标签：font turn 偏移量 bytes nbsp class types pen bsp
1、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；
2、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。
3、函数原型：
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);
4、头文件：#include <unistd.h>
5、示例：
1 #include <stdio.h> 2 #include <fcntl.h> 3 #include <unistd.h> 4 #include <sys/types.h> 5 #include <string.h> 6 7 int main(int argc,char *argv[]) 8 { 9 int fd = open("pread.txt",O_RDWR|O_CREAT,0777); 10 int num = write(fd,"Hello World!\n",strlen("Hello World!\n")); 11 if (num < 0) 12 { 13 perror("write error!\n"); 14 return -1; 15 } 16 int offset = lseek(fd,0,SEEK_CUR); 17 printf("num = %d,offset = %d\n",num,offset); // num = 13,offset = 13; 18 19 pwrite(fd,"My Best Friends!",strlen("My Best Friends!"),6); 20 21 char buf[20] = "",buf1[20] = ""; 22 int ret = read(fd,buf,sizeof(buf)); 23 if (ret < 0) 24 { 25 perror("read error!\n"); 26 return -1; 27 } 28 29 int offset1 = lseek(fd,0,SEEK_CUR); 30 printf("ret = %d,offset1 = %d\n",ret,offset1); // ret = 9,offset1 = 22; 31 32 pread(fd,buf1,sizeof(buf1),6); 33 printf("buf = %s,buf1 = %s\n",buf,buf1);// buf = Friends!,buf1 = My Best Friends! 34 35 return 0; 36 }
标签：font turn 偏移量 bytes nbsp class types pen bsp
原文地址：https://www.cnblogs.com/ys6738-4271-3931/p/11838391.html
