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MODULE 9a_1 - MORE INFO & EXAMPLES ON C FILE I/O 3

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MODULE 19 - C++ FILE I/O

create this, delete that, write this, read that, close this, open that

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9.7 Random Access To Disk Files

Before this you have learned how to read or write data sequentially. In many cases, however, you may need to access particular data somewhere in the middle of a disk file.
Random access is another way to read and write data to disk file. Specific file elements can be accessed in random order.
There are two C I/O functions, fseek() andftell(), that are designed to deal with random access.
You can use the fseek() function to move the file position indicator to the spot you want to access in a file. The prototype for the fseek() function is:

int fseek(FILE *stream, long offset, int whence);

stream is the file pointer with an opened file. offset indicates the number of bytes from a fixed position, specified bywhence, that can have one of the following integral values represented by SEEK_SET,SEEK_CUR and SEEK_END.
If it is successful, the fseek() function return 0, otherwise the function returns a nonzero value.
whence provides theoffset bytes from the file location. whence must be one of the values0, 1, or 2 which represent three symbolic constants (defined in stdio.h) as follows:

Constants	whence	File location
SEEK_SET	0	File beginning
SEEK_CUR	1	Current file pointer position
SEEK_END	2	End of file
Table 9.10: offset bytes

IfSEEK_SET is chosen as the third argument to the fseek() function, theoffset is counted from the beginning of the file and the value of the offset is greater than or equal to zero.
If however,SEEK_END is picked up, then theoffset starts from the end of the file, the value of the offset should be negative.
WhenSEEK_CUR is passed to thefseek() function, theoffset is calculated from the current value of the file position indicator.
You can obtain the value of the current position indicator by calling the ftell() function. The prototype for the ftell() function is,

long ftell(FILE *stream);

stream is the file pointer associated with an opened file. The ftell() function returns the current value of the file position indicator.
The value returned by the ftell() function represents the number of bytes from the beginning of the file to the current position pointed to by the file position indictor.
If theftell() function fails, it returns–1L (that is, a long value of minus 1). Let explore the program example. Create and make sure text file named tesseven.txt is located in the C:\Temp folder before you can execute the program. The contents of the tesseven.txt is,

THIS IS THE FIRST LINE OF TEXT, tesseven.txt file

THIS IS THE SECOND LINE OF TEXT, tesseven.txt file

THIS IS THE THIRD LINE OF TEXT, tesseven.txt file

THIS IS THE FOURTH LINE OF TEXT, tesseven.txt file

The content of tesseven.txt file

1. // random access to a file

2. #include <stdio.h>

3. #include <stdlib.h>

5. enum {SUCCESS, FAIL, MAX_LEN = 120};

7. // function prototypes, seek the file position indicator

8. void PtrSeek(FILE *fptr);

9. // function prototype, tell the file position indicator...

10. long PtrTell(FILE *fptr);

11. // function prototype read and writes...

12. void DataRead(FILE *fptr);

13. int ErrorMsg(char *str);

14.

15. int main(void)

16. {

17. FILE *fptr;

18. char filename[] = "c:\\Temp\\tesseven.txt";

19. int reval = SUCCESS;

20.

21. // if there is some error opening file for reading...

22. if((fptr = fopen(filename, "r")) == NULL)

23. {

24. reval = ErrorMsg(filename);

25. }

26. // if opening is successful..

27. else

28. {

29. // PtrSeek() function call...

30. PtrSeek(fptr);

31. //close the file stream...

32. if(fclose(fptr)==0)

33. printf("%s successfully closed.\n", filename);

34. }

35. // for Borland...

36. // system("pause");

37. return reval;

38. }

39.

40. // PtrSeek() function definition

41. void PtrSeek(FILE *fptr)

42. {

43. long offset1, offset2, offset3, offset4;

44.

45. offset1 = PtrTell(fptr);

46. DataRead(fptr);

47. offset2 = PtrTell(fptr);

48. DataRead(fptr);

49. offset3 = PtrTell(fptr);

50. DataRead(fptr);

51. offset4 = PtrTell(fptr);

52. DataRead(fptr);

53.

54. printf("\nReread the tesseven.txt, in random order:\n");

55. // re-read the 2^nd line of the tesseven.txt

56. fseek(fptr, offset2, SEEK_SET);

57. DataRead(fptr);

58. // re-read the 1^st line of the tesseven.txt

59. fseek(fptr, offset1, SEEK_SET);

60. DataRead(fptr);

61. // re-read the 4^th line of the tesseven.txt

62. fseek(fptr, offset4, SEEK_SET);

63. DataRead(fptr);

64. // re-read the 3^rd line of the tesseven.txt

65. fseek(fptr, offset3, SEEK_SET);

66. DataRead(fptr);

67. }

68.

