# Len\_tag\_key

**URL:** <https://www.ruby-forum.com/t/len-tag-key/230673>\
**Category:** GNU Radio\
**Created:** [June 29, 2014, 10:50am UTC](https://www.ruby-forum.com/t/len-tag-key/230673 "2014-06-29T10:50:37Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![jason\_sam](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/7993a0/32.png) [@jason\_sam](https://www.ruby-forum.com/u/jason_sam)\
**Post date:** [June 29, 2014, 10:50am UTC](https://www.ruby-forum.com/t/len-tag-key/230673/1 "2014-06-29T10:50:37Z")

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Dear all ,  
I use a flowgraph containing : File source , OFDM TX ,throttle, File  
sink  
(I will replace then the file sink by UHD sink to try with USRP)  
I got this error message

FATAL: Missing a required length tag on port 0 at item #0  
thread[thread-per-block[1]: \<block crc32\_bb (14)\>]: Missing length tag.

I tried to solve this problem but I failed :  
in TX in the field called length tag name ,  
I tried to write same input to this field as in the loopback :  
I wrote in this field : len\_tag\_key  
Then I put a variable block with ID :len\_tag\_key , with value :  
“packet\_len”  
Then I put another variable block with ID: packet\_len , with value:87

Also I tried to write in this field : packet\_len & put only one variable  
block with ID packet\_len & value:87 … I got the same error !!  
What is must be written in this field ?!!!

Thanks.

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<div class="post-metadata">

**Author:** ![GP\_2014](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/f08c70/32.png) [@GP\_2014](https://www.ruby-forum.com/u/GP_2014)\
**Post date:** [June 30, 2014, 11:37am UTC](https://www.ruby-forum.com/t/len-tag-key/230673/2 "2014-06-30T11:37:16Z")

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On 06/29/2014 10:49 AM, GP 2014 wrote:

> I tried to write same input to this field as in the loopback :  
> I wrote in this field : len\_tag\_key  
> Then I put a variable block with ID :len\_tag\_key , with value : “packet\_len”  
> Then I put another variable block with ID: packet\_len , with value:87
> 
> Also I tried to write in this field : packet\_len & put only one variable  
> block with ID packet\_len & value:87 .. I got the same error !!  
> What is must be written in this field ?!!!

You must provide a key for the tagged stream tags. Read  
[http://gnuradio.org/doc/doxygen/page\_tagged\_stream\_blocks.html](http://gnuradio.org/doc/doxygen/page_tagged_stream_blocks.html) and the  
OFDM examples.

M