Question

I have a program that reads a file, treat it and put the results in an output file. When I have an argument (input file), I create the output file and write it the content.

I’ve made a fork() in order to redirect the stdout a write() content.

char *program;

program = malloc(80);

sprintf(program, "./program < %s > %s", inputFile, outputFile);   
int st;
switch (fork()) {
  case -1:
       error("fork error");
         case 0:
           /* I close the stdout */
           close (1);

             if (( fd = open(outputfile, O_WRONLY | O_CREAT , S_IWUSR | S_IRUSR | S_IRGRP)==-1)){

                 error("error creating the file \n");
                 exit(1);
             }

             execlp("./program", program,  (char *)0);

             error("Error executing program\n");
              default:
          // parent process - waits for child's end and ends
            wait(&st);
            exit(0);
     //*****************

              }

The child is created properly with the < > stdin and stdout files are created.
But, the child never ends, and when I kill the father, the output file is empty, so the code did not executed.

What is happening?
Thanks!

Answer 1

The functions in the exec family don’t understand redirections.

The way you’re calling execlp, you’re passing one argument to your program: ./program < %s > %s. That’s right, one argument. Of course, execlp doesn’t know what redirections are, and neither does program.

I would replace all your code with:

char *program = malloc(LEN);

snprintf(program, LEN, "./program < %s > %s", inputFile, outputFile);  
system(program);