Creating a header file in C

#include <stdio.h>
#include <stdlib.h>
#include <string.h> //to use string functions such as strcat()

void createFile();// prototyping

int main()
{
    createFile();
    return 0;
}

void createFile(){
    FILE *fp;
    char buff[1025];
    char filename[25];
    int check = 1;


    while(check){
    printf("Enter the name of the file:\t");
    gets(filename);
    if(strlen(filename) > 20 || strlen(filename) < 3){// check filename length
        printf("\nThe name of the file must be between 3 and 20 characters.\n");
        check = 1;
        continue;
        }
        check = 0;
    }

    strcat(filename,".txt");//add file extension txt

    fp = fopen(filename,"w+");

    if(fp == NULL){
            puts("\nThere has been an error creating your file!\n");
            exit(1);
        }

    printf("\nA new file named %s was created successfully.\n",filename);
    printf("Enter your text:\n");
    gets(buff);
    fputs(buff,fp);
    printf("Text has been added successfully to %s",filename);

    fclose(fp);
}