69. // PtrTell() function definition

70. long PtrTell(FILE *fptr)

71. {

72. long reval;

73. // tell the fptr position...

74. reval = ftell(fptr);

75. printf("The fptr is at %ld\n", reval);

76. return reval;

77. }

78.

79. // DataRead() function definition

80. void DataRead(FILE *fptr)

81. {

82. char buff[MAX_LEN];

83. // reading line of text at the fptr position...

84. fgets(buff, MAX_LEN, fptr);

85. // and display the text...

86. printf("-->%s\n", buff);

87. }

88.

89. // error message function definition

90. int ErrorMsg(char *str)

91. {

92. // display this error message...

93. printf("Problem, cannot open %s.\n", str);

94. return FAIL;

95. }

95 lines: Output:

C file I/O open read write: random access and close file stream

We try to open the tesseven.txt file for reading by calling thefopen() function. If successful, we invoke the PtrSeek() function with thefptr file pointer as the argument in line 30.

PtrSeek(fptr);

The definition of our first function PtrSeek() is shown in lines 41-67. The statement in line 45 obtains the original value of the fptr file pointer by calling another function,PtrTell(), which is defined in lines 70–77.
ThePtrTell() function can find and print out the value of the file position indicator with the help of theftell() function.
The third function,DataRead() is called to read one line of characters from the opened file and print out the line of characters on the screen. Line 47 gets the new value of the fptr file position indicator right after the reading and assigns the value to another long variable, offset2.
Then theDataRead() function in line 48 reads the second line of characters from the opened file. Line 49 obtains the value of the file position indicator that points to the first byte of the third line and assigns the value to the third long variableoffset3 and so on for the fourth line of text.
Line 50 calls the DataRead() function to read the third line and print it out on the screen.
From the first portion of the output, you can see the four different values of the file position indicator at four different positions, and the four lines of texts. The four values of the file position indicator are saved byoffset1, offset2,offset3 and offset4 respectively.
Then, we read the lines of text randomly, one line at a time. Firstly read the second line, then the first line, fourth and finally the third one.
C function, called rewind(), can be used to rewind the file position indicator. The prototype for therewind() function is:

void rewind(FILE *stream);

Here,stream is the file pointer associated with an opened file. No value is returned by rewind() function. In fact the following statement of rewind() function:

rewind(fptr);

Is equivalent to this:

(void) fseek(fptr, 0L, SEEK_SET);

Thevoid data type is cast to thefseek() function because therewind() function does not return a value. Study the following program example.
This program also contains example of reading and writing binary data. We create and open the teseight.bin file for writing.

1. // reading, writing, rewind and binary data

2. #include <stdio.h>

3. #include <stdlib.h>

5. enum {SUCCESS, FAIL, MAX_NUM = 5};

7. // function prototypes...

8. void DataWrite(FILE *fout);

9. void DataRead(FILE *fin);

10. int ErrorMsg(char *str);

11.

12. int main(void)

13. {

14. FILE *fptr;

15. // binary type files...

16. char filename[] = "c:\\Temp\\teseight.bin";

17. int reval = SUCCESS;

18.

19. // test for creating, opening binary file for writing...

20. if((fptr = fopen(filename, "wb+")) == NULL)

21. {

22. reval = ErrorMsg(filename);

23. }

24. else

25. {

26. // write data into file teseight.bin

27. DataWrite(fptr);

28. // reset the file position indicator...

29. rewind(fptr);

30. // read data...

31. DataRead(fptr);

32. // close the file stream...

33. if(fclose(fptr)==0)

34. printf("%s successfully closed\n", filename);

35. }

36. // for Borland

37. // system("pause");

38. return reval;

39. }

40.

41. // DataWrite() function definition

42. void DataWrite(FILE *fout)

43. {

44. int i;

45. double buff[MAX_NUM] = { 145.23, 589.69, 122.12, 253.21, 987.234};

46.

47. printf("The size of buff: %d-byte\n", sizeof(buff));

48. for(i=0; i<MAX_NUM; i++)

49. {

50. printf("%5.2f\n", buff[i]);

51. fwrite(&buff[i], sizeof(double), 1, fout);

52. }

53. }

54.

55. // DataRead() function definition

56. void DataRead(FILE *fin)

57. {

58. int i;

59. double x;

60.

61. printf("\nReread from the binary file:\n");

62. for(i=0; i<MAX_NUM; i++)

63. {

64. fread(&x, sizeof(double), (size_t)1, fin);

65. printf("%5.2f\n", x);

66. }

67. }

68.

