These are IO functions defined as part of the C standard.

In the C model, programs communicate with devices by reading and writing a sequence of characters through a construct called a stream. A stream is a channel connecting the program to exactly one open file on a specific device.

A stream consists of an optional high level portion identified by a FILE* that sits on top of a mandatory low level portion that is directly connected to the device driver and is identified by a numerical file descriptor (fd). Programs can choose to send and receive data from the device through the high level FILE* or directly through the low level fd, though most programmers will find using the FILE* interface more convenient most of the time as it supplies formatted input and output operations.

Although all data read or sent to the device through a FILE* is eventually transmitted to the device through the underlying fd, care must be taken if communication with a specific file is done through both the high level FILE* and the low level fd. This is because the high level FILE* portion of the stream may be buffered, meaning data written may not have been immediately sent to the device driver following a write. A subsequent write using the direct fd interface may actually have its data sent to the device driver prior to the previous buffered write using FILE*. To avoid this situation, streams should be flushed (see fflush()) when switching from i/o using FILE* to i/o using fd to ensure that any data in high level buffers are sent to the device driver immediately.

A high level interface can be added to an existing fd by calling fdopen() and the underlying fd can be determined from an existing high level FILE* using fileno().

Aside from read/write operations, stdio also provides a means to randomly access data within files using fseek(), lseek(), rewind(), a means to control the behaviour of the device using ioctl(), a means to detect errors with feof(), ferror() and a means to determine if the device is ready to send or receive data with poll().

At program startup, three streams are opened for you. They are:

• FILE *stdin, fd=0. Open for reading, typically connected to the keyboard. For default program input.
• FILE *stdout, fd=1. Open for writing, typically connected to the primary display. For default program output.
• FILE *stderr, fd=2. Open for writing, also typically connected to the primary display though guaranteed to be unbuffered. For errors.

Insert stuff on writing device drivers, selecting devices for a program

High Level I/O (FILE *)

More discussion at GNU libc

Low Level I/O (fd)

More discussion at GNU libc