Streams API for PHP Extension Authors

Overview

The PHP Streams API introduces a unified approach to the handling of files and sockets in PHP extension. Using a single API with standard functions for common operations, the streams API allows your extension to access files, sockets, URLs, memory and script-defined objects. Streams is a run-time extensible API that allows dynamically loaded modules (and scripts!) to register new streams.

The aim of the Streams API is to make it comfortable for developers to open files, URLs and other streamable data sources with a unified API that is easy to understand. The API is more or less based on the ANSI C stdio family of functions (with identical semantics for most of the main functions), so C programmers will have a feeling of familiarity with streams.

The streams API operates on a couple of different levels: at the base level, the API defines php_stream objects to represent streamable data sources. On a slightly higher level, the API defines php_stream_wrapper objects which "wrap" around the lower level API to provide support for retrieving data and meta-data from URLs. An additional context parameter, accepted by most stream creation functions, is passed to the wrapper's stream_opener method to fine-tune the behavior of the wrapper.

Any stream, once opened, can also have any number of filters applied to it, which process data as it is read from/written to the stream.

Streams can be cast (converted) into other types of file-handles, so that they can be used with third-party libraries without a great deal of trouble. This allows those libraries to access data directly from URL sources. If your system has the fopencookie() or funopen() function, you can even pass any PHP stream to any library that uses ANSI stdio!

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Streams Basics

Using streams is very much like using ANSI stdio functions. The main difference is in how you obtain the stream handle to begin with. In most cases, you will use php_stream_open_wrapper() to obtain the stream handle. This function works very much like fopen, as can be seen from the example below:

php_stream * stream = php_stream_open_wrapper("http://www.php.net", "rb", REPORT_ERRORS, NULL);
if (stream) {
    while(!php_stream_eof(stream)) {
        char buf[1024];
        
        if (php_stream_gets(stream, buf, sizeof(buf))) {
            printf(buf);
        } else {
            break;
        }
    }
    php_stream_close(stream);
}

The table below shows the Streams equivalents of the more common ANSI stdio functions. Unless noted otherwise, the semantics of the functions are identical.

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Streams as Resources

All streams are registered as resources when they are created. This ensures that they will be properly cleaned up even if there is some fatal error. All of the filesystem functions in PHP operate on streams resources - that means that your extensions can accept regular PHP file pointers as parameters to, and return streams from their functions. The streams API makes this process as painless as possible:

PHP_FUNCTION(example_write_hello)
{
    zval *zstream;
    php_stream *stream;
    
    if (FAILURE == zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "r", &zstream))
        return;
    
    php_stream_from_zval(stream, &zstream);

    /* you can now use the stream.  However, you do not "own" the
        stream, the script does.  That means you MUST NOT close the
        stream, because it will cause PHP to crash! */

    php_stream_write(stream, "hello\n");
        
    RETURN_TRUE();
}

PHP_FUNCTION(example_open_php_home_page)
{
    php_stream *stream;
    
    stream = php_stream_open_wrapper("http://www.php.net", "rb", REPORT_ERRORS, NULL);
    
    php_stream_to_zval(stream, return_value);

    /* after this point, the stream is "owned" by the script.
        If you close it now, you will crash PHP! */
}

Since streams are automatically cleaned up, it's tempting to think that we can get away with being sloppy programmers and not bother to close the streams when we are done with them. Although such an approach might work, it is not a good idea for a number of reasons: streams hold locks on system resources while they are open, so leaving a file open after you have finished with it could prevent other processes from accessing it. If a script deals with a large number of files, the accumulation of the resources used, both in terms of memory and the sheer number of open files, can cause web server requests to fail. Sounds bad, doesn't it? The streams API includes some magic that helps you to keep your code clean - if a stream is not closed by your code when it should be, you will find some helpful debugging information in you web server error log.

Note: Always use a debug build of PHP when developing an extension (--enable-debug when running configure), as a lot of effort has been made to warn you about memory and stream leaks.

In some cases, it is useful to keep a stream open for the duration of a request, to act as a log or trace file for example. Writing the code to safely clean up such a stream is not difficult, but it's several lines of code that are not strictly needed. To save yourself the trouble of writing the code, you can mark a stream as being OK for auto cleanup. What this means is that the streams API will not emit a warning when it is time to auto-cleanup a stream. To do this, you can use php_stream_auto_cleanup().

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Streams open options

These constants affect the operation of stream factory functions.

IGNORE_PATH

This is the default option for streams; it requests that the include_path is not to be searched for the requested file.

USE_PATH

Requests that the include_path is to be searched for the requested file.

IGNORE_URL

Requests that registered URL wrappers are to be ignored when opening the stream. Other non-URL wrappers will be taken into consideration when decoding the path. There is no opposite form for this flag; the streams API will use all registered wrappers by default.

IGNORE_URL_WIN

On Windows systems, this is equivalent to IGNORE_URL. On all other systems, this flag has no effect.

ENFORCE_SAFE_MODE

Requests that the underlying stream implementation perform safe_mode checks on the file before opening the file. Omitting this flag will skip safe_mode checks and allow opening of any file that the PHP process has rights to access.

REPORT_ERRORS

If this flag is set, and there was an error during the opening of the file or URL, the streams API will call the php_error function for you. This is useful because the path may contain username/password information that should not be displayed in the browser output (it would be a security risk to do so). When the streams API raises the error, it first strips username/password information from the path, making the error message safe to display in the browser.

STREAM_MUST_SEEK

This flag is useful when your extension really must be able to randomly seek around in a stream. Some streams may not be seekable in their native form, so this flag asks the streams API to check to see if the stream does support seeking. If it does not, it will copy the stream into temporary storage (which may be a temporary file or a memory stream) which does support seeking. Please note that this flag is not useful when you want to seek the stream and write to it, because the stream you are accessing might not be bound to the actual resource you requested.

Note: If the requested resource is network based, this flag will cause the opener to block until the whole contents have been downloaded.

STREAM_WILL_CAST

If your extension is using a third-party library that expects a FILE* or file descriptor, you can use this flag to request the streams API to open the resource but avoid buffering. You can then use php_stream_cast() to retrieve the FILE* or file descriptor that the library requires. The is particularly useful when accessing HTTP URLs where the start of the actual stream data is found after an indeterminate offset into the stream. Since this option disables buffering at the streams API level, you may experience lower performance when using streams functions on the stream; this is deemed acceptable because you have told streams that you will be using the functions to match the underlying stream implementation. Only use this option when you are sure you need it.