fflush(stdin) function does not work - c

Question

I can't seem to figure out what's wrong with this code:
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
#define MAX 100
#define TRUE 1
#define FALSE 0
char sect_cat;
char customer_name[MAX];
char customer_number[MAX]; /* error handling is easier */
int prev_unit = 0;
int current_unit = 0;
int consumed = 0;
int set = FALSE;
float init_bill;
float tax;
float total_bill;
void get_userinfo()
{
printf("Enter sector category: ");
scanf("%c", &sect_cat);
printf("Enter customer name: ");
fflush(stdin);
scanf("%sn", &customer_name);
set = FALSE;
while (set == FALSE)
{
printf("Enter customer number: ");
fflush(stdin);
scanf("%s", customer_number);
int i;
int error;
for (i=0, error=0; i<strlen(customer_number); i++)
{
if (isdigit(customer_number[i]))
{
}
else
{
error = 1;
}
}
if (error == 0)
{
set = TRUE;
}
else
printf("ERROR: Only numbers are allowed\n");
}
printf("Enter previous unit: ");
fflush(stdin);
scanf("%d", &prev_unit);
set = FALSE;
while (set == FALSE)
{
printf("Enter current unit: ");
fflush(stdin);
scanf("%d", &current_unit);
if (prev_unit > current_unit)
{
printf("ERROR: Current unit must be larger than previous unit\n");
}
else
set = TRUE;
}
consumed = current_unit - prev_unit;
}
int main()
{
/* Introduce program to users */
printf("\nThis program computes your electric bill based on these sector categories\n\n");
printf("\tResidential(R)\n");
printf("\tIndustrial(I)\n");
printf("\tCommercial(C)\n\n");
printf("Press any key to continue...");
fflush(stdin);
getchar();
#################### edit
Applying templatetypedef's solution, the program now waits for user input for the customer_name. However entering a string with a space leads to an error, and the program assumes that the word after the space is input for the next prompt.
Enter sector category: r
Enter customer name: George of the Jungle
Enter customer number: ERROR: Only numbers are allowed
Enter customer number: ERROR: Only numbers are allowed
Enter customer number:

Answer 1

The fflush function does not flush data out of an input stream; it is instead used to push data buffered in an output stream to the destination. This is documented here. As seen in this earlier SO question, trying to use fflush(stdin) leads to undefined behavior, so it's best to avoid it.
If you want to eat the newline from the return character entered when the user finished typing in their character, instead consider the following:
scanf("%c%*c", &sect_cat);
This will eat the newline rather than leaving it in stdin.
Hope this helps!

Answer 2

I think that you meant to write fflush(stdout) instead of fflush(stdin).

Answer 3

fflush should work with an output stream, see docs here