read（）返回“不正确”的值

Question

while(  rd = read(fd1, buf, 512) != 0)
{
    len += rd;
    if(readed < 0) 
    perror("read: ");
}

MAN pages says, that read() returns number of read bytes, but in my case this code returns number of blocks(depends of 3rd argument) or number of iterations. for example, I have file with 36 symbols and this code returns 1, when all symbols normally read, if I change 512 to 4, it will return 9, and so on. Please correct me and this code to return number of bytes which read with 512 size blocks

Answer 1

rd = read(fd1, buf, 512) != 0

means

rd = (read(fd1, buf, 512) != 0)

The result of != is always either 0 or 1 .

You probably meant

while ((rd = read(fd1, buf, 512)) != 0)

Answer 2

Incorrect use of brackets, Pl. change your code like below. Hope It will give you the correct result.

while(  (rd = read(fd1, buf, 512)) != 0)
{
    len += rd;
    if(rd < 0) 
    perror("read: ");
}

Answer 3

Did you try to set th rd value outside of while loop condition? Without more bracket i think the Setting and testing the value section in the while condition is not correct.