linux中文件内核数据结构

3.文件io

3.1 文件内核数据结构

3.2 复制文件描述符的内核数据结构

3.3 对指定的描述符打印文件标志

#include "apue.h"
#include <fcntl.h>

int
main(int argc, char *argv[])
{
	int		val;

	if (argc != 2)
		err_quit("usage: a.out <descriptor#>");

	if ((val = fcntl(atoi(argv[1]), F_GETFL, 0)) < 0)
		err_sys("fcntl error for fd %d", atoi(argv[1]));

	switch (val & O_ACCMODE) {
	case O_RDONLY:
		printf("read only");
		break;

	case O_WRONLY:
		printf("write only");
		break;

	case O_RDWR:
		printf("read write");
		break;

	default:
		err_dump("unknown access mode");
	}

	if (val & O_APPEND)
		printf(", append");
	if (val & O_NONBLOCK)
		printf(", nonblocking");
	if (val & O_SYNC)
		printf(", synchronous writes");

#if !defined(_POSIX_C_SOURCE) && defined(O_FSYNC) && (O_FSYNC != O_SYNC)
	if (val & O_FSYNC)
		printf(", synchronous writes");
#endif

	putchar('\n');
	exit(0);
}

结果：

3.4 开启文件状态标志

#include "apue.h"
#include <fcntl.h>

void
set_fl(int fd, int flags) /* flags are file status flags to turn on */
{
	int		val;

	if ((val = fcntl(fd, F_GETFL, 0)) < 0)
		err_sys("fcntl F_GETFL error");

	val |= flags;		/* turn on flags */

	if (fcntl(fd, F_SETFL, val) < 0)
		err_sys("fcntl F_SETFL error");
}

如果将中间的一条语句修改如下（关闭文件标志）：

val &= ~flags;  //turn flags off

3.5 将标志输入复制到标准输出

#include "apue.h"

#define	BUFFSIZE	4096

int
main(void)
{
	int		n;
	char	buf[BUFFSIZE];

    //set_fl(STDOUT_FILENO,O_SYNC) //开启同步写标志
	while ((n = read(STDIN_FILENO, buf, BUFFSIZE)) > 0)
		if (write(STDOUT_FILENO, buf, n) != n)
			err_sys("write error");

	if (n < 0)
		err_sys("read error");

	exit(0);
}

程序运行时，设置O_SYNC会增加系统时间和时钟时间。

4.文件和目录

4.1 access函数的使用

#include "apue.h"
#include <fcntl.h>

int
main(int argc, char *argv[])
{
	if (argc != 2)
		err_quit("usage: a.out <pathname>");
	if (access(argv[1], R_OK) < 0)
		err_ret("access error for %s", argv[1]);
	else
		printf("read access OK\n");
	if (open(argv[1], O_RDONLY) < 0)
		err_ret("open error for %s", argv[1]);
	else
		printf("open for reading OK\n");
	exit(0);
}

8.进程控制

8.1 fork函数实例

#include "apue.h"

int		globvar = 6;		/* external variable in initialized data */
char	buf[] = "a write to stdout\n";

int
main(void)
{
	int		var;		/* automatic variable on the stack */
	pid_t	pid;

	var = 88;
	if (write(STDOUT_FILENO, buf, sizeof(buf)-1) != sizeof(buf)-1)
		err_sys("write error");
	printf("before fork\n");	/* we don't flush stdout */

	if ((pid = fork()) < 0) {
		err_sys("fork error");
	} else if (pid == 0) {		/* child */
		globvar++;				/* modify variables */
		var++;
	} else {
		sleep(2);				/* parent */
	}

	printf("pid = %ld, glob = %d, var = %d\n", (long)getpid(), globvar,
	  var);
	exit(0);
}

运行结果

分析

• 为啥是sizeof(buf)-1:

  

• 为什么出现有两次before fork出现：

8.2 fork之后父子进程对文件的共享

8.3 vfork函数实例

#include "apue.h"

int		globvar = 6;		/* external variable in initialized data */

int
main(void)
{
	int		var;		/* automatic variable on the stack */
	pid_t	pid;

	var = 88;
	printf("before vfork\n");	/* we don't flush stdio */
	if ((pid = vfork()) < 0) {
		err_sys("vfork error");
	} else if (pid == 0) {		/* child */
		globvar++;				/* modify parent's variables */
		var++;
		_exit(0);				/* child terminates */
	}

	/* parent continues here */
	printf("pid = %ld, glob = %d, var = %d\n", (long)getpid(), globvar,
	  var);
	exit(0);
}

运行结果