Chapter 12. Text and Binary File Processing. Instructor: Öğr. Gör. Okan Vardarlı. Copyright 2004 Pearson Addison-Wesley. All rights reserved.

Transcription

1 Chapter 12 Text and Binary File Processing Instructor: Öğr. Gör. Okan Vardarlı Copyright 2004 Pearson Addison-Wesley. All rights reserved.

2 Objectives We will explore the use of standard input, standard output, and program-controlled text files. We will introduce binary files. We will compare the advantages and disadvantages of text and binary files. Copyright 2004 Pearson Addison-Wesley. All rights reserved. 12-2

3 Why Files? Large volume of input data Large volume of output data More permanent storage of data Transfer to other programs Multiple simultaneous input and/or output streams E.g.: Business Data: customer files, payroll files, Scientific Data: weather data, environmental data Image Data: web images, satellite images, medical images, Web Data: HTML, GIF, JPEG, PNG, XML, 12-3

4 Files in C In C, each file is simply a sequential stream of bytes. C imposes no structure on a file. Thus, notions such as a record of a file do not exist as part of the C language. There are two formats for files: Text files: a collection of characters saved in secondary storage (e.g. on a disk). They contain variable length records They must be accessed sequentially, processing all records from the start of file to access a particular record Binary files: binary numbers that are the computer s internal representation of each file component. For example 49 is in a binary file, but it is 4 and 9 in a text file. They contain fixed length records They can be accessed directly (directly accessing the record that is required) Binary files are appropriate for online transaction processing systems, e.g. airline reservation, order processing, banking systems etc. 12-4

5 Files vs. File variables A file variable is a data structure in the C program which represents the file Temporary: exists only when program runs There is a struct called FILE in <stdio.h> Details of the struct are private to the standard C I/O library routines. Information like definitions of useful #define constants such as EOF for End of File are kept here. 12-5

6 Files and Streams C views each file simply as a sequential stream of bytes (Fig. 11.1). Each file ends either with an end-of-file marker or at a specific byte number recorded in a system-maintained, administrative data structure. When a file is opened, a stream is associated with it. Three files and their associated streams are automatically opened when program execution begins the standard input, the standard output and the standard error. Streams provide communication channels between files and programs by Pearson Education, Inc. All Rights Reserved.(Deitel & Deitel 11.2) 12-6

7 Files and Streams (Cont.) The standard library provides many functions for reading data from files and for writing data to files. Function fgetc, like getchar, reads one character from a file. Function fgetc receives as an argument a FILE pointer for the file from which a character will be read. The call fgetc( stdin ) reads one character from stdin the standard input. This call is equivalent to the call getchar(). Function fputc, like putchar, writes one character to a file. Function fputc receives as arguments a character to be written and a pointer for the file to which the character will be written by Pearson Education, Inc. All Rights Reserved.(Deitel & Deitel 11.2) 12-7

8 Files and Streams (Cont.) The function call fputc( 'a', stdout ) writes the character 'a' to stdout the standard output. This call is equivalent to putchar( 'a' ). Several other functions used to read data from standard input and write data to standard output have similarly named fileprocessing functions. The fgets and fputs functions, for example, can be used to read a line from a file and write a line to a file, respectively. In the next several sections, we introduce the file-processing equivalents of functions scanf and printf fscanf and fprintf by Pearson Education, Inc. All Rights Reserved.(Deitel & Deitel 11.2) 12-8

9 Opening a file A file must first be opened properly before it can be accessed for reading or writing. When a file is opened, a stream is associated with the file. Successfully opening a file returns a pointer to (i.e., the address of) a file structure, which contains a file descriptor and a file control block. 12-9

10 Opening a file A file must first be opened properly before it can be accessed for reading or writing. When a file is opened, a stream is associated with the file. Successfully opening a file returns a pointer to (i.e., the address of) a file structure, which contains a file descriptor and a file control block. File is placed in the same folder with the program unless specified. Such as "C:\\test1.txt" 12-10

