Lab 13

1. Lab 13 File I/O & Standard I/O

2. Introduction • Each regular file, device, connection socket, directory…etc, are treated as a file by Linux. • We can perform I/O operations on these different types of files. • Ex : open, read, write … etc • We will start our discussion of these functions on: • Basic file I/O • Standard I/O library NCHU System & Network Lab

3. File I/O • A set of methods dealing with files provided by Linux. • Each opened file is referred to a file descriptor: • fd is a non-negative integer . • creat() and open() functions return a fd to process. • UNIX system shells associate file descriptors : • 0 : standard input • 1 : standard output • 2 : standard error NCHU System & Network Lab

4. File I/O (cont.) • These are functions introduced later : #include <fcntl.h>#include <unistd.h> int open (const char *pathname, int oflag, mode_t mode); int creat (const char *pathname, mode_t mode); int close (int filedes); off_t lseek (int filedes, off_t offset, int whence); ssize_t read (int filedes, void *buf, size_t nbytes); ssize_t write (int filedes, const void *buf, size_t nbytes); NCHU System & Network Lab

5. open() • A file is opened or created by calling open() function. • pathname is the name of the file. • This function has a multitude of options are specified by oflag argument. • mode specifies the access permission of file. #include <fcntl.h>int open (const char *pathname, int oflag, mode_t mode); NCHU System & Network Lab

6. open (cont.) • oflags of open() NCHU System & Network Lab

7. creat() • This create function is equivalent to open (pathname, O_WRONLY|O_CREAT|O_TRUNC, mode) • One deficiency with creat() is that the file is opened only for writing. • A better way is to use open() with O_CREAT instead of creat(). #include <fcntl.h> int creat (const char *pathname, mode_t mode); NCHU System & Network Lab

8. lseek() • Every open file has an associated “current file offset” • A non-negative integer that measures the number of bytes from the beginning of the file. • Read/Write operation increments cur_offset value. • The interpretation of the offset argument depends on the value of whence argument . #include <unistd.h> off_t lseek (int filedes, off_t offset, int whence); NCHU System & Network Lab

9. read()/write() • Data is read from an open file with the read() function. • It will read nbytes bytes into buf from file filedes. • Return values • Count number of read bytes , 0 if EOF, -1 on error #include <unistd.h>ssize_t read (int filedes, void *buf, size_t nbytes);ssize_t write (int filedes, void *buf, size_t nbytes); NCHU System & Network Lab

10. Consistency • Traditional implementations of UNIX system have a buffer cache in the kernel which most disk I/O passes through. • Ex : delayed write • Three functions are provided to ensure consistency of the file system on disk with the data of buffer. • sync() • fsync() • fdatasync() #include <unistd.h> int fsync (int fd);int fdatasync (int fd); void sync(void); NCHU System & Network Lab

11. Consistency (cont.) • Sync () • It simply schedules all the modified block buffers in RAM to be write into disk , and it returns without waiting for write completed. • fsync() • It refers only to a single file , specified by the fd, and waits for the disk writes to complete before returning. • fdatasync() • It affects only data portions of a file. NCHU System & Network Lab

12. Example : Basic I/O #include <unistd.h> #include <sys/stat.h> #include <fcntl.h> #include <stdlib.h> #define BUFFERSIZE 1 int main() { char buf[BUFFERSIZE]; int in,out,readn; in = open("file.in",O_RDONLY); out = open("file.out",O_WRONLY | O_CREAT,S_IRUSR | S_IWUSR); while((readn = read(in,buf,sizeof(buf))) > 0) write(out,buf,readn); exit(0); } NCHU System & Network Lab

13. Standard I/O Library • This library is specified by ISO C standard • Each opened file is associated with a stream. • A file is opened with a FILE structure. • FILE is a structure that contains all information required by the standard I/O library to manage the stream. • Buffer allocation and optimal I/O. NCHU System & Network Lab

14. Buffering of Standard I/O • The goal of the buffering is to use minimum number of read() and write() calls. • Types of buffering : • Fully buffered • Line buffered • The library performs I/O when a newline character is encountered. • Unbuffered NCHU System & Network Lab

15. Buffering of Standard I/O (cont.) • We can change the buffering by calling these two functions before any other operations on the stream: • setbuf() turns buffering on or off with a buffer bufof length BUFSIZ or NULL. • setvbuf() specifies exactly which type of buffering. #include <stdio.h>void setbuf (FILE *restrict fp, char *restrict buf);int setvbuf (FILE *restrict fp, char *restrict buf, int mode, size_t size); NCHU System & Network Lab

16. Opening a Stream • fopen() function opens a specified file and returns a *FILE pointer of the stream. #include <stdio.h>FILE *fopen (char *restrict pathname, char *restrict type); NCHU System & Network Lab

17. Other Functions for Standard I/O #include <stdio.h> Int getc (FILE *fp);int fgetc (FILE *fp);int putc (FILE *fp);int fputc (int c,FILE *fp) char *gets (char *buf);char *fgets (char *buf, int n, FILE *fp);char *puts (char *str);char *fputs (char *str, FILE *fp); int printf (char *format, …);int fprintf (FILE *fp, const char *format, …); int scanf (const char *restrict format, … );int fscanf (FILE *fp, char *format, …); NCHU System & Network Lab

18. File I/O vs. Standard I/O • Standard I/O library ends up calling basic I/O routines . • Standard I/O is specified by ISO C standard, and another one is specified by POSIX.1(Portable Operating System Interface) . • POSIX.1 includes ISO C standard library. • fopen() deals with FILE structure whereas open() uses a file descriptor integer. NCHU System & Network Lab

19. File I/O vs. Standard I/O (cont.) • More flexible buffering than basic I/O that take place with standard library. • Device files only can be opened by open(). NCHU System & Network Lab

20. Example : Standard I/O NCHU System & Network Lab

21. Lab • I/O efficiency • We want to know how does File I/O or standard I/O work to improve I/O efficiency. • Create a 2MB file and copy it into a new output file. • Use “clock()” to measure the process time of each case. • Repeat the step above in different cases and show the result of all : • Basic I/O • write() to output file 1 byte each time • write() to output file 64 bytes each time • open() with O_SYNC set , write()64 bytes each time • write() followed by fsync(), 64 byes each time • Standard I/O • fopen() with setbuf( unbuffered ) • fopen() NCHU System & Network Lab

22. Lab (cont.) • Clock() • Returns the number of clock ticks elapsed since the program was launched. • CLOCKS_PER_SEC represents the number of clocks in a second. #include <stdio.h>#include <time.h> clock_t start,end;start = clock();delay(2000);end = clock();elapsetime = (end-start)/(double)CLOCKS_PERSEC ; NCHU System & Network Lab

23. Lab (cont.) NCHU System & Network Lab

24. Reference • Advanced Programming in the UNIX Environment 2nd Author : Richard Stevens, Stephen A.Rago, Publisher : Addison-Wesley • Beginning Linux ProgrammingAuthor : Richard Stones, Neil Matthew Publisher : Wrox • http://linux.vbird.org/ • http://www.jollen.org/blog/ jollen’s Blog • http://www.cplusplus.com/ C++ Resource Network NCHU System & Network Lab