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: 
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.
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.