Introduction to C Lecture 8: Input / Output Operating System

1. Introduction to CLecture 8: Input / Output Operating System

2. Computer ProgrammingConcepts • Sequence • บน ลง ล่าง, ซ้าย ไป ขาว • Conditions • If, if else, if else if, switch • Iterations • For (statement ; statement ; statement) • While ( statement) do • Do While (statement)

3. Programming Concepts • First, make your program works. • Then make it pretty. • โปรแกรมคอมพิวเตอร์ ความถูกต้องจะต้องมาก่อนความสวยงาม • แต่โปรแกรมต้องสวยงาม

4. Review • Arrays • What is an array? • Array declarations • Array Initializations • Array Accessing

5. Review • String Functions • Length? • Manipulation? • Comparison? • Searching? • What’s required?

6. Review • Functions • What is function? • Why we use function? • How many type of parameter passing? • What are they different?

7. Files in C Information about the file and its state: • Location of the file on the disk • Type of the file • Allowed operations • Present position in the file This information is kept lumped together in a structure. This structure has the generic name FILE, and it is defined in the header file stdio.h. FILE *ifp, *ofp; /* pointers for input and output file */ You do not need to know details of this structure: after assigning a pointer to a file structure, it is used for opening the file, reading from it or writing to it, and finally closing it.

8. Pointers to structure The modes for opening files It is good pratice to test if file was opened sucessfully: ifp = fopen("input.dat", "r"); if( ifp == NULL) { printf("can't open file\n"); return 1; } Example: File i/o Standard method for opening and closing files in C: #include <stdio.h> int main(void) { int a, sum = 0; FILE *ifp, *ofp /* input file pointer, output file pointer */ ifp = fopen(“my_file”, “r”); /* open for reading */ ofp = fopen(“outfile”, “w”); /* open for writing */ ..... fclose(ifp); fclose(ofp);

9. File modes for fopen FILE *ifp;  ifp = fopen(“filename”, “r”); MS-DOS and MS-Windows have different modes for writing binary files: "ab“, “rb“, “wb“, "ab+“, “rb+“, “wb+“

10. In the file i/o functions, a file pointer has to be specified. fprintf(ifp, control_string, other_args); fscanf (ifp, control_string, other_args); example:“%s %12.2f\n” three predefined special file pointers: stdinpoints to: keyboard stdoutpoints to: screen stderrpoints to: screen fprintf(), fscanf() To write to files: function fprintf() analogous to printf() To read from files: function fscanf() analogous to scanf() printf() and scanf() are special cases of file i/o: printf() is equivalent to fprintf(stdout, ....) scanf() is equivalent to fscanf(stdin, ....)

11. EOF (end-of-file) character is not in the usual ASCII character set (0-255), Use int instead of charif EOF is to be read getchar() and putchar(c) are defined as macros: #define getchar() getc(stdin) #define putchar(c) putc((c), stdout) getc(), putc() To write character to file: putc(c,ofp) analogous to putchar(c) To read character from file: getc(ifp) analogous to getchar() int c; c = getc(ifp); putc(c, ofp);

12. ฟังก์ชัน sprintf() และ sscanf() • ฟังก์ชัน sprintf() และ sscanf() เป็นฟังก์ชันสำหรับข้อมูลชนิดสายอักขระของ printf() และ scanf() int sprintf(char *s, const char *format, …); int sscanf(const char *s, const char *format, …);

13. ตัวอย่าง char str1[]=“1 2 3 go”, str2[100], tmp[100]; int a,b,c; … sscanf(str1,”%d%d%d%s”,&a,&b,&c,tmp); sprintf(str2,”%s %s %d %d %d\n”,tmp,tmp,a,b,c); printf(“%s”,str2); Sprintf(str2,”DSC%d.jpg”,i); If (i==999) i=1;

14. Reading From a File • Steps • Declare FILE variable • Open the file (read mode) • Read from the file • Close the file

