This technical note documents the poll method of PRFileDesc. The poll method is not to be confused with the PR_Poll function. The poll method operates on a single NSPR file descriptor, whereas PR_Poll operates on a collection of NSPR file descriptors. PR_Poll uses the poll method behind the scene, but it is also possible to use the poll method directly.
We consider a stack of NSPR I/O layers on top of the network transport. Each I/O layer is represented by a PRFileDesc structure and the protocol of that layer is implemented by a PRIOMethods table. The bottom layer is a wrapper for the underlying network transport. The NSPR library provides a reference implementation of the bottom layer using the sockets API, but you can provide your own implementation of the bottom layer using another network transport API. The poll method is one of the functions in the PRIOMethods table. The prototype of the poll method is
PRInt16 poll_method(PRFileDesc *fd, PRInt16 in_flags, PRInt16 *out_flags);
The purpose of the poll method is to allow a layer to modify that flags that will ultimately be used in the call to the underlying network transport's select (or equivalent) function, and to indicate that a layer is already able to make progress in the manner suggested by the polling flags. The arguments and return value of the poll method are described below.
in_flags [input argument]
The in_flags argument specifies the events at the top layer of the I/O layer stack that the caller is interested in.
- For PR_Recv, you should pass PR_POLL_READ as the in_flags argument to the poll method
- For PR_Send, you should pass PR_POLL_WRITE as the in_flags argument to the poll method
out_flags [output argument]
If an I/O layer is ready to satisfy the I/O request defined by in_flags without involving the underlying network transport, its poll method sets the corresponding event in *out_flags on return. For example, consider an I/O layer that buffers input data. If the caller wishes to test for read ready (that is, PR_POLL_READ is set in in_flags) and the layer has input data buffered, the poll method would set the PR_POLL_READ event in *out_flags. It can determine that without asking the underlying network transport. The current implementation of PR_Poll (the primary user of the poll method) requires that the events in *out_flags reflect the caller's view. This requirement may be relaxed in a future NSPR release. To remain compatible with this potential semantic change, NSPR clients should only use *out_flags as described in the How to use the poll method section below.
If the poll method stores a nonzero value in *out_flags, the return value will be the value of in_flags. (Note: this may change in a future NSPR release if we make the semantic change to *out_flags mentioned above. Therefore, NSPR clients should only use the return value as described in How to use the poll method section below.) If the poll method stores zero in *out_flags, the return value will be the bottom layer's desires with respect to the in_flags. Those are the events that the caller should poll the underlying network transport for. These events may be different from the events in in_flags (which reflect the caller's view) for some protocols.
How to use the poll method
The poll method should only be used with a NSPR file descriptor in non-blocking mode. Most NSPR clients call PR_Poll and do not call the poll method directly. However, PR_Poll can only used with a stack whose bottom layer is NSPR's reference implementation. If you are using your own implementation of the bottom layer, you must call the poll method as follows. Declare two PRInt16 variables to receive the return value and the out_flags output argument of the poll method.
PRInt16 new_flags, out_flags;
If you are going to call PR_Recv, pass PR_POLL_READ as the in_flags argument.
new_flags = fd->methods->poll(fd, PR_POLL_READ, &out_flags);
If you are going to call PR_Send, pass PR_POLL_WRITE as the in_flags argument.
new_flags = fd->methods->poll(fd, PR_POLL_WRITE, &out_flags);
If you are interested in calling both PR_Recv and PR_Send on the same file descriptor, make two separate calls to the poll method, one with PR_POLL_READ as in_flags and the other with PR_POLL_WRITE as in_flags, so that you know what events at the network transport layer PR_POLL_READ and PR_POLL_WRITE are mapped to, respectively. On return, if (new_flags & out_flags) is nonzero, you can try PR_Recv or PR_Send immediately. Otherwise ((new_flags & out_flags) is 0), you should do the following.
- If new_flags contains PR_POLL_READ, you should try PR_Recv or PR_Send when the underlying network transport is readable
- If new_flags contains PR_POLL_WRITE, you should try PR_Recv or PR_Send when the underlying network transport is writable
Important do not use out_flags in any way other than testing if (new_flags & out_flags) is 0. This is how PR_Poll (the primary user and hence the de facto specification of the poll method) uses out_flags.
Original Document Information
- Author: Wan Teh Chang
- Last Updated Date: June 27, 2006