Advanced C 1992

Part II • Managing Data in C

Listing 9.2. continued

}

else

{

switch(chChar)

{

case ‘\n’: case ‘\x0d’:

chChar = ‘\0’; break;

default: /* Change current character to typed character */ szInputLine[nCurrentChar] = chChar;

printf(“%c”, szInputLine[nCurrentChar]); ++nCurrentChar;

break;

}

}

}

printf(“\n”);

return(1);

}

Two parts of EDLINE require closer examination. First, the program declares two character-string buffers to hold temporary file names:

char szTempName[L_tmpnam]; char szNewName[L_tmpnam];

I don’t know how long these buffers should be, so I use the L_tmpnam identifier, which is defined in stdio.h. This identifier is system dependent and is large enough to hold any temporary filename returned by tmpnam().

Now that there is a place to save the temporary file names, I can call the ANSI C tmpnam() function, which returns a unique filename. This filename can be used for any purpose (EDLINE, for example, uses it for a work file). In a multitasking environment, the same name might be returned to more than one program. Your application can handle this situation by getting a new temporary name if a file is

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opened for writing and an error is returned because another application is using the name. There is no need to signal an error to the user; simply recover by getting a new name.

The following code shows how a temporary filename is obtained and opened:

tmpnam(szTempName);

if (strlen(szTempName) == 0)

{

printf(“Couldn’t open a work file...\n”); exit(4);

}

The recovery in a multitasking environment could be coded as follows:

In general, this type of error checking is unnecessary for programs running on the PC under DOS.

When you are finished with your work file, you must delete it. You can do this with the ANSI C remove() function or the unlink() function, which is still found on many C compilers.

ANSI C also supports the tmpfile() function, which creates a temporary file and opens the file. This function does much of the work for the programmer, but it has disadvantages. For example, when the file is closed (whether or not you are finished),

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it is deleted. This makes it impossible to close the file early in the program’s execution and reopen it later. You also cannot specify the mode of the file: it is opened in the binary read/write mode. EDLINE required a temporary work file with the text mode, not the binary mode, so tmpfile() could not be used.

/* Lines of code to use the file */

fclose(TempFile);

The tmpfile() function is handy when the file’s attributes (binary read/write) are suitable to your program’s needs, and when you want the system to automatically delete the file.

Sometimes you will want the temporary file to stay around. If the program crashes, for example, you may be able to recover the user’s data from the temporary work file.

Using a temporary work file is similar to using any other file, except it must be deleted and its name must be unique.

Stream Files and Default File Handles

In C, files can be opened as stream files, which are identified with a pointer to a FILE structure, or as low-level files, which are identified with an integer handle. This section covers stream files. Low-level files are described later in the chapter, under “Low-Level I/O and File Handles.”

A stream file must be opened with one of the following functions:

If a file is opened with open() or one of the other file open functions that return an integer handle, the file is a low-level file, not a stream file. Table 9.1 lists the stream functions that C supports.

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Table 9.1. Stream file functions (including Microsoft’s additions).

continues

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Table 9.1. continued

270

Function Description

vfprintf() Does a formatted write to the specified file. The output is pointed to by a parameter-list pointer.

vprintf() Does a formatted write to stdout. The output is pointed to by a parameter-list pointer.

vsprintf() Does a formatted write to the specified string. Output is pointed to by a parameter-list pointer.

The classification of stream files includes a number of standard files. For every C program, five standard files are always open. Programs can use these files to perform basic file I/O to the screen and the keyboard.

Before describing the standard files, remember that most operating systems enable you to redirect files. Usually, a simple redirection symbol automatically allows a file to be used for input or output; the program uses this file as if it were either the keyboard or the screen.

The stdin File

The stdin file is opened by the operating system and is considered to be the keyboard. If the operating system supports I/O redirection, however, stdin could be a file.

If stdin’s input comes from an I/O redirected file, several problems can occur when the end of the redirected file is reached. First, the program receives an end-of- file (EOF) error. Many programs do not adequately check for EOF, and the user may have a difficult time ending the program. Second, the operating system does not switch back to the keyboard. You can force your program to use the keyboard by opening a file called con: (under DOS on a PC), but this requires a lot of programming.

The following functions use stdin:

getchar() Gets a character from stdin

gets() Gets a string from stdin

scanf() Performs a formatted read from stdin

These functions make your code easier to read and understand. Each has a counterpart that can be used for any other stream file.

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The stdout File

The stdout file is opened by the operating system and is considered to be the screen. If the operating system supports I/O redirection, however, stdout could be a file.

If stdout’s output goes to a redirected file, several problems can occur. The program could receive an end-of-file (EOF) error when the disk is full. Many programs do not adequately check for EOF with stdout, and the user may not realize an error has occurred until the program has finished running. Another problem is that there is no provision to switch back to the screen. You can force your program to use the screen by opening a file called con: (under DOS on a PC), but doing so requires a lot of programming.

The following functions use stdout:

Each stdout function has a counterpart that can be used for any other stream file. And like the stdin functions, the stdout functions enable you to write code that is easier to read and understand.

The stderr File

The stderr file is similar to the stdout file; data written to stderr is displayed on the screen. The major difference is that stderr is used for error messages that you would not want redirected to a file if the user is using I/O redirection. You can use stderr to display messages to the user regardless of whether I/O redirection is used because stderr is never I/O redirected.

No functions use stderr directly. You can use fprintf() to write your error message:

fprintf(stderr,

“The input file is in the old format. Run REFMT first”);

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The error message will not be redirected if the operating system’s I/O redirection is in effect.

Always use stderr for the program’s banner message (the message to the user that describes the program and lists the copyright, author, and so on) and error messages. If you develop one error message function that is always called when an error occurs, you can be sure that the message is written to the correct place—the screen and not a redirected file.

The stdaux File

The stdaux file’s name is a bit confusing. What is the aux device? In ANSI C, the aux device is defined as the main serial communication port (not the console serial communication port). On a PC, aux is defined as COM1:, and stdaux writes to the COM1: port.

The short program in Listing 9.3, called STDAUX.C, writes 100 lines to the stdaux file. Before running this program, initialize the communications port. To do this under DOS on the PC, use the MODE command.

Listing 9.3. STDAUX.C.

/* STDAUX, written 1992 by Peter D. Hipson

*This program uses the stdaux file. It should be run

*only under DOS on a PC.

*/

continues

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Part II • Managing Data in C

Listing 9.3. continued

int i;

for (i = 0; i < 100; i++)

{

/* Because stdaux is opened in the binary mode, * CR/LF must be supplied explicitly, using \n\r */

fprintf(stdaux,

“Line %2d of 100 lines”

“ being written to stdaux by a program.\n\r”, i);

}

return (0);

}

The stdprn File

The stdprn file is easy to understand. It provides a simple way to write to the system printer without having to explicitly open a file. The stdprn file cannot be redirected and therefore should be used only for items that will be printed, probably in response to a user request.

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The short program in Listing 9.4, called STDPRN.C, writes to stdprn. Before running this program, be sure a printer is connected to the primary printer port and is online.

Listing 9.4. STDPRN.C.

/* STDPRN, written 1992 by Peter D. Hipson

* This program uses the stdprn file. It should be run

continues

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