c - while loop won't exit C

Question

This is a working snippet of a while loop:

while(total_bytes_read != fsize){
    while((nread = read(sockd, filebuffer, sizeof(filebuffer))) > 0){
        if(write(fd, filebuffer, nread) < 0){
            perror("write");
            close(sockd);
            exit(1);
        }
        total_bytes_read += nread;
        if(total_bytes_read == fsize) break;
    }
}

This is an example of a NON working snippet of a while loop:

while(total_bytes_read != fsize){
    while((nread = read(sockd, filebuffer, sizeof(filebuffer))) > 0){
        if(write(fd, filebuffer, nread) < 0){
            perror("write");
            close(sockd);
            exit(1);
        }
        total_bytes_read += nread;
    }
}

And also this, is an example of a NON working snippet of a while loop:

while(total_bytes_read < fsize){
    while((nread = read(sockd, filebuffer, sizeof(filebuffer))) > 0){
        if(write(fd, filebuffer, nread) < 0){
            perror("write");
            close(sockd);
            exit(1);
        }
    }
     total_bytes_read += nread;
}

I would like to know why into the 2 snippet above when total_bytes_read is equal to fsize the loop won't exit :O
Thanks in advance!

Answer 1

片段 2 和 3 中的外部循环不会退出，因为内部循环不会退出：total_bytes_read != fsize循环永远没有机会检查其继续条件。

片段 1 工作正常，因为您检查嵌套循环内的相同条件，如果已达到限制计数，则中断。

您可以将这两个条件组合成一个循环，如下所示：

while((total_bytes_read != fsize) && (nread = read(sockd, filebuffer, sizeof(filebuffer))) > 0) {
    ...
}

Answer 2

因为您删除了停止循环的中断

if(total_bytes_read == fsize) break;

Answer 3

不以这种方式编写读取循环是有原因的。（实际上，有很多。）如果您计算 fsize，然后在您阅读时文件更改大小，您的循环将永远不会终止。不要这样做！无需计算文件大小；只需读取数据，直到没有更多数据。