What is the difference between a stream and a file?

Question

The C standard talks about streams. For example the fopen(3) manual page tells that fopen is a stream open function.

Can anybody explain what exactly streams are, and how they relate to files?

Answer 1

In the context of the C Standard Library a stream is a generic interface for performing certain I/O operations. You can read from streams, write to streams, some streams are seekable. Opening a file as a stream is only one way to get a stream as an I/O interface for an application.

Let me quote:

11.1.1 Streams and File Descriptors

When you want to do input or output to a file, you have a choice of two basic mechanisms for representing the connection between your program and the file: file descriptors and streams. File descriptors are represented as objects of type int, while streams are represented as FILE * objects.

File descriptors provide a primitive, low-level interface to input and output operations. Both file descriptors and streams can represent a connection to a device (such as a terminal), or a pipe or socket for communicating with another process, as well as a normal file. [...]

... and further:

12.1 Streams

For historical reasons, the type of the C data structure that represents a stream is called FILE rather than “stream”. Since most of the library functions deal with objects of type FILE *, sometimes the term file pointer is also used to mean “stream”. This leads to unfortunate confusion over terminology in many books on C.

Examples for I/O streams in C:

• Standard Streams: https://linux.die.net/man/3/stdin
• File Streams: https://linux.die.net/man/3/fopen
• Pipes: https://linux.die.net/man/3/popen
• Stream Sockets: https://linux.die.net/man/2/socket

For further reading, also have a look at these links:

• https://www.gnu.org/software/libc/manual/html_mono/libc.html#I_002fO-Overview
• https://www.gnu.org/software/libc/manual/html_mono/libc.html#I_002fO-on-Streams

The stream-based API is built on top of the low-level file descriptor API and provides additional functionality. Some low-level features are however only available via the lower level API, e.g., memory-mapped I/O, non-blocking I/O or "event-driven" I/O:

• https://www.gnu.org/software/libc/manual/html_node/Memory_002dmapped-I_002fO.html
• https://linux.die.net/man/2/poll
• https://linux.die.net/man/4/epoll