socket descriptor vs file descriptor

read(2) and write(2) works both on socket descriptor as well as on file descriptor. In case of file descriptor, User file descriptor table->file table and finally to inode table where it checks for the file type(regular file/char/block), and reads accordingly. In case of char spl file, it gets the function pointers based on the major number of the file from the char devi开发者_如何转开发ce switch and calls the appropriate read/write routines registered for the device. Similarly appropriate read/write routine is called for block special file by getting the function pointers from the block device switch.

Could you please let me know what exatly happens when read/write called on socket descriptor. If read/write works on socket descriptor, we cant we use open instead of socket to get the descriptor?

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As i know in memory, the file descriptor will contains flag to identify the file-system type of this fd. The kernel will invoke corresponding handler function depends on the file-system type. You can see the source read_write.c in linux kernel.

To be speak in brief, the kernel did:

1. In read-write.c, there is a file_system_wrapper function, that call corresponding handler function depends on fd's file type (ext2/ ext3/ socket/ ..)
2. In socket.c, there is a socket_type_wrapper function; that calls corresponding socket handler function depends on socket's type (ipv4, ipv6, atm others)
3. In socket_ipv4.c, there is a protocol_type wrapper function; that calls corresponding protocol handler function depends on protocol tpye (udp/ tcp)
4. In tcp_ip4.c; there is tcp_sendmsg and this function would be called when write to FD of tcp ipv4 type.

Hope this clearly, thanks, Houcheng

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Socket descriptors are associated with file structures too, but a set of file_operations functions for that structures differs from the usual. Initialization and use of those descriptors are therefore different. Read and write part of kernel-level interface just happened to be exactly equivalent.

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read and write are valid for some types of sockets in some states; this all depends on the various structs which are passed around inside the kernel.

In principle, open() could create a socket descriptor, but the BSD sockets API was never defined that way.

There are some other (Somewhat linux-specific) types of file descriptor which are opened by system calls other than open(), for example epoll_create or timerfd_create. These work the same.