Difficulty for a beginner using fopen command in C - c

Question

I am following the C programming tutorial at http://www.cprogramming.com/tutorial/c/lesson10.html. This particular tutorial teaches file I/O in C; in particular, the fopen command is discussed. At one point, they give the following example (which I think should print the contents of file test.txt):
FILE *fp;
fp=fopen("c:\\test.txt", "w");
fprintf(fp, "Testing...\n");
So, I made a text file called test.txt and saved it in my current, working directory (C:\cygwin\home\Andrew\cprogramming). Then I created a c file in this same directory, and it contains the following code:
#include <stdio.h>
int main()
{
FILE *fp;
fp=open("test.txt","w");
fprintf(fp,"Testing...\n");
}
When I compile this c file (which I've called helloworld2.c) using gcc, I get the following messages:
helloworld2.c: In function `main':
helloworld2.c:40: warning: assignment makes pointer from integer without a cast
Then when I try to run the executable, I get:
Segmentation fault (core dumped)
Do you have any ideas about what I should try next?
Thank you very much for your time.

Answer 1

This is because you use open instead of fopen. Open is from the POSIX standard and returns an (integer) handle; fopen returns the memory address of a FILE structure. You cannot use both in an interchangeable way. As it stands, your code implicitly casts the received integer (likely 4) to a FILE* pointer, making it point to the memory address 4. This segfaults your program when fprintf attempts to access it.
fopen is cross-platform, but open is POSIX-only. You may want to stick to fopen for now.

Answer 2

fopen() returns a pointer to a FILE object while open() returns a file descriptor which is a plain int.
Unless you need low-level functions it's usually better to work with fopen and FILE objects.

Answer 3

I'm guessing this was just an unfortunate typo - open() instead of fopen() - which just happens to work well enough to build a final executable (rather than a deliberate attempt to use open())...
You see warning: assignment makes pointer from integer without a cast because there is no "prototype" - a declaration of the argument and return types - for open() in <stdio.h>.
In the absence of such a prototype, the compiler assumes that such a function exists and returns an int, which your code assigns to the pointer variable fp.
It does in fact link successfully because there is a function called open() in the C library, but it does something different (as others have mentioned). But if (for example) you'd written fpen() instead, things would have gone more obviously wrong - it would have failed at the link stage, as there is no library function of that name.
If you compile with more warnings enabled - e.g. -Wall for GCC - you'll get some more helpful errors:
$ gcc -Wall -o helloworld2 helloworld2.c
helloworld2.c: In function 'main':
helloworld2.c:6: warning: implicit declaration of function 'open'
helloworld2.c:6: warning: assignment makes pointer from integer without a cast
helloworld2.c:8: warning: control reaches end of non-void function
$
The warning: implicit declaration of function 'open' tells you that there is a mismatch between the headers you've included, and the function you're trying to use.