Files and Streams

Text and Binary Streams · Byte and Wide 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:

whether you intend to read data from the file or write data to it or both
whether you intend to generate new contents for the file (or create a file if it did not previously exist) or leave the existing contents in place
whether writes to a file can alter existing contents or should only append bytes at the end of the file
whether you want to manipulate a text stream or a binary stream

Once the file is successfully opened, you can then determine whether the stream is byte oriented (a byte stream) or wide oriented (a wide stream). Wide-oriented streams are supported only with Amendment 1. A stream is initially unbound. Calling certain functions to operate on the stream makes it byte oriented, while certain other functions make it wide oriented. Once established, a stream maintains its orientation until it is closed by a call to fclose or freopen.

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:

empty files
space characters at the end of a line
partial lines (by omitting the NL at the end of a file)
characters other than the printable characters, NL, and HT (horizontal tab)

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.

Byte and Wide Streams

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.

By contrast, a wide stream treats a file as a sequence of generalized multibyte characters, which can have a broad range of encoding rules. (Text and binary files are still read and written as described above.) Within the program, the stream looks like the corresponding sequence of wide characters. Conversions between the two representations occur within the Standard C library. The conversion rules can, in principle, be altered by a call to setlocale that alters the category LC_CTYPE. Each wide stream determines its conversion rules at the time it becomes wide oriented, and retains these rules even if the category LC_CTYPE subsequently changes.

Positioning within a wide stream suffers the same limitations as for text streams. Moreover, the file-position indicator may well have to deal with a state-dependent encoding. Typically, it includes both a byte offset within the stream and an object of type mbstate_t. Thus, the only reliable way to obtain a file position within a wide stream is by calling fgetpos, and the only reliable way to restore a position obtained this way is by calling fsetpos.

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. For a wide stream (with Amendment 1), all input takes place as if each character is read by calling fgetwc, and all output takes place as if each character is written by calling fputwc.

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:

an error indicator -- set nonzero by a function that encounters a read or write error
an end-of-file indicator -- set nonzero by a function that encounters the end of the file while reading
a file-position indicator -- specifies the next byte in the stream to read or write, if the file can support positioning requests
a stream state -- specifies whether the stream will accept reads and/or writes and, with Amendment 1, whether the stream is unbound, byte oriented, or wide oriented
a conversion state -- remembers the state of any partly assembled or generated generalized multibyte character, as well as any shift state for the sequence of bytes in the file)
a file buffer -- specifies the address and size of an array object that library functions can use to improve the performance of read and write operations to the stream

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. Five groups of functions can cause state transitions.

Functions in the first three groups are declared in <stdio.h>:

the byte read functions -- fgetc, fgets, fread, fscanf, getc, getchar, gets, scanf, and ungetc
the byte write functions -- fprintf, fputc, fputs, fwrite, printf, putc, putchar, puts, vfprintf, and vprintf
the position functions -- fflush, fseek, fsetpos, and rewind

Functions in the remaining two groups are declared in <wchar.h>:

the wide read functions -- fgetwc, fgetws, fwscanf, getwc, getwchar, ungetwc, and wscanf
the wide write functions -- fwprintf, fputwc, fputws, putwc, putwchar, vfwprintf, vwprintf, and wprintf,

For the stream s, the call fwide(s, 0) is always valid and never causes a change of state. Any other call to fwide, or to any of the five groups of functions described above, causes the state transition shown in the state diagram. If no such transition is shown, the function call is invalid.

The state diagram shows how to establish the orientation of a stream:

The call fwide(s, -1), or to a byte read or byte write function, establishes the stream as byte oriented.
The call fwide(s, 1), or to a wide read or wide write function, establishes the stream as wide oriented.

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

You cannot call a read function if the last operation on the stream was a write.
You cannot call a write function if the last operation on the stream was a read, unless that read operation set the end-of-file indicator.

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

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