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Files and Streams

Text and Binary Streams · Controlling Streams · Stream States

A program communicates with the target environment by reading and writing files (ordered sequences of bytes). A file can be, for example, a data set that you can read and write repeatedly (such as a disk file), a stream of bytes generated by a program (such as a pipeline), or a stream of bytes received from or sent to a peripheral device (such as the keyboard or display). The latter two are interactive files. Files are typically the principal means by which to interact with a program.

You manipulate all these kinds of files in much the same way -- by calling library functions. You include the standard header <stdio.h> to declare most of these functions.

Before you can perform many of the operations on a file, the file must be opened. Opening a file associates it with a stream, a data structure within the Standard C library that glosses over many differences among files of various kinds. The library maintains the state of each stream in an object of type FILE.

The target environment opens three files prior to program startup. You can open a file by calling the library function fopen with two arguments. The first argument is a filename, a multibyte string that the target environment uses to identify which file you want to read or write. The second argument is a C string that specifies:

Text and Binary Streams

A text stream consists of one or more lines of text that can be written to a text-oriented display so that they can be read. When reading from a text stream, the program reads an NL (newline) at the end of each line. When writing to a text stream, the program writes an NL to signal the end of a line. To match differing conventions among target environments for representing text in files, the library functions can alter the number and representations of characters transmitted between the program and a text stream.

Thus, positioning within a text stream is limited. You can obtain the current file-position indicator by calling fgetpos or ftell. You can position a text stream at a position obtained this way, or at the beginning or end of the stream, by calling fsetpos or fseek. Any other change of position might well be not supported.

For maximum portability, the program should not write:

If you follow these rules, the sequence of characters you read from a text stream (either as byte or multibyte characters) will match the sequence of characters you wrote to the text stream when you created the file. Otherwise, the library functions can remove a file you create if the file is empty when you close it. Or they can alter or delete characters you write to the file.

A binary stream consists of one or more bytes of arbitrary information. You can write the value stored in an arbitrary object to a (byte-oriented) binary stream and read exactly what was stored in the object when you wrote it. The library functions do not alter the bytes you transmit between the program and a binary stream. They can, however, append an arbitrary number of null bytes to the file that you write with a binary stream. The program must deal with these additional null bytes at the end of any binary stream.

Thus, positioning within a binary stream is well defined, except for positioning relative to the end of the stream. You can obtain and alter the current file-position indicator the same as for a text stream. Moreover, the offsets used by ftell and fseek count bytes from the beginning of the stream (which is byte zero), so integer arithmetic on these offsets yields predictable results.

A byte stream treats a file as a sequence of bytes. Within the program, the stream looks like the same sequence of bytes, except for the possible alterations described above.

Controlling Streams

fopen returns the address of an object of type FILE. You use this address as the stream argument to several library functions to perform various operations on an open file. For a byte stream, all input takes place as if each character is read by calling fgetc, and all output takes place as if each character is written by calling fputc.

You can close a file by calling fclose, after which the address of the FILE object is invalid.

A FILE object stores the state of a stream, including:

Do not alter any value stored in a FILE object or in a file buffer that you specify for use with that object. You cannot copy a FILE object and portably use the address of the copy as a stream argument to a library function.

Stream States

The valid states, and state transitions, for a stream are shown in the diagram.

Each of the circles denotes a stable state. Each of the lines denotes a transition that can occur as the result of a function call that operates on the stream. (The ``wide'' states are never entered in the Embedded C++ library.) Three groups of functions can cause state transitions. All are declared in <stdio.h>:

The state diagram shows that you must call one of the position functions between most write and read operations:

Finally, the state diagram shows that a position operation never decreases the number of valid function calls that can follow.

See also the Table of Contents and the Index.

Copyright © 1989-2002 by P.J. Plauger and Jim Brodie. All rights reserved.

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