15. ฟังก์ชัน fopen() และ fclose() • ในเชิงนามธรรม แฟ้มข้อมูล คือ “สายของอักขระ” ดังนั้น ต้องมีตัวดำเนินการจัดการเปิด และ ปิด

16. Declare File Variable • Format: FILE *name; • FILE type defined in <stdio.h> • Variable must be a pointer • E.g., FILE *infileptr;

17. Open the File • Format: fptr = fopen(“file”,”r”); • E.g, infileptr = fopen(“my_file”,”r”); • “my_file” – file name • “r” – specifies read mode • fptr is always set to the beginning of the file • Check for successful file open • if ((infileptr = fopen(“my_file”,”r”)) != NULL)

18. ตัวอย่าง #include <stdio.h> int main() { int a, sum=0; FILE *ifp, *ofp; ifp = fopen(“Input.txt”,”r”); ofp = fopen(“output.txt”,”w”); … while (fscanf(ifp, “%d”, &a)==1 ) sum+=a; fprintf(ofp,”The sum is %d.\n”,sum); … fclose(ifp); fclose(ofp); }

19. Read from the File • One character at a time • Format: c = getc(ifp) • getc reads from file pointer (ifp) and returns a character • Similar to getchar • Example • while ((c = getc(ifp)) != EOF) • Continues reading characters until it reaches the end of the file • c = fgetc(ifp) does the same thing

20. Read from the File • Multiple characters at a time • Format: fgets (str, len, ifp) • Reads at most len – 1 chars from file pointer (ifp) and store into the char array pointed to by str • If new line or end of file encountered, reading stops • Null char added to end of array • Example • char line[100], *s; • while (fgets(s, 100, ifp) != EOF)

21. Read from the File • Structured input • Format: fscanf(fptr,”cntrl”, args) • Reads from file pointed to by file pointer (fptr) • Uses control string just like scanf • Args must be pointers • Example • while (fscanf(ifp, “%d %s\n”, &num, str) != EOF) • Reads an integer, string and new line from each line of the file pointed to by infileptr

22. Close the File • Format: fclose(fptr); • Does everything to break the file connection

23. File Reading Example • Sum the numbers in a file #include <stdio.h> void main () { int num, sum = 0; FILE *inptr; if ((inptr = fopen(“infile”, “r”)) != NULL) { while(fscanf(inptr, “%d”, &num) != EOF) sum += num; fclose(inptr); printf(“The sum of the numbers is %d\n”,sum); } }

24. Programming Exercise • Write a program that • Reads a list of first and last names from the file phonebook • The first and last names are stored one per line • Prints each entry as last name, first name

25. Writing to a File • Steps • Declare FILE variable • Same as before • Open the file (write mode) • Write to the file • Close the file • Same as before

26. Open the File • Format: fptr = fopen(“file”,”w”); • E.g, infileptr = fopen(“my_file”,”w”); • Creates new file • Overwrites file if it exists • Starts writing at the beginning of the file • fopen(“file”, “a”) appends a file • Creates new file if it doesn’t exist • Starts writing from the beginning • Starts writing at the end of an existing file

27. Write to the File • One character at a time • Format: putc(char, fptr) • putc writes a character to file pointer (fptr) Similar to putchar • Example • putc(‘\n’, outfileptr); • Writes a newline to the file • fputc(char, fptr) does the same thing

28. Write to the File • Multiple characters at a time • Format: fputs (str, fptr) • Writes the null-terminated string to the file • Does not write the null character • Example • char s[6] = “Hello\0”; • fputs(s, outfileptr);

29. Write to the File • Structured output • Format: fprintf(fptr,”cntrl”, args) • Writes to the file pointed to by file pointer (fptr) • Uses control string just like printf • Example • fprintf(outfileptr, “%d %s\n”, num, str); • Writes an integer and string to the file

30. File Writing Example • Write the sum of the numbers to a file #include <stdio.h> void main () { int num, sum = 0; FILE *inptr, *outptr; if ((inptr = fopen(“infile”, “r”)) != NULL) { while(fscanf(inptr, “%d”, &num) != EOF) sum += num; fclose(inptr); if ((outptr = fopen(“outfile”, “w”)) != NULL) { fprintf(outptr,“The sum of the numbers is %d\n”,sum); fclose(outptr); } } }

