函数pread、pwrite
pread()、pwrite()函数与read()、write()函数的区别在于是否更新当前文件偏移量； pread：相当于调用lseek后再调用read函数；
调用pread时，无法中断其定位和读操作，且不更新当前文件偏移量。pwrite()函数与此相同。
函数原型：
#include <unistd.h>
ssize_t pread(int fd,void *buf,size_t nbytes,off_t offset);
ssize_t pwrite(int fd,void *buf,size_t nbytes,off_t offset);
测试程序
#include<stdio.h>
#include<fcntl.h>
#include<unistd.h>
#include<sys/types.h>
#include<string.h>
int main(int argc
,char *argv
[])
{
int fd
= open("pread.txt",O_RDWR
|O_CREAT
,0777);
int num
= write(fd
,"Hello world!\n",strlen("Hello world!\n"));
if(num
<0){
printf("write error\n");
return -1;
}
int offset
= lseek(fd
,0,SEEK_CUR);
printf("num = %d,offset = %d\n",num
,offset
);
pwrite(fd
,"My Best Friends!",strlen("My Best Friends!"),6);
char buf
[20]="",buf1
[20]="";
int ret
= read(fd
,buf
,sizeof(buf
));
if(ret
<0)
{
printf("read error!\n");
return -1;
}
int offset1
= lseek(fd
,0,SEEK_CUR);
printf("ret = %d,offset1 = %d\n",ret
,offset1
);
pread(fd
,buf1
,sizeof(buf1
),6);
printf("buf = %s,buf1 = %s\n",buf
,buf1
);
return 0;
}
如果用write和read则不会取修改文件的偏移量
