What does a file pointer point to in C?

Question

I am trying to understand input and output files in C. In the beginning, when we want to open a file to read, we declare a file pointer as follows:

FILE *fptr1 = fopen( "filename", "r")

I understand that FILE is a data structure in the stdio.h library and that it contains information about the file. I also know that the fopen() function returns a FILE structure. But, is that the purpose of the pointer. It just points to a bunch of information about the file? I've been reading into this and I have heard the term "file streams" floating around a bit. I understand that it is a an interface of communication with the file (find it vague, but I'll take it). Is that what the pointer points to in simple terms - a file stream? In the above code example, would the pointer be pointing to an input file stream?

Thank you!

Answer 1

The FILE structure is intended to be opaque. In other words, you are not supposed to look into it if you want your programs to remain portable.

Further, FILE is always used through a pointer, so you don't even need to know its size.

In a way, you can consider it a void * for all intents and purposes.

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Now, if you are really interested on what the FILE type may hold, the C standard itself explains it quite well! See C11 7.21.1p2:

(...) FILE which is an object type capable of recording all the information needed to control a stream, including its file position indicator, a pointer to its associated buffer (if any), an error indicator that records whether a read/write error has occurred, and an end-of-file indicator that records whether the end of the file has been reached; (...)

So as you see, at least it contains stuff like:

• The position inside the file
• A pointer to a buffer
• Error flags
• EOF flag

It mentions (as you do) streams. You can find some more details about it in section 7.21.2 Streams:

Input and output, whether to or from physical devices such as terminals and tape drives, or whether to or from files supported on structured storage devices, are mapped into logical data streams, whose properties are more uniform than their various inputs and outputs. Two forms of mapping are supported, for text streams and for binary streams.

(...)

A binary stream is an ordered sequence of characters that can transparently record internal data. (...)

As we can read, a stream is an ordered sequence of characters. Note that it does not say whether this sequence is finite or not! (More on that later)

So, how do they relate to files? Let's see section 7.21.3 Files:

A stream is associated with an external file (which may be a physical device) by opening a file, which may involve creating a new file. Creating an existing file causes its former contents to be discarded, if necessary. If a file can support positioning requests (such as a disk file, as opposed to a terminal), then a file position indicator associated with the stream is positioned at the start character number zero) of the file, unless the file is opened with append mode in which case it is implementation-defined whether the file position indicator is initially positioned at the beginning or the end of the file. The file position indicator is maintained by subsequent reads, writes, and positioning requests, to facilitate an orderly progression through the file.

(...)

See, when you open a "disk file" (the typical file in your computer), you are associating a "stream" (finite, in this case) which you can open/read/write/close/... through fread() and related functions; and the data structure that holds all the required information about it is FILE.

However, there are other kinds of files. Imagine a pseudo-random number generator. You can conceptualize it as an infinite read-only file: every time you read it gives you a different value and it never "ends". Therefore, this file would have an infinite stream associated with it. And some operations may not make sense with it (e.g. maybe you cannot seek it, i.e. move the file position indicator).

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This only serves as a quick introduction, but as you can see, the FILE structure is an abstraction over the concept of a file. If you want to learn more about this kind of thing, the best you can do is reach for a good book on Operating Systems, e.g. Modern Operating Systems from Tanenbaum. This book also refers to C, so even better.