31. Programming Exercise • Write a program that • Reads a list of first and last names from the file phonebook • The first and last names are stored one per line • Writes each entry as last name, first name to the file phonebook2 • The entries are stored one per line

32. Moving around in files When reading from or writing to a file, the system keeps track of thecurrent position in the file. Subsequent calls to fprintf() or fscanf() can then carry on where they left off, rather than starting again from the beginning. However it is often useful to access a file at some random position instead. rewind(file_ptr); resets the position indicator to start of file ftell(file_ptr); returns the current value of file position indicator: i.e. no. of bytes from start of file fseek(file_ptr, offset, place); moves file position indicator to specified position The fseek function moves the indicator to offset bytes from place: place can be SEEK_SET, SEEK_CUR or SEEK_END (actually 0, 1 or 2) for beginning, current position or end of file espectively. Offset can be negative as well as positive.

33. Notice we have to back up 2 chars, because the act of reading in one char increments the file position indicator by 1. Example of moving within a file /* Write a file backwards */ #include <stdio.h> #define MAXSTRING 100 int main(void) { char fname[MAXSTRING]; int c; FILE *ifp;   fprintf(stdout, "\nInput a filename: "); scanf("%s", fname); ifp = fopen(fname, "r"); /* open for reading */ fseek(ifp, 0, SEEK_END); /* move to end of file */ fseek(ifp, -1, SEEK_CUR); /* back up one char */ while (ftell(ifp) > 0) { c = getc(ifp); /* move ahead one char */ putchar(c); fseek(ifp, -2, SEEK_CUR); /* back up 2 chars */ } return 0; }

34. ls > junk.dat writes a directory listing to the file junk.dat, (using the redirection operator > ) UNIX example: Executing system commands System commands are executed by use of the function system(). Example: system(“date”); passes the string “date” to the operating system, which executes it before returning control to the program. The following code asks the system to write the directory to a temporary file, then reads it in from the temporary file and writes out to the screen in uppercase.

35. Integer, not char, to read EOF-character hard-coded string length limitunlikely to be exceeded! The function tmpnam(NULL) returns a name for a temporary file (that doesn’t exist already), e.g. temp.dat writes into the string command_string equivalent to “ls > temp.dat” command string “ls > temp.dat” is sent to the system open the temporary file for reading directory information from it read from file one character at a time, convert to uppercase and put out to the screen delete the temporary file Example #include <ctype.h> #include <stdio.h> #include <stdlib.h> int main(void) { int c; char command_string[100], *tmp_filename; FILE *ifp;   tmp_filename = tmpnam(NULL);sprintf(command_string, "ls > %s", tmp_filename); system(command_string); ifp = fopen(tmp_filename, "r"); while ((c = getc(ifp)) != EOF) putchar(toupper(c)); remove(tmp_filename); return 0; }

36. Open a pipe to the operating system... ... only for reading ... ... and ask it to send a directory listing read from the pipe one character at a time, convert to uppercase and put out to the screen ... and close the pipe Pipes In Unix, you can use “pipes” to communicate directly with the operating system; this avoids the need for the temporary file in the previous example. Example: uppercase2.c #include <ctype.h> #include <stdio.h> int main(void) { int c; FILE *ifp; ifp = popen("ls", "r"); /* open pipe */ while ((c = getc(ifp)) != EOF) putchar(toupper(c)); pclose(ifp); return 0; }

37. argcis the number of arguments argv[] are the arguments Arguments to main( ) Two arguments, conventionally called argc and argv, can be used with main() to communicate with the operating system. argc counts the arguments (including the program name itself), argv[] is an array of strings corresponding to the arguments. Filenames are often passed in this way. Example: echoinput.c /* Echoing command line arguments */#include <stdio.h>int main(int argc, char *argv[]) { int i; printf("argc = %d\n", argc); for (i = 0; i < argc; ++i) printf("argv[%d] = %s\n", i, argv[i]); return 0;} echoinput file1 file2 then argc will be 3; argv[0] will be echoinput argv[1] will be file1 argv[2] will be file2

