MDN’s new design is in Beta! A sneak peek:

« Gecko Plugin API Reference « Plug-in Side Plug-in API


Requests a range of bytes from a seekable stream. This initiates a read operation; the actual data is received through subsequent calls to NPP_WriteReady() and NPP_Write().


#include <npapi.h>
NPError NPN_RequestRead(NPStream*    stream,
                        NPByteRange* rangeList);


The function has the following parameters:

Stream of type NP_SEEK from which to read bytes.
Range of bytes in the form of a linked list of NPByteRange objects, each of which specifies a request for a range of bytes.


  • If successful, the function returns NPERR_NO_ERROR.
  • If unsuccessful, the function returns an error code. For possible values, see Error Codes.


For a seekable stream, the browser sends data only in response to requests by the plug-in. The plug-in calls NPN_RequestRead() to request data from a seekable stream.

The plug-in can use this function to make one or more requests for ranges of bytes. These requests result in subsequent calls to NPP_WriteReady() and NPP_Write(). For multiple requests, the function creates a linked list of NPByteRange structures, each of which represents a separate request.

If the plug-in requests multiple ranges (either through a list of NPByteRange objects in a single call to NPN_RequestRead(), or multiple calls to NPN_RequestRead()), the browser can write individual ranges in any order, and with any number of NPP_WriteReady() and NPP_Write() calls.

The plug-in must allocate NPByteRange objects, which the browser copies if necessary. The plug-in can free these as soon as the call returns.

Seekable streams are created by calling NPP_NewStream() with NP_SEEK as the stype mode.

  • The plug-in can call NPN_RequestRead() on streams that were not initially in NP_SEEK mode as long as the stream is inherently seekable; NPN_RequestRead() automatically changes the mode to NP_SEEK.
  • If the stream is not inherently seekable, the stream must have been put in NP_SEEK mode initially (since the browser must cache all the stream data on disk in order to access it randomly).
  • If NPN_RequestRead() is called on a stream that is not inherently seekable and not initially in mode NP_SEEK, it returns the error code NPERR_STREAM_NOT_SEEKABLE.

Typically, the only streams that are inherently seekable are those from in-memory or on-disk data, or from HTTP servers that support byte-range requests.

See also

Document Tags and Contributors

 Contributors to this page: teoli, kscarfone, Sheppy, jswisher, Pmash
 Last updated by: teoli,