69. // ErrorMsg() function definition

70. int ErrorMsg(char *str)

71. {

72. printf("Cannot open %s.\n", str);

73. return FAIL;

74. }

74 lines, Output:

C file I/O open read write read write binary file and rewind

This program writes five values of the double data type into a binary file named teseight.bin and then rewind the file position indicator and re-read the fivedouble values from the binary file.
The two functions, DataWrite() andDataRead(), that perform the writing and reading, declared in lines 8 and 9. Theenum names, SUCCESS,FAIL, and MAX_NUM, are defined in line 5 with values 0, 1, and 5 respectively.
The statement in line 20, tries to create and open a binary file calledteseight.bin for both reading and writing.
If thefopen() function is successful, theDataWrite() function is called in line 27 to write four double data items, into the opened binary file, according to the definition of the DataWrite() function.
Thefwrite() function in line 51 does the writing. Right after the execution of theDataWrite() function, the file position indicator is reset to the beginning of the binary file by calling the rewind() function in line 29 because we want to re read all five double data items written to the file.
Thefread() function is used to perform the reading operation. The output from running the program shows the five double data items before the writing and after the reading as well.
As you learned, two C library functions scanf()/scanf_s() and printf()/printf_s() can be used to read or write formatted data through the standard I/O (that is,stdin and stdout). For C disk file I/O functions, there are two equivalent functions;fscanf()/fscanf_s() andfprintf()/fprintf_s() functions allow the programmer to specify I/O streams.

The prototype for the fscan() function is:

int fscanf(FILE *stream, const char *format,…);

stream is the file pointer associated with an opened file. format, which usage is similar to the scanf() function, is achar pointer pointing to a string that contains the format specifiers. If successful, the fscan() function returns the number of data items read. Otherwise, the function returns EOF.
The prototype for the fprintf() function is:

int fprintf(FILE *stream, const char *format, …);

Here,stream is the file pointer associated with an opened file. format, is similar as in the printf() function, is achar pointer pointing to a string that contains the format specifiers.
If successful, the fprintf() function returns the number of formatted expressions. Otherwise, the function returns a negative value.
Let try a program example. Firstly create testcal.txt file with the following data and save it.

23 12 33 10 4 6 44 31 7 50

Then create another text file named testavg.txt for writing the average value computed from data read from testcal.txt file. Then compile and run the following program.

/* C Program to calculate the average of a list of numbers. */

/* calculate the total from one file, output the average into another file */

#include <stdio.h>

/* for exit() */

#include <stdlib.h>

int main(void)

{

int value, total = 0, count = 0;

/* fileptrIn and fileptrOut are variables of type (FILE *) */

FILE * fileptrIn, * fileptrOut;

char filenameIn[100], filenameOut[100];

printf("Please enter an input filename (use path if needed):\n");

scanf("%s", filenameIn);

printf("Please enter an output filename (use path if needed):\n");

scanf("%s", filenameOut);

/* open files for reading, "r" and writing, "w" */

if((fileptrIn = fopen(filenameIn, "r")) == NULL)

{

printf("Error opening %s for reading.\n", filenameIn);

exit (1);

}

else

printf("Opening %s for reading is OK.\n", filenameIn);

if((fileptrOut = fopen(filenameOut, "w")) == NULL)

{

printf("Error opening %s for writing.\n", filenameOut);

exit (1);

}

else

printf("Opening %s for writing is OK.\n", filenameOut);

/* fscanf */

printf("\nCalculate the total...\n");

while(EOF != fscanf(fileptrIn, "%i", &value))

{

total += value;

++count;

}/* end of while loop */

/* write the average value to the file. */

/* fprintf */

printf("Calculate the average...\n\n");

fprintf(fileptrOut, "Average of %i numbers = %f \n", count, total/(double)count);

printf("Average of %i numbers = %f \n\n", count, total/(double)count);

printf("Check also your %s file content\n", filenameOut);

if(fclose(fileptrIn) == 0)

printf("%s closed successfully\n", filenameIn);

if(fclose(fileptrOut) == 0)

printf("%s closed successfully\n", filenameOut);

return 0;

}

Output:

C file I/O open read and process external data file

C & C++ programming tutorials

Further C file i/o reading and digging:

The source code for this Module is: C file input/output program source codes.
For C++ and MFC (Windows GUI programming) it is called Serialization and the topics are inSingle Document Interface (SDI) andMultiple Document Interface (MDI).
Check the best selling C / C++ books at Amazon.com.
Wide character/Unicode is discussed Character Sets, Unicode & Locale and the implementation using Microsoft C is discussedWindows Users & Groups C programming.
Implementation specific information for Microsoft can be foundMicrosoft C Run-Time Tutorials and More Win32 Windows C Run-Time programming Tutorials.

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