Introduction to Computer Programming Lecture 18 Binary Files

Transcription

1 Introduction to Computer Programming Lecture 18 Binary Files Assist.Prof.Dr. Nükhet ÖZBEK Ege University Department of Electrical&Electronics Engineering 1

2 RECALL: Text File Handling in C Step 0: Include stdio.h Step 1: Declare a file handler (a.k.a. file pointer) as FILE * for each file Step 2: Open file using fopen() Step 3: Check if file is opened successfully Step 4a: Use fscanf() for input Step 4b: Use fprintf() for output Step 5: Close file using fclose() 2

3 Binary Files When text files are used for storage of data, a program must spend extra effort to convert the stream of characters from an input file into integers and vice versa for output files If the file is not to be read by a human, we can avoid this unnecessary translation 3

4 Binary Files A binary file is a file created by executing a program that stores directly in the file the computer s internal representation of each file component 4

5 Binary File Example FILE *binaryp; int i; binaryp = fopen( nums.bin, wb ); for (i = 2; i <= 500; i+=2) fwrite(&i, sizeof(int), 1, binaryp); fclose(binaryp); 5

6 Binary File Example FILE *binaryp; int i; Binary file declaration is the same as text file declaration binaryp = fopen( nums.bin, wb ); for (i = 2; i <= 500; i+=2) fwrite(&i, sizeof(int), 1, binaryp); fclose(binaryp); 6

7 Binary File Example FILE *binaryp; int i; b character is used to denote that the file is a binary file binaryp = fopen( nums.bin, wb ); for (i = 2; i <= 500; i+=2) fwrite(&i, sizeof(int), 1, binaryp); fclose(binaryp); 7

8 Binary File Example FILE *binaryp; int i; binaryp = fopen( nums.bin, wb ); for (i = 2; i <= 500; i+=2) fwrite(&i, sizeof(int), 1, binaryp); fclose(binaryp); fclose function is used just as for text files 8

9 Binary File Example FILE *binaryp; int i; binaryp = fopen( nums.bin, wb ); for (i = 2; i <= 500; i+=2) fwrite(&i, sizeof(int), 1, binaryp); fclose(binaryp); a different stdio library function is used 9

10 fwrite function fwrite function has 4 input parameters fwrite(&i, sizeof(int), 1, binaryp); Address of the first memory cell whose contents are to be copied to the file 10

11 fwrite function fwrite function has 4 input parameters fwrite(&i, sizeof(int), 1, binaryp); Number of bytes to copy to the file for one component 11

12 sizeof operator sizeof operator can be applied to any data type to find the number of bytes used for storage of the data type sizeof operator can be applied to both builtin and user-defined types E.g. printf( An integer requires %d bytes,sizeof(int)); 12

13 fwrite function fwrite function has 4 input parameters fwrite(&i, sizeof(int), 1, binaryp); Number of values to write to the file 13

14 fwrite function fwrite function has 4 input parameters fwrite(&i, sizeof(int), 1, binaryp); Pointer to the file which is opened for writing before 14

15 Notes on fwrite It is possible to write the contents of an entire array using just one fwrite call E.g., int score[10]; fwrite(score, sizeof(int), 10, binaryp); 15

16 fread function fread function also requires four parameters 1. Address of the first memory cell to fill 2. Size of one value 3. Maximum number of elements to copy from the file into memory 4. File pointer to a binary file opened in mode rb fread function returns an integer indicating how many elements it copied from the file 16

17 Notes It is important not to mix file types A binary file created or written using fwrite must be read using fread A text file created or written using fprintf must be read using fscanf 17

18 Binary File Example /* FREAD.C: This program opens a file named FREAD.OUT and writes 25 characters to the file. It then tries to open the file and read in 25 characters. If the attempt succeeds, the program displays the number of actual items read. */ #include <stdio.h> int main( void ) { FILE *stream; int list[25]; int i, numread, numwritten; /* Open file in binary mode: */ stream = fopen( "fread.out", "wb" ); if( stream == NULL ) { printf( "Problem opening the file\n" ); return 1; } 18

19 Binary File Example (cntd) for ( i = 0; i < 25; i++ ) list[i] = i; /* Write 25 characters to stream */ numwritten = fwrite( list, sizeof( int ), 25, stream ); printf( "Wrote %d items\n", numwritten ); fclose( stream ); stream = fopen( "fread.out", "rb" ); if( stream == NULL ) { printf( "File could not be opened\n" ); return 2; } 19

20 Binary File Example (cntd) /* Attempt to read in 25 characters */ numread = fread( list, sizeof( int ), 25, stream ); printf( "Number of items read = %d\n", numread ); fclose( stream ); } return 0; 20

21 Structures and Binary Files Structures can also be written to and read from binary files using fwrite and fread functions Structures are written or read as a package with one function Structure arrays can also be written with one function 21

22 Data I/O Using Text and Binary Files Suppose we have a structure like the following: typedef struct { char name[10]; double diameter; int moons; } planet_t; 22

23 Data I/O Using Text and Binary Files (Opening for reading) Text File I/O FILE *fp; fp = fopen( planets.txt, r ); Binary File I/O FILE *fp; fp = fopen( planets.bin, rb ); 23

24 Data I/O Using Text and Binary Files (Opening for writing) Text File I/O FILE *fp; fp = fopen( planets.txt, w ); Binary File I/O FILE *fp; fp = fopen( planets.bin, wb ); 24

25 Data I/O Using Text and Binary Files (Read one structure from file) Suppose: planet_t p; Text File I/O fscanf(fp, %s %lf %d, p.name,&(p.diameter),&(p.moons)); Binary File I/O fread(&p,sizeof(planet_t), 1, fp); 25