11 Opening a file The statement: FILE *fptr1, *fptr2 ; declares that fptr1 and fptr2 are pointer variables of type FILE. They will be assigned the address of a file descriptor, that is, an area of memory that will be associated with an input or output stream. Whenever you are to read from or write to the file, you must first open the file and assign the address of its file descriptor (or structure) to the file pointer variable. Pointer arithmetic can NOT be applied to the file pointers

12 Opening a file FILE *fopen(const char *path, const char *mode); The fopen() function opens the file whose name is the string pointed to by path and associates a stream with it. The argument mode points to a string beginning with one of the following sequences (see next slide). In the example below, "r" stands for read only. Let s open an existing file named mydata.txt FILE *fptr1 ; fptr1 = fopen ( "mydata.txt", "r") ; Copyright 2004 Pearson Addison-Wesley. All rights reserved

13 Basic File Opening Modes: Mode Meaning fopen Returns if FILE- Exists Not Exists r Reading NULL w Writing Over write on Existing a Append r+ w+ a+ Reading + Writing Reading + Writing Reading + Appending New data is written at the beginning overwriting existing data Open a text file for update (reading and writing), first truncating the file to zero length New data is appended at the end of file Create New File Create New File Create New File Create New File Create New File 12-13

14 File opening modes by Pearson Education, Inc. All Rights Reserved.(Deitel & Deitel 11.5) 12-14

15 Testing for Successful Open If the file was not able to be opened, then the value returned by the fopen routine is NULL. For example, let's assume that the file mydata does not exist. Then: FILE *fptr1 ; fptr1 = fopen ( "mydata.txt", "r") ; if (fptr1 == NULL) { printf ("File 'mydata' did not open.\n") ; 12-15

16 Common Programming Errors by Pearson Education, Inc. All Rights Reserved.(Deitel & Deitel 11.5) 12-16

17 Reading formatted data from a file int fscanf ( FILE * stream, const char * format,... ); INPUT: Format string is analogous to scanf format string %d for integer, %c for character etc. And must have an argument for each format specifier. OUTPUT: On success, fscanf returns the number of items read; can be 0 if the pattern doesn't match. On failure, returns EOF

18 Reading From Files In the following segment of C language code: int a, b ; FILE *fptr1, *fptr2 ; fptr1 = fopen ( "mydata.txt", "r" ) ; fscanf ( fptr1, "%d%d", &a, &b) ; the fscanf function would read values from the file "pointed" to by fptr1 and assign those values to a and b. In the segment of code shown the file mydata is opened and the file pointer fptr1 is assigned to the open file. Then the fscanf function scans for two integers from this file and assigns the values read to the variables a and b

19 End of File (EOF) Status EOF: a special status value Returned by scanf and fscanf when end of data is reached defined in stdio.h #define EOF (some negative value) I/O library routines use EOF in various ways to signal end of file. Your programs can check for EOF EOF is a status, not an input value! 12-19

20 End of File The end-of-file indicator informs the program when there are no more data (no more bytes) to be processed. There are a number of ways to test for the end-of-file condition. One is to use the feof function which returns a true or false condition: fscanf (fptr1, "%d", &var) ; if ( feof (fptr1) ) { printf ("End-of-file encountered.\n ); 12-20

21 End of File There are a number of ways to test for the end-of-file condition. Another way is to use the value returned by the fscanf function: int istatus ; istatus = fscanf (fptr1, "%d", &var) ; if ( istatus == EOF ) { printf ("End-of-file encountered.\n ) ; 12-21

22 Going to the beginning of a file void rewind ( FILE * stream ); EFFECT Moves file pointer to beginning of file Resets end-of-file indicator Reset error indicator Forgets any virtual characters from ungetc E.g. rewind(fptr1); 12-22

23 Writing a formatted string to a file int fprintf ( FILE * stream, const char * format,... ) INPUT The format string is same as for printf Must have an argument for each specifier in the format OUTPUT / EFFECT On success, returns the number of character written On failure, returns a negative number 12-23

24 Writing a formatted string to a file Likewise in a similar way, in the following segment of C language code: int a = 5, b = 20 ; FILE *fptr2 ; fptr2 = fopen ( "results.txt", "w" ) ; fprintf ( fptr2, "%d %d\n", a, b ) ; the fprintf functions would write the values stored in a and b to the file "pointed" to by fptr

