1 1 I/O Management!2 Goals of this Lecture!• Help you to learn about:"• The Unix stream concept"• Standard C I/O functions"• Unix system-level functions for I/O"• How the standard C I/O functions use the Unix system-level functions"• Additional abstractions provided by the standard C I/O functions"Streams are a beautiful Unix abstraction"2 3 Stream Abstraction!• Any source of input or destination for output"• E.g., keyboard as input, and screen as output"• E.g., files on disk or CD, network ports, printer port, …"• Accessed in C programs through file pointers"• E.g., FILE *fp1, *fp2; • E.g., fp1 = fopen("myfile.txt", "r"); • Three streams provided by stdio.h"• Streams stdin, stdout, and stderr!• Typically map to keyboard, screen, and screen"• Can redirect to correspond to other streams"• E.g., stdin can be the output of another program"• E.g., stdout can be the input to another program"4 Sequential Access to a Stream!• Each stream has an associated file position"• Starting at beginning of file (if opened to read or write)"• Or, starting at end of file (if opened to append)"• Read/write operations advance the file position"• Allows sequencing through the file in sequential manner"• Support for random access to the stream"• Functions to learn current position and seek to new one"file file3 5 Standard I/O Functions!• Portability"• Generic I/O support for C programs"• Specific implementations for various host OSes"• Invokes the OS-specific system calls for I/O"• Abstractions for C programs"• Streams"• Line-by-line input"• Formatted output "• Additional optimizations"• Buffered I/O"• Safe writing"File System Stdio Library Appl Prog user OS 6 Example: Opening a File !• FILE *fopen("myfile.txt", "r") • Open the named file and return a stream"• Includes a mode, such as “r” for read or “w” for write"• Creates a FILE data structure for the file"• Mode, status, buffer, …"• Assigns fields and returns a pointer"• Opens or creates the file, based on the mode"• Write (ʻwʼ): create file with default permissions"• Read (ʻrʼ): open the file as read-only"• Append (ʻaʼ): open or create file, and seek to the end"4 7 Example: Formatted I/O!• int fprintf(fp1, "Number: %d\n", i)"• Convert and write output to stream in specified format"• int fscanf(fp1, "FooBar: %d", &i) • Read from stream in format and assign converted values"• Specialized versions"• printf(…) is just fprintf(stdout, …) • scanf(…) is just fscanf(stdin, …) 8 Layers of Abstraction!Disk!Driver!Storage!File System!disk blocks"variable-length segments"hierarchical file system"Operating"System"Stdio Library!FILE * stream"Appl Pgm!User"process"int fd!File descriptor:"An integer that"uniquely identifies"an open file"5 9 System-Level Functions for I/O!int creat(char *pathname, mode_t mode); • Create a new file named pathname, and return a file descriptor"int open(char *pathname, int flags, mode_t mode); • Open the file pathname and return a file descriptor"int close(int fd); • Close fd int read(int fd, void *buf, int count); • Read up to count bytes from fd into the buffer at buf ""int write(int fd, void *buf, int count); • Writes up to count bytes into fd from the buffer at buf int lseek(int fd, int offset, int whence); • Assigns the file pointer of fd to a new value by applying an offset"10 Example: open() • Converts a path name into a file descriptor"• int open(const char *pathname, int flags, mode_t mode); • Arguments"• Pathname: name of the file"• Flags: bit flags for O_RDONLY, O_WRONLY, O_RDWR • Mode: permissions to set if file must be created"• Returns"• File descriptor (or a -1 if an error)"• Performs a variety of checks"• E.g., whether the process is entitled to access the file"• Underlies fopen()6 11 Example: read() • Reads bytes from a file descriptor"• int read(int fd, void *buf, int count); • Arguments"• File descriptor: integer descriptor returned by open() • Buffer: pointer to memory to store the bytes it reads"• Count: maximum number of bytes to read • Returns"• Number of bytes read"• Value of 0 if nothing more to read"• Value of -1 if an error"• Performs a variety of checks"• Whether file has been opened, whether reading is okay"• Underlies getchar() , fgets(), scanf() , etc."12 Example: A Simple getchar() int getchar(void) { char c; if (read(0, &c, 1) == 1) return c; else return EOF; } • Read one character from stdin • File descriptor 0 is stdin • &c points to the buffer"• 1 is the number of bytes to read"• Read returns the number of bytes read "• In this case, 1 byte means success"7 13 Making getchar() More Efficient!• Poor performance reading one byte at a time"• Read system call is accessing the device (e.g., a disk)"• Reading one byte from disk is very time consuming"• Better to read and write in larger chunks!• Buffered I/O"• Read a large chunk from disk into a buffer"• Dole out bytes to the user process as needed"• Discard buffer contents when the stream is closed"• Similarly, for writing, write individual bytes to a buffer"• And write to disk when full, or when stream is closed"• Known as “flushing” the buffer"14 Better getchar() with Buffered I/O!int getchar(void) { static char base[1024]; static char *ptr; static int cnt = 0; if (cnt--) return *ptr++; cnt = read(0, base, sizeof(base)); if (cnt <= 0) return EOF; ptr = base; return getchar(); } persistent variables base ptr But, many functions may read (or write) the stream…8 15 Details of FILE in stdio.h (K&R 8.5)!#define OPEN_MAX 20 /* max files open at once */ typedef struct _iobuf { int cnt; /* num chars left in buffer */ char *ptr; /* ptr to next char in buffer */ char *base; /* beginning of buffer */ int flag; /* open mode flags, etc. */ char fd; /* file descriptor */ } FILE; extern FILE _iob[OPEN_MAX]; #define stdin (&_iob[0]) #define stdout (&_iob[1]) #define stderr (&_iob[2]) 16 A Funny Thing About Buffered I/O!int main(void) { printf("Step 1\n"); sleep(10); printf("Step 2\n");
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