Question

I’m using the “GCC C Compiler” as my compiler and I have a program that takes in inputs as stdin using “fgets” and then I’m using multiple printf’s to print results due to certain inputs.

However, my problem is I want the output to occur between the fgets, which they do reside in my code, however currently nothing prints until I return from main and the program ends.

Input Code:

int get_inputs(char** operands, char* delim) {
  if (fgets(input,sizeof(input),stdin) == NULL) return 0;  /* End of file */

  /* Parse with StringParse, returns number of substrings */
  return StringParse(input, operands, delim, 2, "+-*/^ ");
}

Output Code: (In a While(1) loop)

count = get_inputs(operands, delim);

switch(count) {
case 0:
    printf("User Terminated\n");
    return 0;  /* User Terminated */

case 1: /* Single Value Input */
    accumulator = atof(operands[0]);
    printf("%g\n", accumulator);
    break;

case 2:
    if(strlen(operands[0]) == 0) { /* Operation First use Accumulator as input */
        accumulator = doMath(accumulator, atof(operands[1]), delim[0]);
        printf("%g\n", accumulator);
    }
    else { /* Two new values, replace Accumulator */
        accumulator = doMath(atof(operands[0]), atof(operands[1]), delim[0]);
        printf("%g\n", accumulator);
    }
    break;

default:
    printf("Invalid Input\n"); /* Invalid Input or Error */
    break;
}

Every other function is just doing math or string parsing.

Thanks in Advance!

Answer 1

This is because stdout is buffered to improve performance. The data is only pushed to the output pipe in larger chunks. To force this to happen at a certain point, add

fflush(stdout);

In the code which is writing to stdout.