Question

I have a problem with going over an if statement that the code should enter in c:

void getInput(void)
    {
        static size_t _read = 0;
        memset(line, 0, _read);
        do{
            memset(line, 0, _read);
            write(STDOUT_FILENO, ">> ", 3);
            _read = read(STDIN_FILENO, line, MAXLINE - 1);
            if (line[0] == '\n' || line[0] == '\r')
                continue;
        }while(line[_read - 1] != '\n');
        line[_read] = '\0';
    }

The problem is at the line “if(line[0] == ‘\n’ || line[0] == ‘\r’)” here is an extract from the debugger.

getInput () at main.c:29
29          if (line[0] == '\n' || line[0] == '\r')
1: line[0] = 10 '\n'
(gdb) step
31      }while(line[_read - 1] != '\n');
1: line[0] = 10 '\n'
(gdb) list 29
24      memset(line, 0, _read);
25      do{
26          memset(line, 0, _read);
27          write(STDOUT_FILENO, ">> ", 3);
28          _read = read(STDIN_FILENO, line, MAXLINE - 1);
29          if (line[0] == '\n' || line[0] == '\r')
30              continue;
31      }while(line[_read - 1] != '\n');
32      line[_read] = '\0';
33  }

As you can see it does not go to the continue statement, but rather straight to the while statement. I did think that the continue would go to the while statement but i still dont understand why it doesnt step on the continue first.

Thanks

Answer 1

If you are compiling with optimizations enabled, it is entirely possible that the jump instruction for the if is simply going straight to the while, rather than bothering with the continue, which would just be another simple jump instruction.