25 Closing Files The statements: fclose ( fptr1 ) ; fclose ( fptr2 ) ; will close the files and release the file descriptor space and I/O buffer memory

26 Reading and Writing Files #include <stdio.h> int main ( ) { FILE *outfile, *infile ; int b = 5, f ; float a = 13.72, c = 6.68, e, g ; outfile = fopen ("testdata.txt", "w") ; if (outfile == NULL) printf ("File 'testdata.txt' cannot be opened.\n"); else fprintf (outfile, "%6.2f%2d%5.2f", a, b, c) ; fclose (outfile) ; 12-26

27 Reading and Writing Files infile = fopen ("testdata.txt", "r") ; if (infile == NULL) printf ("File 'testdata.txt' cannot be opened.\n"); else{ fscanf (infile,"%f %d %f", &e, &f, &g) ; printf (" \n"); printf ("%6.2f,%2d,%5.2f\n", e, f, g) ; fclose (outfile) ; ; return 0; , 5,

28 //Reading from a file character by character #include <stdio.h> int main ( ) { FILE *infile ; char c; infile = fopen ("testdata.txt", "r") ; if (infile == NULL) printf ("File 'testdata.txt' cannot be opened.\n"); else{ printf (" \n"); c=getc(infile); while (c!= EOF) { putchar(c); //Print character to the screen c=getc(infile); fclose (infile) ; return 0; 12-28

29 //Appending data to an existing file then list it to the screen #include <stdio.h> int main ( ) { FILE *infile ; char c; infile = fopen ("testdata.txt", "a") ;//Open file for appending if (infile == NULL) printf ("File 'testdata.txt' cannot be opened.\n"); else fprintf(infile,"%c%s and a number:%d",'\n',"another line",3); fclose (infile) ; infile = fopen ("testdata.txt", "r") ;//Open file for reading if (infile == NULL) printf ("File 'testdata.txt' cannot be opened.\n"); else{ printf (" \n"); do{ if ((c = fgetc(infile))!= EOF) putchar(c); //Print character to the screen else break; while (1); fclose (infile) ; printf("\n"); return 0; 12-29

30 //Overwriting existing data from the beginning of the file #include <stdio.h> int main ( ) { FILE *infile ; char c; infile = fopen ("testdata.txt", "w+") ; if (infile == NULL) printf ("File 'testdata.txt' cannot be opened.\n"); else fprintf(infile,"here we go"); fclose (infile) ; infile = fopen ("testdata.txt", "r") ; if (infile == NULL) printf ("File 'testdata.txt' cannot be opened.\n"); else{ printf (" \n"); do { c=getc(infile); if (!feof(infile)) putchar(c); //Print character to the screen while (!feof(infile)); fclose (infile) ; printf("\n"); return 0; Previous file started with " 13.72" Now data at the beginning is overwritten

31 //Writing to and reading from a file #include <stdio.h> int main () { FILE *outfile, *infile ; int b = 5, f ; float a = 13.72, c = 6.68, e, g ; char line[80]; char *status; outfile = fopen ("testdata.txt", "w") ; if (outfile == NULL) printf ("File 'testdata.txt' cannot be opened.\n"); else{ fprintf (outfile, "%6.2f%2d%5.2f", a, b, c) ; fprintf (outfile, "%6.2f%2d%5.2f", a+1, b+1, c+1) ; fclose (outfile) ; infile = fopen("testdata.txt","r"); if (infile == NULL) printf ("File 'testdata.txt' cannot be opened.\n"); else{ printf (" \n"); fscanf (infile,"%f %d %f", &e, &f, &g) ; printf ("%6.2f%2d%5.2f\n", e, f, g) ; fscanf (infile,"%f %d %f", &e, &f, &g) ; printf ("%6.2f%2d%5.2f\n", e, f, g) ; fclose(infile); printf("\nlet's try to read full line:\n"); infile = fopen("testdata.txt","r"); if (infile == NULL) printf ("File 'testdata.txt' cannot be opened.\n"); else{ printf (" \n"); do{ status=fgets(line,80,infile); if (status!= NULL) printf("%s\n",line); while (status!= NULL); //Not EOF yet fclose(infile); return 0; 12-31

