Beginning-Linux-Programming-03

2017-11-08 Linux Programming 阅读量

文件操作

文件和设备
系统调用
库函数
底层文件访问
管理文件
标准I/O库
格式化输入输出
文件和目录的维护
扫描目录
错误及处理
/proc 文件系统
fcntl和mmap

Linux 文件结构

5个基本函数————open，close，read，write和ioctl.

底层文件访问

0: 标准输入
1：标准输出
2：标准错误

write 系统调用

1 2	#include <unistd> size_t write(int fildes, const void *buf, size_t nbytes);

作用：把缓冲区buf的前nbytes个字节写入与文件描述符fildes关联的文件中。
如：

#include <unistd.h>
#include <stdlib.h>
int main()
{
    if ((write(1, "Here is some data\n", 18)) != 18)
        write(2, "A write error has occurred on file descriptor 1\n",46);
    exit(0);
}

read 系统调用

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#include <unistd>
size_t read(int fildes,void *buf, size_t nbytes);

从与文件相关联的文件读入nbytes个字节的数据，并把他们放入到数据区buf中。

如：

#include <unistd.h>
#include <stdlib.h>
int main()
{
    char buffer[128];
    int nread;
    nread = read(0, buffer, 128);
    if (nread == -1)
        write(2, "A read error has occurred\n", 26);
    if ((write(1,buffer,nread)) != nread)
        write(2, "A write error has occurred\n",27);
    exit(0);
}

open

To create a new file descriptor, you need to use the open system call.

#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
int open(const char *path, int oflags);
int open(const char *path, int oflags, mode_t mode);

In simple terms, open establishes an access path to a file or device. If successful, it returns a file descriptor
that can be used in read , write , and other system calls.

For example,

1	open (“myfile”, O_CREAT, S_IRUSR\|S_IXOTH);

close

You use close to terminate the association between a file descriptor, fildes , and its file. The file
descriptor becomes available for reuse. It returns 0 if successful and –1 on error.

1 2	#include <unistd.h> int close(int fildes);

ioctl

It provides an interface for controlling the behavior of devices and
their descriptors and configuring underlying services.

1 2	#include <unistd.h> int ioctl(int fildes, int cmd, ...);

For example, the following call to ioctl on Linux turns on the keyboard LEDs:

1	ioctl(tty_fd, KDSETLED, LED_NUM\|LED_CAP\|LED_SCR);

The Standard I/O Library

fopen , fclose
fread , fwrite
fflush
fseek
fgetc , getc , getchar
fputc , putc , putchar
fgets , gets
printf , fprintf , and sprintf
scanf , fscanf , and sscanf

fopen

1 2	#include <stdio.h> FILE fopen(const char filename, const char *mode);

fopen opens the file named by the filename parameter and associates a stream with it. The mode
parameter specifies how the file is to be opened. It’s one of the following strings:

“r” or “rb” : Open for reading only
“w” or “wb” : Open for writing, truncate to zero length
“a” or “ab” : Open for writing, append to end of file
“r+” or “rb+” or “r+b” : Open for update (reading and writing)
“w+” or “wb+” or “w+b” : Open for update, truncate to zero length
“a+” or “ab+” or “a+b” : Open for update, append to end of file

fread

1 2	#include <stdio.h> size_t fread(void ptr, size_t size, size_t nitems, FILE stream);

The fread library function is used to read data from a file stream. Data is read into a data buffer given
by ptr from the stream, stream . Both fread and fwrite deal with data records. These are specified by
a record size, size , and a count, nitems , of records to transfer. The function returns the number of items
(rather than the number of bytes) successfully read into the data buffer. At the end of a file, fewer than
nitems may be returned, including zero.

fwrite

1 2	#include <stdio.h> size_t fwrite (const void ptr, size_t size, size_t nitems, FILE stream);

The fwrite library call has a similar interface to fread . It takes data records from the specified data
buffer and writes them to the output stream. It returns the number of records successfully written.

fcolse

1 2	#include <stdio.h> int fclose(FILE *stream);

The fclose library function closes the specified stream , causing any unwritten data to be written.

fflush

1 2	#include <stdio.h> int fflush(FILE *stream);

The fflush library function causes all outstanding data on a file stream to be written immediately.

fseek

1 2	#include <stdio.h> int fseek(FILE *stream, long int offset, int whence);

fgetc, getc, and getchar

#include <stdio.h>
int fgetc(FILE *stream);
int getc(FILE *stream);
int getchar();

fgets and gets

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#include <stdio.h>
char *fgets(char *s, int n, FILE *stream);
char *gets(char *s);

The fgets function reads a string from an input file stream .

Formatted Input and Output

printf, fprintf, and sprintf

#include <stdio.h>
int printf(const char *format, ...);
int sprintf(char *s, const char *format, ...);
int fprintf(FILE *stream, const char *format, ...);

scanf, fscanf, and sscanf

#include <stdio.h>
int scanf(const char *format, ...);
int fscanf(FILE *stream, const char *format, ...);
int sscanf(const char *s, const char *format, ...);

File and Directory Maintenance

chmod

1 2	#include <sys/stat.h> int chmod(const char *path, mode_t mode);

chown

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#include <sys/types.h>
#include <unistd.h>
int chown(const char *path, uid_t owner, gid_t group);

unlink, link, and symlink

#include <unistd.h>
int unlink(const char *path);
int link(const char *path1, const char *path2);
int symlink(const char *path1, const char *path2);

mkdir and rmdir

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#include <sys/types.h>
#include <sys/stat.h>
int mkdir(const char *path, mode_t mode);

1 2	#include <unistd.h> int rmdir(const char *path);

Advanced Topics: fcntl and mmap

fcntl

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#include <fcntl.h>
int fcntl(int fildes, int cmd);
int fcntl(int fildes, int cmd, long arg);

fcntl(fildes, F_DUPFD, newfd) : This call returns a new file descriptor with a numerical
value equal to or greater than the integer newfd . The new descriptor is a copy of the descriptor
fildes . Depending on the number of open files and the value of newfd , this can be effectively
the same as dup(fildes) .
fcntl(fildes, F_GETFD) : This call returns the file descriptor flags as defined in fcntl.h .
These include FD_CLOEXEC , which determines whether the file descriptor is closed after a suc-
cessful call to one of the exec family of system calls.
fcntl(fildes, F_SETFD, flags) : This call is used to set the file descriptor flags, usually just
FD_CLOEXEC .
fcntl(fildes, F_GETFL) and fcntl(fildes, F_SETFL, flags) : These calls are used,
respectively, to get and set the file status flags and access modes. You can extract the file access
modes by using the mask O_ACCMODE defined in fcntl.h . Other flags include those passed in a
third argument to open when used with O_CREAT . Note that you can’t set all flags. In particular,
you can’t set file permissions using fcntl .

mmap

1 2	#include <sys/mman.h> void mmap(void addr, size_t len, int prot, int flags, int fildes, off_t off);