How can i use execl() system call to execute tr command in Unix?

Question

I m trying to translate all small characters of file to capital latter using tr command with exec system call of unix.

execl("/usr/bin/tr","tr [a-z] [A-Z] < emp.lst",NULL);

Answer 1

   #include <stdio.h>
   #include <stdlib.h>
   #include <sys/types.h>
   #include <unistd.h>
   #include <sys/wait.h>
   #include <unistd.h>
   #include <ctype.h>
   #define MAX 5000
   int main( int argc, char *argv[])
     {  int pipen[2];
       pipe(pipen);
       int pipe2[2];
       pipe(pipe2);
       char buf[MAX];
           pid_t child_pid;


         if((child_pid = fork()) < 0 )
         {
            perror("fork failure");
            exit(1);
         }

      if(child_pid == 0)
    {    
      close(pipen[1]);
          close(pipe2[0]);
          dup2(pipen[0],0);
          dup2(pipe2[1],1);
          close(pipen[0]);
      close(pipe2[1]);
          execl("/usr/bin/tr","/usr/bin/tr","[:lower:]", "[:upper:]",NULL);

    }

    else
   {
    FILE* inputfile;
    inputfile = fopen("in","r");
     char c;
     if (inputfile) {
     int i=0;
     while ((c = getc(inputfile)) != EOF){
    buf[i]=c;
            i++;
            if(i>=MAX){break;
        buf[i-1]='\0';
    }
    }
     buf[i]='\0';    
     fclose(inputfile);
  }

  close(pipen[0]);
  close(pipe2[1]);
  write(pipen[1],buf,sizeof(buf));
  close(pipen[1]);
  read(pipe2[0],&buf,sizeof(buf));
  close(pipe2[1]);
  printf("----->\n%s\n",buf);

  }

   return 0;
}

This is one way to solve this problem,As I know we can directly write

execl("/bin/ls","ls","/home/Martin/Documents","-l",NULL); or something but cant 

directly write tr command which you have mentioned, That is why I have used dup2 and fork system calls to do the job as you want. I am also new to these stuff .If anything went wrong please elaborate :)