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244 lines
8.8 KiB
Plaintext
244 lines
8.8 KiB
Plaintext
An Overview of the PHP Streams abstraction
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==========================================
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$Id$
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Please send comments to: Wez Furlong <wez@thebrainroom.com>
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Note: this doc is preliminary and is intended to give the reader an idea of
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how streams work and should be used.
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Why Streams?
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============
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You may have noticed a shed-load of issock parameters flying around the PHP
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code; we don't want them - they are ugly and cumbersome and force you to
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special case sockets and files everytime you need to work with a "user-level"
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PHP file pointer.
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Streams take care of that and present the PHP extension coder with an ANSI
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stdio-alike API that looks much nicer and can be extended to support non file
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based data sources.
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Using Streams
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=============
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Streams use a php_stream* parameter just as ANSI stdio (fread etc.) use a
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FILE* parameter.
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The main functions are:
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PHPAPI size_t php_stream_read(php_stream * stream, char * buf, size_t count);
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PHPAPI size_t php_stream_write(php_stream * stream, const char * buf, size_t
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count);
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PHPAPI int php_stream_eof(php_stream * stream);
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PHPAPI int php_stream_getc(php_stream * stream);
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PHPAPI char *php_stream_gets(php_stream * stream, char *buf, size_t maxlen);
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PHPAPI int php_stream_close(php_stream * stream);
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PHPAPI int php_stream_flush(php_stream * stream);
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PHPAPI int php_stream_seek(php_stream * stream, off_t offset, int whence);
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PHPAPI off_t php_stream_tell(php_stream * stream);
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These (should) behave in the same way as the ANSI stdio functions with similar
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names: fread, fwrite, feof, fgetc, fgets, fclose, fflush, fseek, ftell.
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Opening Streams
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===============
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Ultimately, I aim to implement an fopen_wrapper-like call to do this with
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minimum fuss.
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Currently, mostly for testing purposes, you can use php_stream_fopen to open a
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stream on a regular file.
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PHPAPI php_stream * php_stream_fopen(const char * filename, const char *
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mode);
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This call behaves just like fopen(), except it returns a stream instead of a
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FILE *
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Casting Streams
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===============
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What if your extension needs to access the FILE* of a user level file pointer?
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You need to "cast" the stream into a FILE*, and this is how you do it:
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FILE * fp;
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php_stream * stream; /* already opened */
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if (php_stream_cast(stream, PHP_STREAM_AS_STDIO, &fp, 1) == FAILURE) {
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RETURN_FALSE;
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}
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The prototype is:
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PHPAPI int php_stream_cast(php_stream * stream, int castas, void ** ret, int
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show_err);
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The show_err parameter, if non-zero, will cause the function to display an
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appropriate error message of type E_WARNING if the cast fails.
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castas can be one of the following values:
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PHP_STREAM_AS_STDIO - a stdio FILE*
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PHP_STREAM_AS_FD - a generic file descriptor
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PHP_STREAM_AS_SOCKETD - a socket descriptor
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If you ask a socket stream for a FILE*, the abstraction will use fdopen to
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create it for you. Be warned that doing so may cause buffered data to be lost
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if you mix ANSI stdio calls on the FILE* with php stream calls on the stream.
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If your system has the fopencookie function, php streams can synthesize a
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FILE* on top of any stream, which is useful for SSL sockets, memory based
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streams, data base streams etc. etc.
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NOTE: There might be situations where this is not desireable, and we need to
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provide a flag to inform the casting routine of this.
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You can use:
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PHPAPI int php_stream_can_cast(php_stream * stream, int castas)
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to find out if a stream can be cast, without actually performing the cast, so
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to check if a stream is a socket you might use:
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if (php_stream_can_cast(stream, PHP_STREAM_AS_SOCKETD) == SUCCESS) {
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/* it's a socket */
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}
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Stream Internals
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================
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There are two main structures associated with a stream - the php_stream
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itself, which holds some state information (and possibly a buffer) and a
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php_stream_ops structure, which holds the "virtual method table" for the
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underlying implementation.
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The php_streams ops struct consists of pointers to methods that implement
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read, write, close, flush, seek, gets and cast operations. Of these, an
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implementation need only implement write, read, close and flush. The gets
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method is intended to be used for non-buffered streams if there is an
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underlying method that can efficiently behave as fgets. The ops struct also
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contains a label for the implementation that will be used when printing error
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messages - the stdio implementation has a label of "STDIO" for example.
