read/write的包裹函数

《Unix环境高级编程》

ssize_t writen (int fd, const void *buf, size_t num)  
{  
ssize_t res;  
size_t n;  
const char *ptr;  
  
  
n = num;  
ptr = buf;  
while (n > 0) {  
/* 开始写*/   
     if ((res = write (fd, ptr, n)) <= 0) {  
      if (errno == EINTR)  
       res = 0;  
      else  
       return (-1);  
     }  
  
  
     ptr += res;/* 从剩下的地方继续写     */   
     n -= res;  
}  
  
  
return (num);  
}  

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ssize_t readn (int fd, void *buf, size_t num)  
{  
ssize_t res;  
size_t n;  
char *ptr;  
  
  
n = num;  
ptr = buf;  
while (n > 0) {  
     if ((res = read (fd, ptr, n)) == -1) {  
      if (errno == EINTR)  
       res = 0;  
      else  
       return (-1);  
     }  
     else if (res == 0)  
      break;  
  
  
     ptr += res;  
     n -= res;  
}  
  
  
return (num - n);  
}