The C Programming Language P8.5

#include 
#define PERMS 0666

#define NULL 0
#define EOF (-1)
#define BUFSIZE 1024
#define OPEN_MAX 20/* max #files open at once */

typedef struct _iobuf {
	int cnt; /* characters left */
	char *ptr; /* next character position */
	char *base; /* location of buffer */
	int flag; /* mode of file access */
	int fd; /* file descriptor */
} FILE;
extern FILE _iob[OPEN_MAX];
#define stdin (&_iob[0])
#define stdout (&_iob[1])
#define stderr (&_iob[2])

enum _flags {
	_READ = 01, /* file open for reading */
	_WRITE = 02, /* file open for writing */
	_UNBUF = 04, /* file is ubuffered */
	_EOF = 010, /* EOF has occured on this file */
	_ERR = 020 /* error occurred on this file */
};
int _fillbuf(FILE *);
int _flushbuf(int, FILE *);
FILE *fopen(char *name, char *mode);
int fflush(FILE *);
int fclose(FILE *);
int fseek(FILE *fp, long offset, int origin);

#define feof(p) (((p)->flag&_EOF)!=0)
#define ferror(p) (((p)->flag&_ERR)!=0)
#define fileno(p) ((p)->fd)

#define getc(p) (--(p)->cnt>=0\
		?(unsigned char) *(p)->ptr++:_fillbuf(p))

#define putc(x,p) (--(p)->cnt>=0\
		?*(p)->ptr++=(x):_flushbuf((x),p))

#define getchar() getc(stdin)
#define putchar(x) putc((x),stdout)

FILE _iob[OPEN_MAX] = { { 0, (char *) 0, (char *) 0, _READ, 0 }, { 0,
		(char *) 0, (char *) 0, _WRITE, 1 }, { 0, (char *) 0, (char *) 0, _WRITE
		| _UNBUF, 2 } };

int fseek(FILE *fp, long offset, int origin) {
	int result;
	if (!(fp->flag & _UNBUF) && fp->base != NULL) {
		if (fp->flag & _WRITE) {
			if (fflush(fp))
				return EOF;
		} else if (fp->flag & _READ) {
			if (origin == 1) {
				if (offset >= 0 && offset <= fp->cnt) {
					fp->cnt -= offset;
					fp->ptr += offset;
					fp->flag &= ~_EOF;
					return 0;
				} else
					offset -= fp->cnt;
			}
			fp->cnt = 0;
			fp->ptr = fp->base;
		}
	}
	result = (lseek(fp->fd, offset, origin) < 0);
	if (result == 0)
		fp->flag &= ~_EOF;
	return result;
}

int _flush(FILE *fp) {
	return _flushbuf(EOF, fp);
}

int fflush(FILE *fp) {
	int retVal = 0;
	FILE *fpp = _iob;
	if (fp != NULL) {
		return _flush(fp);
	} else {
		while (fpp++ < _iob + OPEN_MAX)
			if (_flush(fpp) == EOF)
				retVal = EOF;
	}
	return retVal;
}
int fclose(FILE *fp) {
	if (fp == NULL)
		return EOF;
	if (fp < _iob || fp >= _iob + OPEN_MAX)
		return EOF;

	_flush(fp);

	if (fp->base != NULL)
		free(fp->base);
	fp->base = fp->ptr = NULL;
	fp->cnt = 0;
	fp->flag = 0;
	close(fp->fd);
	return 0;
}

int _fillbuf(FILE *fp) {
	int bufsize;
	if ((fp->flag & (_READ | _EOF | _ERR)) != _READ)
		return EOF;
	bufsize = (fp->flag & _UNBUF) ? 1 : BUFSIZE;
	if (fp->base == NULL)
		if ((fp->base = (char *) malloc(bufsize)) == NULL)
			return EOF;
	fp->ptr = fp->base;
	fp->cnt = read(fp->fd, fp->ptr, bufsize);
	if (--fp->cnt < 0) {
		if (fp->cnt == -1)
			fp->flag |= _EOF;
		else
			fp->flag |= _ERR;
		fp->cnt = 0;
		return EOF;
	}
	return (unsigned char) *fp->ptr++;
}

int _flushbuf(int c, FILE *fp) {
	int bufsize;
	int num_written;

	if (fp < &_iob[0] || fp >= &_iob[OPEN_MAX])
		return EOF;
	if ((fp->flag & (_WRITE | _EOF | _ERR)) != _WRITE)
		return EOF;
	bufsize = (fp->flag & _UNBUF) ? 1 : BUFSIZE;

	if (fp->base == NULL) {
		if ((fp->base = (char *) malloc(bufsize)) == NULL) {
			fp->flag |= _ERR;
			return EOF;
		}
	} else {
		num_written = fp->ptr - fp->base;
		if (write(fp->fd, fp->base, num_written) != num_written) {
			fp->flag |= _ERR;
			return EOF;
		}
	}

	fp->ptr = fp->base;
	fp->cnt = bufsize;

	if (--fp->cnt < 0) {
		if (fp->cnt == -1)
			fp->flag |= _EOF;
		else
			fp->flag |= _ERR;
		return EOF;
	}
	if(c!=EOF)
	*fp->ptr++ = c;
	return c;
}

FILE *fopen(char *name, char *mode) {
	int fd;
	FILE *fp;
	if (*mode != 'r' && *mode != 'w' && *mode != 'a')
		return NULL;
	for (fp = _iob; fp < _iob + OPEN_MAX; fp++)
		if (fp->flag & (_READ ^ _WRITE))
			break;
	if (fp >= _iob + OPEN_MAX)
		return NULL;

	if (*mode == 'w')
		fd = creat(name, PERMS);
	else if (*mode == 'a') {
		if ((fd = open(name, O_WRONLY, 0)) == -1)
			fd = creat(name, PERMS);
		lseek(fd, 0L, 2);
	} else
		fd = open(name, O_RDONLY, 0);
	if (fd == -1)
		return NULL;
	fp->fd = fd;
	fp->cnt = 0;
	fp->base = NULL;
	fp->ptr = NULL;
	fp->flag = (*mode == 'r') ? _READ : _WRITE;
	return fp;
}

int main(void) {
	int c;
	FILE *fp;

	fp = fopen("/root/apu/cl/c8/test1.c", "r");
	fseek(fp, 2, 0);
	while ((c = getc(fp)) != EOF) {
		if (putchar(c) == EOF)
			exit(1);
	}
	fflush((FILE *) NULL);

	return 0;
}