32 //Writing to and reading from a file #include <stdio.h> #include <string.h> int main(){ FILE *fp; char sentence[25]; int i; //Ceate a new file fp = fopen("tenlines.txt","w"); if (fp == NULL) printf ("File cannot be opened.\n"); else{ strcpy(sentence,"some thing."); for (i = 1;i <= 10;i++) fprintf(fp,"line#:%d %s\n",i,sentence); fclose(fp); printf("file is created successfully.\n"); fp = fopen("tenlines.txt","r"); //Let's read from the file if (fp == NULL) printf (" File cannot be opened.\n"); else{ for (i = 1;i <= 10;i++) { fgets(sentence,25,fp); printf("%s",sentence); fclose(fp); printf("\nlet's try a better way to read.\n"); fp = fopen("tenlines.txt","r"); if (fp == NULL) printf ("File 'TenLines.txt' cannot be opened.\n"); else{ int c; do { c = fgets(sentence,25,fp); if (c!= NULL) printf("%s",sentence); while (c!= NULL); //Not EOF yet fclose(fp); return 0; 12-32

33 Copying a file //Create a file then make a backup copy of it #include <stdio.h> int main ( ) { FILE *file1,*file2 ; char c; file1 = fopen ("test1.txt", "w") ; if (file1 == NULL) printf ("File 'test1.txt' cannot be opened.\n"); else fprintf (file1, "A few word.") ; fclose (file1) ; printf("content of test1.txt:\n"); file1 = fopen ("test1.txt", "r") ; if (file1 == NULL) printf ("File 'test1.txt' cannot be opened.\n"); else for(c=getc(file1);!feof(file1);c=getc(file1)) putc(c,stdout); printf("\n"); rewind(file1); //Move file pointer to the beginning file2 = fopen ("test2.txt", "w") ;//Backup file is openning if (file2 == NULL) printf ("File 'test2.txt' cannot be opened.\n"); else for(c=getc(file1);c!=eof;c=getc(file1)) putc(c,file2);//copy data from file1 to file2 fclose (file2); fclose (file1); printf("\ncontent of test2.txt:\n"); file2 = fopen ("test2.txt", "r") ; if (file1 == NULL) printf ("File 'test2.txt' cannot be opened.\n"); else do { c=getc(file2); if (!feof(file2)) putc(c,stdout); while (!feof(file2)); //Not EOF yet printf("\n"); fclose (file2); return 0; 12-33

34 What's wrong with this? #include <stdio.h> int main(){ FILE *fp = fopen ( "mydata.txt", "r" ) ; char state[3]; if (fp==null) printf("file mydata.txt cannot be opened"); else while(fscanf(fp,"%s", state)!= EOF) printf("i read: %s\n",state); return 0; 12-34

35 Buffer overruns Data is written to locations past the end of the buffer Hackers can exploit to execute arbitrary code User can always create an input longer than fixed size of buffer Don't use: scanf, fscanf, gets Use functions that limit the number of data read Use: fgets 12-35

36 Moving to a location fseek function Description The C library function sets the file position of the stream to the given offset. Declaration int fseek(file *stream, long int offset, int whence) Parameters stream This is the pointer to a FILE object that identifies the stream. offset This is the number of bytes to offset from whence. whence This is the position from where offset is added. It is specified by one of the following constants Origin constants: - SEEK_SET : seek from the beginning of the file (go byte) - SEEK_CUR: seek from the current file position (go byte) - SEEK_END: seek from the end of the file (go byte) Return Value This function returns zero if successful, or else it returns a non-zero value

37 Moving to a location #include <stdio.h> int main () { FILE * fp = fopen("myfile.txt", "w" ); fputs ( "This is an apple.", fp ); fseek ( fp, 9, SEEK_SET ); fputs ( " sam", fp ); fclose ( fp ); return(0); Open myfile.txt, you will see the content of it changed to: This is a sample 12-37

38 Where is the pointer? ftell function Description The C library function returns the current file position of the given stream. Declaration long int ftell(file *stream) Parameters stream This is the pointer to a FILE object that identifies the stream. Return Value This function returns the current value of the position indicator. If an error occurs, -1L is returned, and the global variable errno is set to a positive value