26 Data I/O Using Text and Binary Files (Read N structure from file) Suppose: planet_t p[n]; Text File I/O for(i=0; i < N; i++) fscanf(fp, %s %lf %d, p[i].name,&(p[i].diameter),&(p[i].moons); Binary File I/O fread(p,sizeof(planet_t), N, fp); 26

27 Data I/O Using Text and Binary Files (Write one structure to file) Suppose: planet_t p; Text File I/O fprintf(fp, %s %lf %d, p.name,p.diameter,p.moons); Binary File I/O fwrite(&p,sizeof(planet_t), 1, fp); 27

28 Data I/O Using Text and Binary Files (Write N structure from file) Suppose: planet_t p[n]; Text File I/O for(i=0; i < N; i++) fprintf(fp, %s %lf %d, p[i].name, p[i].diameter, p[i].moons); Binary File I/O fwrite(p,sizeof(planet_t), N, fp); 28

29 Data I/O Using Text and Binary Files (Closing) Text File I/O fclose(fp); Binary File I/O fclose(fp); 29

30 Two other file operations functions that may be useful int remove(const char *filename); Defined in <stdio.h> Deletes the file specified by filename int rename(const char *oldname, const char *newname); Defined in <stdio.h> Changes the name of a file from oldname to newname 30

31 Disadvantages of Binary Files A binary file created on one computer is rarely readable on another type of computer Since a binary file can be read only by a specialized computer program, a person cannot proofread the file A binary file cannot be created or modified in a word processor, so a program that expects binary file input cannot be tested until the program that produces the binary file is complete 31

32 Random Files Random file access refers to the ability to read or modify information directly at any given position in a file This is done by getting and setting a file position indicator, which represents the current access position in the file associated with a given stream 32

33 Random Access Files Records in a file created with fprintf are not necessarily the same length Individual records of a randomly accessed file are normally fixed in length and may be accessed directly (thus quickly) without searching through records This makes random access files appropriate for systems that require rapid access to specific data 33

34 Random Access Files Every record in a randomly accessed file normally has the same length The exact location of a record relative to the beginning of the file can be calculated as a function of the record key 100 bytes 100 bytes 100 bytes 100 bytes 100 bytes etc byte offsets 34

35 Random Access Files Normally, structs are used in random access files Example program A program to process bank accounts Each record has an account number used as the record key, a last name, a first name and a balance First a random access file of 100 records is created with 0 for account number, NULL for first name and last name and 0.0 for balance 35

36 Example creating a random accessed file (from Deitel) /* Creating a randomly accessed file sequentially */ #include <stdio.h> #define NRECORDS 100 struct clientdata { int acctnum; char lastname[15]; char firstname[15]; double balance; }; 36

37 Example creating a random accessed file (from Deitel) int main() { int i; struct clientdata blankclient = {0, "", "", 0.0}; FILE *cfptr; if ( (cfptr = fopen("credit.dat", "wb")) == NULL) printf("file could not be opened.\n"); for (i = 1 ; i <= NRECORDS ; i++) fwrite(&blankclient, sizeof(struct clientdata), 1, cfptr); } fclose(cfptr); return 0; 37

38 Example writing data randomly to a random accessed file (from Deitel) /* Writing data randomly to a random accessed file */ #include <stdio.h> #define NRECORDS 100 struct clientdata { int acctnum; char lastname[15]; char firstname[15]; double balance; }; int main() { struct clientdata client = {0, "", "", 0.0}; FILE *cfptr; 38

39 Example writing data randomly to a random accessed file (from Deitel) if ( (cfptr = fopen("credit.dat", "r+b")) == NULL) printf("file could not be opened.\n"); printf("enter account number (1 to %d, 0 to end input)\n? ", NRECORDS); scanf("%d", &client.acctnum); 39

40 Example writing data randomly to a random accessed file (from Deitel) while(client.acctnum!= 0) { printf("enter lastname, firstname, balance\n? "); fscanf(stdin, "%s%s%lf", client.lastname, client.firstname, &client.balance); fseek(cfptr, (client.acctnum - 1) * sizeof(struct clientdata), SEEK_SET); fwrite(&client, sizeof(struct clientdata), 1, cfptr); printf("enter account number (1 to %d, 0 to end input)\n? ", NRECORDS); scanf("%d", &client.acctnum); } fclose(cfptr); } return 0; 40

41 fseek function int fseek (FILE *stream, long int offset, int whence) offset is the number of bytes from location whence in the file pointed to by stream whence can have one of three values SEEK_SET seek starts at the beginning of the file SEEK_CUR seek starts at the current location in the file SEEK_END seek starts at the end of the file 41

42 Example reading data from a random accessed file (from Deitel) /* Reading data from a random accessed file */ #include <stdio.h> struct clientdata { int acctnum; char lastname[15]; char firstname[15]; double balance; }; int main() { struct clientdata client = {0, "", "", 0.0}; FILE *cfptr; 42

43 Example reading data from a random accessed file (from Deitel) if ( (cfptr = fopen("credit.dat", "rb")) == NULL) printf("file could not be opened.\n"); printf("%-6s%-16s%-11s%10s\n","acct", "Last Name", "First Name", "Balance"); while(fread(&client, sizeof(struct clientdata), 1, cfptr)) if(client.acctnum!= 0) printf("%-6d%-16s%-11s%10.2f\n", client.acctnum, client.lastname, client.firstname, client.balance); fclose(cfptr); } return 0; 43