Optional read from STDIN in C

Question

My application is basically a shell which expects an input of type cmd [x] , where cmd is constant and x is optional. So cmd 1 is legal as well as cmd by itself - then I assume a default parameter for x .

I am doing this:

char cmd[64];
scanf("%s", cmd);
int arg;
scanf("%d", &arg); // but this should be optional

How can I read the integer parameter, and set it to a default if none is currently available in the prompt? I do not want the prompt to wait for additional input if it was not given in the original command.

I tried several versions using fgetc() and getchar() and comparing them to EOF but to no avail. Each version I tried ends up waiting on that optional integer parameter.

Answer 1

The easy way:

   char b[73]; //powers of 2 magic is evil.
    if(fgets(b,sizeof b,stdin) != NULL) {
      char cmd[59]; //59, no command will ever be > 58. Ever I say.
      int arg;
      if(sscanf(b,"%58s %d",cmd,&arg) == 2) {
         //handle cmd and arg
      } else if(sscanf(b,"%58s",cmd) == 1) {
         //handle cmd only
      } else {
       // :-/
      }
    }

Answer 2

Simple answer, you can't. The C runtime takes input from the OS, but doesn't control it. To do something like this you will need to interact directly with the OS using platform specific APIs.

Answer 3

Are you reading line-by-line? Can't you just read the whole command until you reach a "\\n" (newline)? If you get two tokens before the newline, it is a command and the argument; if you read only one, it is the command only and you set the second argument to the default.

Answer 4

Here's a program that works (sorry my previous answer was made in haste).

int main(){
    char cmd[100], line[100];
    int man = 0;
    printf("OK: ");
    fgets(line, 100, stdin);
    int num = sscanf(line,"%s %d",cmd,&man);
    if (num==1)printf("one! %s %d\n", cmd, man);
    else if (num==2)printf("two! %s %d\n", cmd, man);
}

fgets reads the line (with bounds checking), and sscanf will assess whether one or two tokens were entered.