Q:

Don't know why I'm getting a 'conflicting types' error and a 'implicit declaration error' in C.

I'm writing a program to generate a string of random uppercase letters, then take user input of uppercase letters, along with a character form the user. For any instance of the user input letter in the random string, it replaces that letter with the character entered by the user. 
For example, s1 = {BDHFKYL} s2 = {YEIGH} c = '*' 
Output = BD*FK*L
Anyway, I'm getting a conflicting types error at the function declaration under main and I implicit decoration at the function call. Does anyone know why? Thank you in advance. 
#include <stdio.h>
#include <stdlib.h>

void fillS1(char x[42]);

void fillS2(char x[22]);

void strfilter(char a[], char b[], char c);

int main(int argc, const char * argv[])
{
char s1[42];
char s2[22];

fillS1(s1);

//printf("%s\n", s1);

puts(s1);

fillS2(s2);

strFilter(s1, s2, '*');  //Implicit declaration here

return 0;
}

void fillS1(char x[])
{
for (int i = 0; i < 40; i++)
    x[i] = 'A' + random() % 26;
x[40] = (char)0;
}

void fillS2(char x[]){

int i = 0;

printf("Please enter at least 2 capital letters and a maximum of 20.\n");
while (( x[i] = getchar()) != '\n' ) {

    i++;

    }

x[i] = '\0';

if (i < 3) {
    printf("You need at least two letters");
}

else if (i > 21){
    printf("You cannot have more than twenty letters");
}

/*else if (((i > 'a')) || ((i < 'z'))){
    printf("You many only have capital letters");
}   */

 else puts(x);
}

void strFilter(char a[], char b[], char c){ //Conflicting types error here
int i = 0;
int n = 0;

while (n < 21) {

    for (i = 0; i < 41; i++) {
        if (a[i] == b[n]){
            c = a[i];
        }
    }
    i = 0;
    n++;
}

puts(a);
}

A:

You have a mismatch in your identifiers: strFilter (with a capital F) is not the same as strfilter (with a lowercase f). So, change your predeclaration from this:
void strfilter(char a[], char b[], char c);

to this:
void strFilter(char a[], char b[], char c);