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The idea is that a stream implementation defines a php_stream_ops struct, and
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associates it with a php_stream using php_stream_alloc.
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As an example, the php_stream_fopen() function looks like this:
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PHPAPI php_stream * php_stream_fopen(const char * filename, const char * mode)
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{
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FILE * fp = fopen(filename, mode);
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php_stream * ret;
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if (fp) {
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ret = php_stream_alloc(&php_stream_stdio_ops, fp, 0, 0, mode);
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if (ret)
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return ret;
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fclose(fp);
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}
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return NULL;
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}
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php_stream_stdio_ops is a php_stream_ops structure that can be used to handle
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FILE* based streams.
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A socket based stream would use code similar to that above to create a stream
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to be passed back to fopen_wrapper (or it's yet to be implemented successor).
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The prototype for php_stream_alloc is this:
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PHPAPI php_stream * php_stream_alloc(php_stream_ops * ops, void * abstract,
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size_t bufsize, int persistent, const char * mode)
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ops is a pointer to the implementation,
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abstract holds implementation specific data that is relevant to this instance
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of the stream,
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bufsize is the size of the buffer to use - if 0, then buffering at the stream
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level will be disabled (recommended for underlying sources that implement
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their own buffering - such a FILE*),
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persistent controls how the memory is to be allocated - persistently so that
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it lasts across requests, or non-persistently so that it is freed at the end
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of a request (it uses pemalloc),
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mode is the stdio-like mode of operation - php streams places no real meaning
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in the mode parameter, except that it checks for a 'w' in the string when
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attempting to write (this may change).
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The mode parameter is passed on to fdopen/fopencookie when the stream is cast
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into a FILE*, so it should be compatible with the mode parameter of fopen().
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Writing your own stream implementation
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======================================
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First, you need to figure out what data you need to associate with the
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php_stream. For example, you might need a pointer to some memory for memory
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based streams, or if you were making a stream to read data from an RDBMS like
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mysql, you might want to store the connection and rowset handles.
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The stream has a field called abstract that you can use to hold this data.
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If you need to store more than a single field of data, define a structure to
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hold it, allocate it (use pemalloc with the persistent flag set
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appropriately), and use the abstract pointer to refer to it.
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For structured state you might have this:
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struct my_state {
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MYSQL conn;
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MYSQL_RES * result;
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};
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struct my_state * state = pemalloc(sizeof(struct my_state), persistent);
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/* initialize the connection, and run a query, using the fields in state to
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* hold the results */
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state->result = mysql_use_result(&state->conn);
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/* now allocate the stream itself */
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stream = php_stream_alloc(&my_ops, state, 0, persistent, "r");
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/* now stream->abstract == state */
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Once you have that part figured out, you can write your implementation and
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define the your own php_stream_ops struct (we called it my_ops in the above
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example).
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For example, for reading from this wierd mysql stream:
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static size_t php_mysqlop_read(php_stream * stream, char * buf, size_t count)
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{
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struct my_state * state = (struct my_state*)stream->abstract;
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if (buf == NULL && count == 0) {
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/* in this special case, php_streams is asking if we have reached the
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* end of file */
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if (... at end of file ...)
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return EOF;
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else
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return 0;
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}
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/* pull out some data from the stream and put it in buf */
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... mysql_fetch_row(state->result) ...
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/* we could do something strange, like format the data as XML here,
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and place that in the buf, but that brings in some complexities,
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such as coping with a buffer size too small to hold the data,
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so I won't even go in to how to do that here */
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}
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Implement the other operations - remember that write, read, close and flush
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are all mandatory. The rest are optional. Declare your stream ops struct:
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php_stream_ops my_ops = {
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php_mysqlop_write, php_mysqlop_read, php_mysqlop_close,
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php_mysqlop_flush, NULL, NULL, NULL,
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"Strange mySQL example"
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}
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Thats it!
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Take a look at the STDIO implementation in streams.c for more information
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about how these operations work.
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The main thing to remember is that in your close operation you need to release
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and free the resources you allocated for the abstract field. In the case of
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the example above, you need to use mysql_free_result on the rowset, close the
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connection and then use pefree to dispose of the struct you allocated.
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You may read the stream->persistent field to determine if your struct was
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allocated in persistent mode or not.
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vim:tw=78
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