Hey, long time no see!
Been a bit busy recently, so haven't had much time to get online.
It's been a while since I did any low level C programming too. But there are a number of things that are wrong with your code.
Firstly, the file name will not be an integer, it will be a c-style string - an array of characters.
Secondly, you need to get the file name as a c-style string (%s), not as int (%d).
Thirdly, you can't use the + operator to append text to a string in C. In C you need to use strncat.
Finally: If the file fails to open, you have the program outputting an error message via printf, but immediately afterwards, you attempt to write to the file anyway. This is the cause of the seg-fault/crash you are seeing.
What is happening is the call to fopen is failing (because of a combination of the problems above), you output an error message and then immediately attempt to write to an invalid file handle... Then Boom!
At the point of failure, it might be wiser to exit the program. Or separate the code for the two states with an if..else...
Here's a heavily commented and updated version of your code for your consideration:
(The comments should hopefully explain everything I've done)
Code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h> /* for strncat */
#include <linux/limits.h> /* for PATH_MAX */
#define PI 3.14
int main(void)
{
/* Get the user to enter the file name as a string */
char raw_name[PATH_MAX];
printf ("Want to make a File?\nType a File Name now. :)\n");
scanf ("%s", raw_name);
/* Concatenate .txt onto the end of the file name
But what if the user already specified .txt? This would append .txt again!
Just something for you to consider! Maybe add a function to check the filename and see if we need to append an extension?!
Anyway, strncat returns a char * which we assign to the variable file_name.
The first parameter (raw_name) is the destination string we want to append additional characters to
The second param (".txt") is the source string we want to append to the destination string
The third parameter is the number of bytes to copy from the source string (2nd parameter). ".txt" is 4 characters, so we require 4*sizeof(char) bytes
*/
char *file_name = strncat(raw_name, ".txt", 4*sizeof(char));
/* Open the file */
FILE *f_ptr = fopen (file_name, "w");
/* If the file opened successfully, write to the file and close it again */
if(f_ptr)
{
fprintf (f_ptr, "Hello World!!\n");
fflush (f_ptr);
fclose (f_ptr);
}
else
{
printf ("Could not write to file.\n");
/* return EXIT_FAILURE; /* Optionally, you might want to exit with an error status */
}
/*Rest of Program goes here*/
return EXIT_SUCCESS; /* EXIT_SUCCESS and EXIT_FAILURE are defined in stdlib.h */
}
BTW: If you actually want to append text to the end of the file, you might also want to consider opening the file using the a or a+ flag. The w flag opens the file for writing, but will always overwrite the file from the beginning. Whereas a will append to the end of the file (or create a new file - if a file with the specified name does not exist) and a+ will append to the end of the file, but ONLY if the file already exists - If my memory serves me correctly!
Hope this has helped. Any further questions, fire away!
Thanks for any help, guys....... :3
)
Man, Lowercase "O" and 0 REALLY confuse me..... 
