unix环境高级编程 FILE I/O笔记
/*
* Default file access permissions for new files.
*/
#define FILE_MODE (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)
FILE_MODE是在apue.h中定义的宏变量
文件所有者(owner)有读、写全新；文件组成员只读权限和其他用户只读
S_IRUSR等的header file为 sys/stat.h
S_IRUSR 00400 owner has read permission S_IWUSR 00200 owner has write permission S_IXUSR 00100 owner has execute permission S_IRWXG 00070 mask for group permissions S_IRGRP 00040 group has read permission S_IWGRP 00020 group has write permission S_IXGRP 00010 group has execute permission S_IRWXO 00007 mask for permissions for others (not in group) S_IROTH 00004 others have read permission S_IWOTH 00002 others have write permission S_IXOTH 00001 others have execute permission
#include<fcntl.h>
#include<sys/stat.h>
#include<stdio.h>
char buf1[]="abcdefghij";
char buf2[]="ABCDEFGHIJ";
#define FILE_MODE (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)
int main(void)
{
//file descriptor
int fd;
if((fd=creat("file.hole",FILE_MODE))<0)
printf("create error\n");
if(write(fd,buf1,10)!=10)
printf("buf1 write error\n");
/*offset now=10 */
if(lseek(fd,16384,SEEK_SET)==-1)
printf("lseek error");
/*offset now =16384 */
if(write(fd,buf2,10)!=10)
printf("buf2,write error\n");
/*offset now =16394 */
exit(0);
}
od -c file.hole
0000000 a b c d e f g h i j \0 \0 \0 \0 \0 \0
0000020 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0
*
0040000 A B C D E F G H I J
0040012
每一行输出的前七位为八进制表示40000表示成10进制 (40000)oct=4*(8^4)=2^14=16384
od command:
Write an unambiguous representation, octal bytes by default, of FILE to standard output. With more than one FILE argument, concatenate them in the listed order to form the input. With no FILE, or when FILE is -, read standard input