39 //Let s see how many bytes does file.txt have #include <stdio.h> int main () { FILE *fp; int len; fp = fopen("file.txt", "r"); if( fp == NULL ) { printf ("Error opening file"); return (-1); fseek(fp, 0, SEEK_END); len = ftell(fp); fclose(fp); printf("total size of file.txt = %d bytes\n", len); return(0); 12-39

40 Removing a file int remove ( const char * filename ) OUTPUT On success, returns 0 On failure, returns a non-zero value 12-40

41 Renaming a file int rename ( const char * oldname, const char * newname ) OUTPUT On success, returns 0 On failure, returns a non-zero value 12-41

42 Binary Files As we mentioned earlier, binary files containing binary numbers that are the computer s internal representation of each file component. For example 49 is in a binary file, but it is 4 and 9 in a text file. They contain fixed length records, usually arrays or structures They can be accessed directly (directly accessing the record that is required) Binary files are appropriate for online transaction processing systems, e.g. airline reservation, order processing, banking systems etc

43 In a Random Access File Data Data unformatted (stored as "raw bytes") in random access files All records of the same type have a fixed length Data not human readable

44 by Pearson Education, Inc. All Rights Reserved.(Deitel & Deitel 11.2)

45 11.5 Random-Access Files (Cont.) Fixed-length records enable data to be inserted in a random-access file without destroying other data in the file. Data stored previously can also be updated or deleted without rewriting the entire file by Pearson Education, Inc. All Rights Reserved.

46 Opening Binary Files Add b to the fopen mode string binaryp = fopen( nums.bin, wb ); wb : write binary rb : read binary ab : append binary 12-46

47 Binary I/O Functions There are following two functions, which can be used for binary input and output: fread and fwrite If you want to test while reading a binary file, either test right after you read a record with feof command or test numbers of items read to make sure end of file is not reached yet

48 Write to a file - fwrite function Description The C library function writes data from the array pointed to, by ptr to the given stream. Declaration size_t fwrite (const void * ptr, size_t size, size_t count, FILE * stream) Parameters ptr This is the pointer to the array of elements to be written. size This is the size in bytes of each element to be written. nmemb This is the number of elements, each one with a size of size bytes. stream This is the pointer to a FILE object that specifies an output stream. Return Value This function returns the total number of elements successfully returned as a size_t object, which is an integral data type. If this number differs from the nmemb parameter, it will show an error

49 Reading Binary Files - fread function Description The C library function reads data from the given stream into the array pointed to, by ptr. Declaration size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream) Parameters ptr This is the pointer to a block of memory with a minimum size of size*nmemb bytes. size This is the size in bytes of each element to be read. nmemb This is the number of elements, each one with a size of size bytes. stream This is the pointer to a FILE object that specifies an input stream. Return Value The total number of elements successfully read are returned as a size_t object, which is an integral data type. If this number differs from the nmemb parameter, then either an error had occurred or the End Of File was reached

50 Binary File I/O Writing binary files fwrite function E.g: fwrite(&i, sizeof(int), 1, binaryp); &i: the address of the first memory cell whose contents are copied to the file (here i is an integer variable). sizeof(int) : the number of bytes to copy to the file for one component. sizeof can be applied to both built-in and userdefined types. 1:the number of values to write to the binary file. binaryp:file pointer Reading binary files fread function E.g: fread(&i, sizeof(int), 1, binaryp); &i: the address of the first memory cell whose contents are copied from the file (here i is an integer variable). Other parameters are used similar way 12-50

51 //Creating a text file with fwrite function #include<stdio.h> int main () { FILE *fp; char str[] = "This is a test."; //Print the content of the file file.txt #include <stdio.h> int main () { FILE *fp; int c; fp = fopen( "file.bin", "wb" ); fwrite(str, sizeof(str), 1, fp ); fclose(fp); return(0); fp = fopen("file.bin","rb"); while(1) { c = fgetc(fp); if( feof(fp) ) { break ; printf("%c", c); fclose(fp); return(0); Note: You might see the content of this file since all data is in ASCII format

