output from execvp, dup2 it, and put it into a string?

I know I use dup2, but I'm not sure how.

I have these throughout my code:

char* globalString;   //a global string I want to put the output into
char* myString = "one two three ";
char* append = "echo four";
int myPipe[2];  //my pipe  
pipe(myPipe);

then I call my function and I want to pass the write end of pipe into it.

 myfunction( ... , [pointer to write end of pipe]); //i don't know how    

  //ignoring previous code


  cpid = fork();
  if(cpid < 0){
     //Fork wasn't successful 
     perror("fork");
     return -1;
  }

  //in the child
  if(cpid == 0){

     execvp(...);  // in this example, this prints "four" to stdout        

     //execvp returned, wasn't successful
     perror("exec");

     fclose(stdin);  

     exit(127);
  }

//then more code happens
}

at the end, I want the output from exec to be put into globalString. Then I put globalString into myString, so that myString is "one two three four"

thank you.

Upvotes: 0

Views: 2254

Answers (1)

AShelly
AShelly

Reputation: 35580

The snippet I use for getting output from a spawned process is:

pid_t pid = 0;
int pipefd[2];

pipe(pipefd); //create a pipe
pid = fork(); //spawn a child process
if (pid == 0)
{
   // Child. redirect std output to pipe, launch process
   close(pipefd[0]);
   dup2(pipefd[1], STDOUT_FILENO);
   execv(my_PROCESSNAME, args);
}
//Only parent gets here. make tail nonblocking.
close(pipefd[1]);
fcntl(pipefd[0], F_SETFL, fcntl(pipefd[0], F_GETFL) | O_NONBLOCK);

child_process_output_fd = pipefd[0];  //read output from here
child_process_pid = pid;  //can monitor this for completion with `waitpid`

Upvotes: 1

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