Exercise 8.4 - implement fseek#
Question#
The standard library function:
int fseek(FILE *fp, long offset, int origin)
inprogress
#include <fcntl.h>
#include <stdarg.h>
// The functions available from stdio.h are implemented here.
//#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/syscall.h>
#ifdef NULL
#undef NULL
#endif
#define NULL 0
#define EOF (-1)
#define BUFSIZ 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 the buffer */
int flag; /* mode of the 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 = 03, /* file is unbuffered */
_EOF = 010, /* EOF has occurred on this file */
_ERR = 020, /* error occurred on this file */
};
int _fillbuf(FILE *);
int _flushbuf(int, FILE *);
#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)
#define PERMS 0666 /* RW for owner, group and others */
/* fopen: open file, return file ptr */
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)) == 0)
break; /* found free slot */
if (fp >= _iob + OPEN_MAX) /* no free slots */
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) /* couldn't access name */
return NULL;
fp->fd = fd;
fp->cnt = 0;
fp->base = NULL;
fp->flag = (*mode == 'r') ? _READ : _WRITE;
return fp;
}
/* _fillbuf: allocate and fill input buffer */
int _fillbuf(FILE *fp)
{
int bufsize;
if ((fp->flag & (_READ | _EOF | _ERR)) != _READ)
return EOF;
bufsize = (fp->flag & _UNBUF) ? 1: BUFSIZ;
if (fp->base == NULL) /* no buffer yet */
if ((fp->base = (char *) malloc(bufsize)) == NULL)
return EOF; /* can't get buffer */
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++;
}
/* _flushbuf: flush a buffer */
/* http://clc-wiki.net/wiki/K%26R2_solutions:Chapter_8:Exercise_3 */
int _flushbuf(int c, FILE *f) {
int num_written, bufsize;
unsigned char uc = c;
if ((f->flag & (_WRITE | _EOF | _ERR)) != _WRITE) {
return EOF;
}
if (f->base == NULL && ((f->flag & _UNBUF) == 0)) {
/* no buffer yet */
if ((f->base = malloc(BUFSIZ)) == NULL)
/* could not allocate a buffer, so try unbuffered */
f->flag |= _UNBUF;
else {
f->ptr = f->base;
f->cnt = BUFSIZ - 1;
}
}
if (f->flag & _UNBUF) {
/* unbuffered write */
f->ptr = f->base = NULL;
f->cnt = 0;
if (c == EOF) {
return EOF;
}
num_written = write(f->fd, &uc, 1);
bufsize = 1;
} else {
/* buffered write */
/* TODO: Senthil Kumaran - What should be the f-> ptr?*/
/*if ( c!= EOF) {
f->ptr = uc;
} */
bufsize = (int)(f->ptr - f->base);
num_written = write(f->fd, f->base, bufsize);
f->ptr = f->base;
f->cnt = BUFSIZ - 1;
}
if (num_written == bufsize)
return c;
else {
f->flag |= _ERR;
return EOF;
}
}
FILE _iob[OPEN_MAX] = { /* stdin, stdout, stderr */
{ 0, (char *) 0, (char *) 0, _READ, 0 },
{ 0, (char *) 0, (char *) 0, _WRITE, 1 },
{ 0, (char *) 0, (char *) 0, _WRITE | _UNBUF, 2 }
};
/* fflush */
int fflush(FILE *f) {
int retval;
int i;
FILE *fp;
retval = 0;
if (f == NULL) {
/* flush all the output streams */
for (fp = _iob; fp < _iob + OPEN_MAX; fp++)
if ( (fp->flag & _WRITE) == 0 && fflush(fp) == -1)
retval = -1;
} else {
if ((f->flag & _WRITE) == 0)
return -1;
_flushbuf(EOF, f);
if (f->flag & _ERR)
retval = -1;
}
return retval;
}
/* fclose */
int fclose(FILE *f)
{
int fd;
if (f == NULL)
return -1;
fd = f->fd;
fflush(f);
f->cnt = 0;
f->ptr = NULL;
if (f->base != NULL)
free(f->base);
f->base = NULL;
f->flag = 0;
f->fd = -1;
return close(fd);
}
/* fseek */
/* http://clc-wiki.net/wiki/K%26R2_solutions:Chapter_8:Exercise_4 */
#define SEEK_SET 0
#define SEEK_CUR 1
#define SEEK_END 2
int fseek(FILE *f, long offset, int whence) {
int result;
if ((f->flag & _UNBUF) == 0 && f->base != NULL) {
/* deal with buffering */
if (f->flag & _WRITE) {
if (fflush(f))
return EOF;
} else if (f->flag & _READ) {
/* Reading so thrash buffer, do some housekeeping first
*/
if (whence == SEEK_CUR) {
/* fix offset so that it's from the last
* character the user read (not the last character that was
* actually read)
*/
if (offset >=0 && offset <= f->cnt) {
/* easy shortcut */
f->cnt -= offset;
f->ptr += offset;
f->flag &= ~_EOF; /* If successful, clear EOF */
return 0;
} else {
offset -= f->cnt;
}
}
f->cnt = 0;
f->ptr = f->base;
}
}
result = (lseek(f->fd, offset, whence) < 0);
if (result == 0)
f->flag &= ~_EOF; /* If successful, clear EOF */
return result;
}
int main(int argc, char *argv[]) {
int c;
while ((c =getchar()) != 'x') {
putchar(c);
}
}