52 Searching Binary Files - fseek function Description The C library function int fseek(file *stream, long int offset, int whence) sets the file position of the stream to the given offset. Declaration int fseek(file *stream, long int offset, int whence) Parameters stream This is the pointer to a FILE object that identifies the stream. offset This is the number of bytes to offset from whence. whence This is the position from where offset is added. It is specified by one of the following constants Return Value This function returns zero if successful, or else it returns a non-zero value

53 //Creating a binary file with a structure //Saving in the order of account number //Creating a binary file with a structure #include <stdio.h> struct custinfo{ int accountno; char lastname[15]; char firstname[10]; double balance; ; int main(){ FILE *cfptr; struct custinfo cust; //Read created file if ( ( cfptr = fopen( "data.bin", "rb" ) ) == NULL ) printf( "File cannot be opened.\n" ); printf( "\\%-6s %-16s%-11s%10s\n", "AcctNo", "lastname", "firstname", "balance" ); do{ fread(&cust, sizeof(struct custinfo), 1,cfPtr ); if (!feof(cfptr)) //Do following if you don t want to see empty records //if (!feof(cfptr) && cust.accountno!=0) printf( "%-6d %-16s%-11s%10.2f\n", cust.accountno, cust.lastname, cust.firstname, cust.balance); while (!feof(cfptr)); fclose( cfptr ); return 0; if ( ( cfptr = fopen( "data.bin", "wb" ) ) == NULL ) printf( "File cannot be opened.\n" ); else { printf( "Enter account#: ( in a range 1-100, to exit 0) \n? " ); scanf( "%d", &cust.accountno ); while (cust.accountno!= 0 ) { printf( "Enter lastname, firstname, balance:\n? " ); fscanf( stdin, "%s%s%lf", cust.lastname, cust.firstname, &cust.balance);//stdin is keyboard fseek(cfptr,(cust.accountno-1)*sizeof(struct custinfo),seek_set); fwrite(&cust, sizeof(struct custinfo), 1,cfPtr ); printf( "Enter account #: \n? " ); scanf( "%d", &cust.accountno); fclose(cfptr); 12-53

54 Random Access Binary files have two features that distinguish them from text files: You can jump instantly to any structure in the file, which provides random access as in an array. You can change the contents of a structure anywhere in the file at any time. C supports the file-of-structures concept very cleanly. Once you open the file you can read a structure, write a structure, or seek to any structure in the file. This file concept supports the concept of a file pointer. When the file is opened, the pointer points to record 0 (the first record in the file). Any read operation reads the currently pointed-to structure and moves the pointer down one structure. Any write operation writes to the currently pointed-to structure and moves the pointer down one structure. fseek moves the pointer to the requested record. Keep in mind that C thinks of everything in the disk file as blocks of bytes read from disk into memory or read from memory onto disk. C uses a file pointer, but it can point to any byte location in the file. You therefore have to keep track of things

55 Finding a specific data //This program reads the required element from a file #include <stdio.h> #define SIZE 5 int main(){ FILE *binaryp; int x,i,numbers[size]={3,5,9,12,1; //open for writing binary binaryp = fopen("nums.bin", "wb"); if (binaryp == NULL) printf ("File 'nums.bin' cannot be opened.\n"); else { if(fwrite(numbers, sizeof (int), SIZE, binaryp)!=size) {printf("error in writing the data."); return 1; else printf("file created sucessfully.\n\n"); fclose(binaryp); //Open for reading binary binaryp = fopen("nums.bin", "rb"); if (binaryp == NULL) printf ("File 'nums.bin' cannot be opened.\n"); else { if (fread(numbers, sizeof (int),size, binaryp)!=size) { printf("error in reading file."); return 1; else printf("content of file:\n"); for (i=0; i<size;i++) printf("%d ",numbers[i]); printf("\nenter the index of the element number 0-%d:",SIZE-1); do { scanf("%d",&i); if (i<0 i>=size) printf("invalid index. please reenter.\n"); while(i<0 i>=size); //if user entered 0, pointer will stay at the beginning fseek(binaryp,i*sizeof(int),seek_set); if (fread(&x, sizeof (int),1, binaryp)!=1) { printf("error in reading file."); return 1; else printf("the value is:%d",x); fclose(binaryp); printf("\n"); return 0; 12-55

