请注意,对于复制常规文件,您可以跳过EAGAIN的东西,因为常规文件总是阻止I / O.但是不可避免地,如果你写这个代码,有人会使用它在其他类型的文件描述符,所以认为它是一个免费的.

有一个文件特定的GNU cp优化,我没有在这里烦恼,对于0个字节的长块而不是写你只是通过寻求结束扩展输出文件.

void block(int fd,int event) {
    pollfd topoll;
    topoll.fd = fd;
    topoll.events = event;
    poll(&topoll,1,-1);
    // no need to check errors - if the stream is bust then the
    // next read/write will tell us
}

int copy_data_buffer(int fdin,int fdout,void *buf,size_t bufsize) {
    for(;;) {
       void *pos;
       // read data to buffer
       ssize_t bytestowrite = read(fdin,buf,bufsize);
       if (bytestowrite == 0) break; // end of input
       if (bytestowrite == -1) {
           if (errno == EINTR) continue; // signal handled
           if (errno == EAGAIN) {
               block(fdin,POLLIN);
               continue;
           }
           return -1; // error
       }

       // write data from buffer
       pos = buf;
       while (bytestowrite > 0) {
           ssize_t bytes_written = write(fdout,pos,bytestowrite);
           if (bytes_written == -1) {
               if (errno == EINTR) continue; // signal handled
               if (errno == EAGAIN) {
                   block(fdout,POLLOUT);
                   continue;
               }
               return -1; // error
           }
           bytestowrite -= bytes_written;
           pos += bytes_written;
       }
    }
    return 0; // success
}

// Default value. I think it will get close to maximum speed on most
// systems,short of using mmap etc. But porters / integrators
// might want to set it smaller,if the system is very memory
// constrained and they don't want this routine to starve
// concurrent ops of memory. And they might want to set it larger
// if I'm completely wrong and larger buffers improve performance.
// It's worth trying several MB at least once,although with huge
// allocations you have to watch for the linux 
// "crash on access instead of returning 0" behavIoUr for Failed malloc.
#ifndef FILECOPY_BUFFER_SIZE
    #define FILECOPY_BUFFER_SIZE (64*1024)
#endif

int copy_data(int fdin,int fdout) {
    // optional exercise for reader: take the file size as a parameter,// and don't use a buffer any bigger than that. This prevents 
    // memory-hogging if FILECOPY_BUFFER_SIZE is very large and the file
    // is small.
    for (size_t bufsize = FILECOPY_BUFFER_SIZE; bufsize >= 256; bufsize /= 2) {
        void *buffer = malloc(bufsize);
        if (buffer != NULL) {
            int result = copy_data_buffer(fdin,fdout,buffer,bufsize);
            free(buffer);
            return result;
        }
    }
    // could use a stack buffer here instead of failing,if desired.
    // 128 bytes ought to fit on any stack worth having,but again
    // this could be made configurable.
    return -1; // errno is ENOMEM
}

打开输入文件：

int fdin = open(infile,O_RDONLY|O_BINARY,0);
if (fdin == -1) return -1;

打开输出文件是很巧妙的.作为基础,你想要：

int fdout = open(outfile,O_WRONLY|O_BINARY|O_CREAT|O_TRUNC,0x1ff);
if (fdout == -1) {
    close(fdin);
    return -1;
}

但有混淆因素：

>你需要特殊情况下文件是一样的,我不记得如何做可移植的.
>如果输出文件名是目录,则可能需要将文件复制到目录中.
>如果输出文件已经存在(使用O_EXCL打开以确定此错误并检查EEXIST是否有错误),则可能需要执行与cp -i不同的操作.
>您可能希望输出文件的权限反映输入文件的权限.
>您可能需要复制其他平台特定的元数据.
>您可能希望或可能不希望取消输出文件的错误.

显然,所有这些问题的答案可能是“做同样的cp”.在这种情况下,原始问题的答案是“忽略我或任何其他人所说的一切,并使用cp的来源”.

获取文件系统的集群大小的Btw是无用的.在您通过磁盘块大小后,您将几乎总是看到速度随着缓冲区大小而增加.