bugAVR C Runtime Library - Bugs: bug #35240, Incomplete Description of Calling...


bug #35240: Incomplete Description of Calling Convention

Submitted by:  Don Kinzer <dkinzer>
Submitted on:  Wed 04 Jan 2012 11:42:04 PM UTC  
Category:  Documentation Severity:  3 - Normal
Priority:  9 - Immediate Item Group:  Documentation
Status:  None Assigned to:  None
Percent Complete:  0% Open/Closed:  Open
Release:  1.6.7 Fixed Release:  None

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Sun 04 Nov 2012 07:49:44 PM UTC, comment #1: 

For a complete description of the calling conversion, see the avr-gcc AVI in the GCC Wiki:


Georg-Johann Lay <gjlayde>
Wed 04 Jan 2012 11:42:04 PM UTC, original submission:  

The avr-libc documentation section "11.14 What registers are used by the C compiler?" is incomplete in that it does not describe the case where a function returns a large (>64 bit) data item.  As far as I can tell, in this (perhaps unusual) case r25:24 contains a pointer to a buffer in which to return the data and all parameters (if any) are shifted by two registers (i.e. a first uint16_t parameter is in r23:22, etc.).  Large pointer support may change the register use in this case.

This observation applies to the documentation accompanying avr-libc v1.6.7 and v1.8.0.

Don Kinzer <dkinzer>


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  • -email is unavailable- added by arcanum (Updated the item)
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    Follows 1 latest change.

    Date Changed by Updated Field Previous Value => Replaced by
    2012-11-16 arcanum Priority5 - Normal => 9 - Immediate

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