Advanced C 1992

Part II • Managing Data in C

Listing 9.1. continued

case ‘b’:

printf(“Note the bytes written always” “ equal the string’s length\n\n”);

break;

}

return (0);

}

TEXTFILE enables you to open the file in text mode or binary mode so you can see how the modes differ. The ftell() function returns the number of bytes in the file, without regard to the file’s mode. All strings in TEXTFILE have the same number of characters, according to strlen(), but the number of bytes written in the text mode depends on the number of newline characters in the string.

Virtually all files saved by editors and other programs are equivalent to a text file. Many DOS programs cannot read a file that does not have the newline and carriage return combination.

Creating and Using Temporary Work Files

When you create a temporary work file, remember the following simple rules:

• The filename must be unique. This uniqueness can be guaranteed by using

tmpfile() or tmpnam().

•The file must be deleted when you have finished using it. If you create the file using tmpfile(), the operating system deletes the file for you. If you create it with tmpnam() and an explicit open, your program must delete the file.

Few programs can store all their data in memory. You cannot be sure of the amount of memory available to your program for data storage, and you therefore won’t know if there will be enough memory to load the data the program requires.

Many larger programs with large data objects do not even try to save all their data in memory. They read the data, index the data, then write the data to a temporary work file. Listing 9.2, EDLINE, is a simple editor that reads a text file, provides editing

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capabilities, then writes the program’s buffers out to the file when the user ends the program. Because this editor is simple, it supports only line-number editing.

Listing 9.2. EDLINE.C.

/* EDLINE, written 1992 by Peter D. Hipson

*This program is a simple line-oriented editor. If

*your compiler supports memory models, use the

*large model.

int EditLine(char * szInputLine); /* Used to edit a given line */

continues

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Part II • Managing Data in C

Listing 9.2. continued

/* First, get the filename to edit */

if (argc >= 2)

{

DataFile = fopen(argv[1], “rt”);

if (DataFile == NULL)

{

printf(“ERROR: File ‘%s’ couldn’t be opened.\n”, argv[1]);

}

else

{

strcpy(szFileName, argv[1]);

}

}

while (DataFile == NULL)

{

printf(“\nPlease enter name of file to edit: “);

gets(szFileName);

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DataFile = fopen(szFileName, “rt”);

if (DataFile == NULL)

{

printf(“ERROR: File ‘%s’ couldn’t be opened.\n”, szFileName);

}

}

printf(“\n”);

/* Next, get a temporary filename, read the original file, and

*write it to the work file. Create a line-number index so that

*you can access the records.

*/

tmpnam(szTempName);

if (strlen(szTempName) == 0)

{

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

}

WorkFile = fopen(szTempName, “w+t”);

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

{

lLineIndex[i] = DELETED_LINE;

}

nMaxLines = 1; lLineIndex[nMaxLines] = 0;

while(fgets(szInputLine, sizeof(szInputLine), DataFile))

{

lLineIndex[nMaxLines++] = ftell(WorkFile); fputs(szInputLine, WorkFile);

}

fclose(DataFile);

continues

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Part II • Managing Data in C

Listing 9.2. continued

printf(“Total lines in file %d.\n”, nMaxLines - 1);

szCommand[0] = ‘\0’;

while(szCommand[0] != ‘q’) // Quit without saving (use w command to // save)

{

printf(“Command? “);

gets(szCommand);

strlwr(szCommand);

nEndLine = -1;

sscanf(&szCommand[1], “%d%d”, &nStartLine,

&nEndLine);

if (nEndLine < nStartLine)

{

nEndLine = nStartLine;

}

if (nEndLine >= nMaxLines)

{

nEndLine = (nMaxLines - 1);

}

switch(szCommand[0])

{

case ‘e’: /* Edit the specified line */

if (nStartLine == 0)

{

printf(“Line number must be 1 to %d\n”, nMaxLines);

}

else

{

260

if (nStartLine < nMaxLines && lLineIndex[nStartLine] != DELETED_LINE)

{

fseek(WorkFile, lLineIndex[nStartLine], SEEK_SET);

fgets(szInputLine, sizeof(szInputLine), WorkFile);

if (EditLine(szInputLine))

{

fseek(WorkFile, 0, SEEK_END);

lLineIndex[nStartLine] = ftell(WorkFile);

fputs(szInputLine, WorkFile);

}

}

}

break;

case ‘l’: /* List the specified line */

if (nStartLine == 0)

{

nStartLine = 1;

while(nStartLine < nMaxLines)

{

if (lLineIndex[nStartLine] != DELETED_LINE)

{

fseek(WorkFile, lLineIndex[nStartLine], SEEK_SET);

continues

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Part II • Managing Data in C

Listing 9.2. continued

fgets(szInputLine, sizeof(szInputLine), WorkFile);

printf(“%4d - %s”, nStartLine, szInputLine);

}

else

{

printf(“%4d ***DELETED LINE***\n”, nStartLine);

}

++nStartLine;

}

nStartLine = 0;

}

else

{

while(nStartLine <= nEndLine)

{

if (lLineIndex[nStartLine] != DELETED_LINE)

{

fseek(WorkFile, lLineIndex[nStartLine], SEEK_SET);

fgets(szInputLine, sizeof(szInputLine), WorkFile);

printf(“%4d - %s”, nStartLine, szInputLine);

}

else

{

printf(“%4d ***DELETED LINE***\n”, nStartLine);

}

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++nStartLine;

}

}

break;

case ‘d’: /* Delete the specified line */

if (nStartLine > 0 && nStartLine < nMaxLines)

{

printf(“Do you really want to delete line %d? (y|n) “, nStartLine);

chChar = getche();

printf(“\n”);

if (chChar == ‘y’ || chChar == ‘Y’)

{

lLineIndex[nStartLine] = DELETED_LINE;

}

}

break;

case ‘w’: /* Write; continue editing? */

szNewName[0] = ‘\0’;

tmpnam(szNewName);

if (strlen(szNewName) == 0)

{

printf(“Error getting a temporary file name...\n”);

}

rename(szFileName, szNewName);

DataFile = fopen(szFileName, “wt”);

nStartLine = 1;

continues

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Part II • Managing Data in C

Listing 9.2. continued

while(nStartLine < nMaxLines)

{

if (lLineIndex[nStartLine] != DELETED_LINE)

{

fseek(WorkFile, lLineIndex[nStartLine], SEEK_SET);

fgets(szInputLine, sizeof(szInputLine), WorkFile);

fputs(szInputLine, DataFile);

}

++nStartLine;

}

nStartLine = 0;

fclose(DataFile);

/* In this version, the original file is simply deleted.

*A better programming practice is to rename it to .BAK

*so the user can recover the original file.

*

*Question:

*When renaming to .BAK, does the user recover from the

*last save or the original file?

*/

remove(szNewName); /* Could be renamed to .BAK */

break;

case ‘q’: /* Quit, with no save */

break;

default:

printf(“Error: the command ‘%c’ is not supported!\n”, szCommand[0]);

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