CSCI 171

1. CSCI 171 Presentation 12 Files

2. Working with files • File Streams – sequence of data that is connected with a specific file • Text Stream – Made up of lines of characters • Each line has 0 – 255 characters ending in a carriage return • Carriage return is converted to operating systems end of line character • Used for text files • Binary stream – data read and written unchanged (in binary format) • Used for binary files

3. File names • Names must adhere to operating system rules • 8.3 for DOS • 256 characters for Windows 95, 98, NT • 256 for most UNIX systems • Letters, Numbers, and Special characters are allowed in names

4. File names • Strings are used for file names • Recall the slash (\) has special meaning to C • Examples: • char * fileName = “c:\\winnt\\myfile.txt”;char * fileName = “c:/winnt/myfile.txt”;

5. Opening files • You cannot use a file until it has been opened • The fopen function is located in stdio.h • Accepts 2 argument: • filename (string) • mode (string) • Returns a file pointer

6. Available modes • What mode a file is opened in dictates how the file is opened • Files may be opened for reading or writing • Files may be opened in text mode or binary mode

7. Available Text Modes • Modes are available for to open a file for read and write

8. Available Binary Modes • Modes are available for to open a file for read and write

9. The fopen function #include <stdio.h> void main( void ) { FILE * fptr; char * fileName = "c:/temp/test.txt"; char * mode = "r"; if ((fptr = fopen(fileName, mode)) == NULL) { printf("Opening file %s caused an error.", fileName); exit(1); } //Processing continues }

10. Types of files • Two main types • Text files • Can be viewed by user • Read by other applications (db, spreadsheet, other programs in other languages, etc.) • Binary files • Cannot be viewed by user • Read by other programs

11. Types of files • Three main structures to data files • Specified number of records • Usually specified in first record • for loops • Trailer or sentinel signal • Usually specified in last record • while loops • End-of-file indicator • Every file contains a system-readable end-of-file • feof function in conjunction with while loops

12. Text file output • Functions in stdio.h • fprintf • most common • fputs • putc • no fputc • must include file name

13. fprintf • fprintf works like the printf function • fprintf allows you to tell the compiler what file you are using • printf automatically uses stdout • prototype comparison: • int fprintf(FILE *fp, char *fmt, …); • int printf(char *fmt, …);

14. fprintf example • #include <stdio.h> • #include <stdlib.h> • int main( void ) { • FILE * fp; • int x = 3; • if ((fp = fopen("c:/test.txt", "w")) == NULL) { • printf("Error opening file!"); • exit(1); • } • fprintf(fp, "The value of x is: %d", x); • return 0; • }

15. Text file input • Functions in stdio.h • fscanf • most common • fgets • getc and fgetc • identical • must include file name

16. fscanf • fscanf works like the scanf function • fscanf allows you to tell the compiler what file you are using • scanf automatically uses stdout • prototype comparison: • int fscanf(FILE *fp, char *fmt, …); • int scanf(char *fmt, …);

17. fscanf example • //Assume test.txt is a text file with an integer in it: • #include <stdio.h> • #include <stdlib.h> • int main( void ) { • FILE * fp; • int x = 0; • if ((fp = fopen("c:/test.txt", "r")) == NULL) { • printf("Error opening file!"); • exit(1); • } • fscanf(fp, "%d", &x); • printf("The value of x is: %d", x); • return 0; • }

18. Binary file output • fwrite() function • prototype: • fwrite(void *buf, int size, int count, FILE *fp) • buf points to the data • size tells the size of the objects to be written • count tells how many objects are to be written • fp tells which file is being written to

19. fwrite example • #include <stdio.h> • #include <stdlib.h> • int main( void ) { • FILE * fp; • int x[5] = {1, 2, 3, 4, 5}; • if ((fp = fopen("c:/test.dat", "wb")) == NULL) { • printf("Error opening file!"); • exit(1); • } • fwrite(x, sizeof(int), 5, fp); • return 0; • }

20. Binary file input • fread() function • prototype: • fread(void *buf, int size, int count, FILE *fp) • buf points to the data • size tells the size of the objects to be read • count tells how many objects are to be read • fp tells which file is being read from

21. fread example • #include <stdio.h> • #include <stdlib.h> • int main( void ) { • FILE * fp; • int x[5], i; • if ((fp = fopen("c:/test.dat", "rb")) == NULL) { • printf("Error opening file!"); • exit(1); • } • fread(x, sizeof(int), 5, fp); • for (i = 0; i < 5; i++) • printf("%d ", x[i]); • return 0; • }

22. Closing and flushing files • All files should be flushed and closed prior to program termination • flushing writes the data from the buffer to the actual file • files written in blocks, streams written in characters do data is buffered prior to being written • closing literally closes the file – needs to be reopened to be used again • all files are closed by the compiler at program termination, but this process should be handled by the programmer

23. fclose • fclose will flush and close a file • prototype: • int fclose(FILE *fp); • fcloseall will flush and close all but the standard files • Prototype int fcloseall( ); • Not ANSI standard • will not close stdout, stdin, etc.