38. Numeric arguments Only strings can be passed via the command line. In order to get numeric input, they must be converted to numers: atoi() for converting a string to an integer atof() for converting a string to a double #include <stdio.h>int main(int argc, char *argv[]) { int i; double x; i = atoi(argv[1]); x = atof(argv[2]); printf(“Input = %d, %lf\n",i,x); return 0; } Call with: ./program 123 4.567

39. The C compiler cc [filename.c] Options include: -o [output file name] -lm to include the maths library -E to get output from preprocessor only (no compilation). The compiler works in three stages: it preprocesses (replacing the #include, #define etc); it compiles; it links all of the pieces of code together into the executable program. For a large program you can compile a small piece at a time, and link it to larger, pre-compiled parts. To compile only, use: cc -c [filename.c]This creates object files (whose names end in .o), which can then be linked later

40. ar= archive ruv= replace, update, verbose object files to go in the library g_lib.a= output archive file name C Compiler: Libraries It is possible to create libraries, which are (generally large) collections of useful object code ready for linking, so they don’t need to be compiled over and over again for different programs. To do this with a set of .o files, ar ruv g_lib.a gfopen.o gfclose.o gcalloc.o....... This makes a library file called g_lib.a. Follow this by ranlib g_lib.a which organises the file in a form that is useful for the linker. To make the functions available to other programs, add the library tothe list of compiled files: cc -o pgm main.c file1.c g_lib.a

41. Conditional compilation The preprocessor can be used to exclude code from compilation: #define DEBUG 1 /*set debug to true at top of file*/......#if DEBUG printf(“debug: a = %d\n”, a);#endif Because DEBUG has value 1, the printf statements will be compiled. The statements can be turned off by setting DEBUG = 0 at the start. An alternative is #ifdef DEBUG ....#endifwhich will include the lines if DEBUG is defined at all at the top. There is also an #ifndef, which includes lines if a name is not defined. There is also an #undef function to remove previous definitions, in order to prevent clashes. To match the #if, there is an #else; there is also #elif, which is short for else if.

42. Preprocessor macros with arguments #define SQ(x) ((x) * (x)) will replace, e.g., SQ(a + b) by ((a + b) * (a + b)) . Note here that all of the parentheses are needed! #define min(x, y) (((x) < (y)) ? (x) : (y)) will replace an expression such asm = min(u, v)bym = (((u) < (v)) ? (u) : (v))The arguments of m can be arbitrary expressions of comparable type. Warning: Debugging code that contains macros with arguments can be very difficult!

43. Writing large programs A large program will normally be stored as a collection of .h and .c files in its own directory. If we are developing a program called pgm, we make a header file pgm.h, which contains #includes, #defines and a list of function prototypes. All .c files should then have #include “pgm.h” at the top. Note the use of quotes rather than in < >, when you include your own header files. The compiler then looks in the current directory rather than in the directory for standard C header files. Compile with the usual command, listing all of the .c files and libraries, but not the .h files, which are included automatically by the #include preprocessor commands.

44. this is only compiled if MYHEADERFILE1 is not defined (i.e. if the header file has not been processed before) Header files in large programs Often the same header files are included in several different source code files. How to avoid multiple definitions when the header file is compiled repeatedly? Let us assume we have a header file called MyHeaderFile1.h.: #ifndef MYHEADERFILE1#define MYHEADERFILE1 .... function prototypes; .... definitions; #endif Thus, the material in the file is only included if it hasn’t been included already.

45. End • Strictly follow the C-syntax • Don‘t forget to prototype all your functions • Use indentation to get your bracketing right • Use parentheses to clarify operator precedence • Use typecasting (double) in integer divisions • Use pointers to pass variables to functions