56 Updating a specific data //Updating a binary file with a structure #include <stdio.h> struct custinfo{ int accountno; char lastname[15]; char firstname[10]; double balance; ; int main(){ FILE *cfptr; struct custinfo cust; int acct; double transaction; if ( ( cfptr = fopen( "data.bin", "rb+" ) ) == NULL ) printf( "File cannot be opened.\n" ); else { printf( "Enter account# to be updated: ( in a range 1-100, to exit 0) \n? " ); scanf( "%d", &acct ); while (acct!= 0 ) { fseek(cfptr,(acct-1)*sizeof(struct custinfo),seek_set); fread(&cust, sizeof(struct custinfo), 1,cfPtr ); if (cust.accountno==0) printf("account has no information.\n"); else printf( "%-6d %-16s%-11s%10.2f\n",cust.accountNo, cust.lastname, cust.firstname, cust.balance); See next page 12-56

57 Updating a specific data (continued) printf("enter transaction + or - value:"); scanf("%lf",&transaction); cust.balance+=transaction; //update balance printf( "%-6d %-16s%-11s%10.2f\n",cust.accountNo, cust.lastname, cust.firstname, cust.balance); fseek(cfptr,(acct-1)*sizeof(struct custinfo),seek_set); fwrite(&cust, sizeof(struct custinfo), 1,cfPtr ); printf( "Enter account #: \n? " ); scanf( "%d", &acct); fclose(cfptr); //Read updated file if ( ( cfptr = fopen( "data.bin", "rb" ) ) == NULL ) printf( "File cannot be opened.\n" ); else { printf( "\n%-6s %-16s%-11s%10s\n", "AcctNo", "lastname", "firstname", "balance" ); do{ fread(&cust, sizeof(struct custinfo), 1,cfPtr ); if (!feof(cfptr)) //if (!feof(cfptr) && cust.accountno!=0) DO THIS IF YOU DON'T WANT TO SEE EMPTY RECORDS printf( "%-6d %-16s%-11s%10.2f\n",cust.accountNo, cust.lastname, cust.firstname, cust.balance); while (!feof(cfptr)); fclose( cfptr ); return 0; 12-57

58 #include <stdio.h> int main () { FILE *fp; int i,x,y=99; fp = fopen("file.bin", "wb+"); if( fp == NULL ) { printf ("Error opening file"); return (-1); //Create file with integers from 1-10 for(i=1;i<=10;i++) fwrite(&i,sizeof(int),1,fp); fseek(fp, 0, SEEK_SET); //Rewind to the beginning while (fread(&x,sizeof(int),1,fp)==1) printf("%d\n",x); printf("file is created.\n\n"); fclose(fp); fp = fopen("file.bin", "rb+"); if( fp == NULL ) { printf ("Error opening file"); return 1; printf("beginning pointer:%ld\n",ftell(fp)); if (fread(&x,sizeof(int),1,fp)==1) {printf("first value:%d\n",x); printf("pointer after reading:%ld\n",ftell(fp)); else return 1; fseek(fp,0,seek_cur); if(fwrite(&y,sizeof(int),1,fp)==1) printf("pointer after writing:%ld\n",ftell(fp)); else printf("file is not updated.\n\n"); rewind(fp); while (fread(&x,sizeof(int),1,fp)==1) printf("%d\n",x); fclose(fp); return(0);

59 Advantages of Binary files Assume that two bytes are used store an int value. 244 ( ) In text files, Write: Read: It takes more time. It takes more space. (Three bytes versus two) Precision Binary files also usually have faster read and write times than text files, because a binary image of the record is stored directly from memory to disk (or vice versa). In a text file, everything has to be converted back and forth to text, and this takes time

60 Disadvantages of Binary files A binary file created on one computer is rarely readable on another type of computers. A binary file can not be created or modified in a word processor. fread fwrite fscanf fprintf 12-60

61 References J.R. Hanly & E.B. Koffman, Problem Solving and Program Design in C (6 th Ed.), Addison- Wesley, 2010 P.J. Deitel & H.M. Deitel, C How to Program (5 th Ed.), Pearson Education, Inc.,