文件描述符
Unix上用文件描述符，指代文件。
文件描述符是整型
0/1/2分别代表标准输入，输出，错误
低级I/O
#include "syscalls.h"
/* getchar: simple buffered version */
int getchar(void)
{
static char buf[BUF SIZ];
static char *bufp = buf;
static int n = 0;
if (n == 0)
{
/* buffer is empty */
n = read(0, buf, sizeof buf);
bufp = buf;
}
return (--n >= 0) ? (unsigned char) *bufp++ : EOF;
}
Open/Creat/Close/Unlink
#include <fcntl.h>
int fd;
int open(char *name, int flags, int perms);
int creat(char *name, int perms);
#include <stdio.h>
#include <stdarg.h>
/* error: print an error message and die */
void error(char *fmt, ...)
{
va_list args;
va_start(args, fmt);
fprintf(stderr, "error: ");
vprintf(stderr, fmt, args);
fprintf(stderr, "\n");
va_end(args);
exit(1);
}
unlink将文件name从文件系统删除
随机访问
long lseek(int fd, long offset, int origin);
#include "syscalls.h"
/*get: read n bytes from position pos */
int get(int fd, long pos, char *buf, int n)
{
if (lseek(fd, pos, 0) >= 0) /* get to pos */
return read(fd, buf, n);
else
return -1;
}
例子－实现Fopen and Getc
#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 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 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 putcher(x) putc((x), stdout)
#include <fcntl.h>
#include "syscalls.h"
#define PERMS 0666 /* RW for owner, group, others */
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;
}
#include "syscalls.h"
/* _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++;
}
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 }
};
例子－列举目录
目录是一个文件，包含文件名列表，和文件位置。
文件位置用索引号指示。
目录项通常仅仅包含：the filename and an inode number.
文件ｉ节点存放除文件名外文件信息
Dirent
opendir
readdir
closedir
The Dirent structure contains the inode number and the name.
The maximum length of a filename component is NAME_MAX,
which is a system-dependent value.
opendir returns a pointer to a structure called DIR,
analogous to FILE, which is used by readdir and closedir.
// dirent.h.
#define NAME_MAX 14 /* longest filename component; */
/* system-dependent */
typedef struct
{
/* portable directory entry */
long ino; /* inode number */
char name[NAME_MAX+1]; /* name + '\0' terminator */
}
Dirent;
typedef struct
{
/* minimal DIR: no buffering, etc. */
int fd; /* file descriptor for the directory */
Dirent d; /* the directory entry */
}
DIR;
DIR *opendir(char *dirname);
Dirent *readdir(DIR *dfd);
void closedir(DIR *dfd);
char *name;
struct stat stbuf;
int stat(char *, struct stat *);
stat(name, &stbuf);
<sys/stat.h>
<sys/types.h>
struct stat /* inode information returned by stat */
{
dev_t st_dev; /* device of inode */
ino_t st_ino; /* inode number */
short st_mode; /* mode bits */
short st_nlink; /* number of links to file */
short st_uid; /* owners user id */
short st_gid; /* owners group id */
dev_t st_rdev; /* for special files */
off_t st_size; /* file size in characters */
time_t st_atime; /* time last accessed */
time_t st_mtime; /* time last modified */
time_t st_ctime; /* time originally created */
};
#define S_IFMT 0160000 /* type of file: */
#define S_IFDIR 0040000 /* directory */
#define S_IFCHR 0020000 /* character special */
#define S_IFBLK 0060000 /* block special */
#define S_IFREG 0010000 /* regular */
<sys/dir.h>
#ifndef DIRSIZ
#define DIRSIZ 14
#endif
struct direct
{
/* directory entry */
ino_t d_ino; /* inode number */
char d_name[DIRSIZ]; /* long name does not have '\0' */
};
一个存储空间分配器
