fenilgmehta / cs744-pa1-pa2-shell-threads-synchronization Goto Github PK

File to see

• pa1_shell/shell_fm.c

Steps to reproduce

1. Create a file called a.txt with content as follows

   1
   3
   2
   

2. Compile the shell
3. Execute

   # Technique 1 to see the infinite loop
   ./shell_fm <<< "executeCommands a.txt"
   
   # Technique 2 to see the infinite loop
   ./shell_fm < a.txt

Description

Mostly, the problem is in the myShellInput(...) function.

1. Use int instead of char to store the value returned by getchar(). So, convert static char ch; to static int ch;. Read the docs of getchar() to know why, and also refer https://stackoverflow.com/a/7119500
2. After debugging I found that getchar() does not return EOF even after reading the file completely. Instead, it starts reading the file again from the start. I did not understand why this is happening, and according to my understanding of getchar() docs it should return EOF on reaching the end of file.
   • One solution to this is to use read(STDIN_FILENO, &ch, 1) instead of getchar()

     bool myShellInput(char *userInput) {
         static char ch;
         static bool eofFlag;
     
         eofFlag = 0;
         char *userInputOld = userInput;
         if (read(STDIN_FILENO, &ch, 1) <= 0) return false;
     
         while (ch != '\n') {
             if (eofFlag == 1) {
                 *userInput = '\0';
                 printflush(stderr, "DEBUG: finished myShellInput(...)\n");
                 return userInput != userInputOld;  // this returns false if not input read
             }
             *userInput = ch;
             ++userInput;
             if (read(STDIN_FILENO, &ch, 1) <= 0) {
                 eofFlag = 1;
             }
         }
     
         *userInput = '\0';
         return true;
     }