C Program to convert contents of a file to upper case

Program

#define _GNU_SOURCE
#include<stdio.h>
#include<stdlib.h>
#include<ctype.h>
int file_to_upper(FILE *);
void main()
{
	FILE *fp;
	int status;
	char file_name[30];
	printf("Enter the file name you would like to convert:\n");
	scanf(" %s", file_name);
	fp = fopen(file_name, "r+");
	status = file_to_upper(fp);
	if(status == 1)
		printf("File %s converted to upper case successfully\n", file_name);
	else
		printf("Unable to convert.");
}
int file_to_upper(FILE *fp)
{
	char ch;
	if (fp == NULL)
	{
		puts("Error!!! Cannot open file");
		return -1;
	}
	do
	{
		ch = fgetc(fp);
		if ((ch >= 'a') && (ch <= 'z'))
		{
			ch = toupper(ch);
			fseek(fp, -1, SEEK_CUR);
			fputc(ch, fp);
		}
	}
	while (ch != EOF);
	fclose(fp);
	return 1;
}

Output

Enter the file name you would like to convert:
source.txt
File source.txt converted to upper case successfully

source.txt before converting

source.txt after converting