Add attribute values support to XML generation

[999Marius]

PR Description

This PR adds support for including attribute values in XML generation within libiio. The new functionality allows XML output to contain both attribute names and their values.

Currently, iio-emu has separate XML generation logic that duplicates functionality in libiio.
These commits add support for including attribute values in XML generation. This is the first step toward integrating iio-emu functionality into libiio.

Changes

• Add attribute value storage arrays to iio_device, iio_channel and iio_buffer structures
• Extend XML generation functions to optionally include attribute values
• Add iio_context_get_xml_with_values() public API
• Add -e flag to iio_genxml utility
• Implement iio_context_refresh_attr_values() to populate value arrays
• Implement iio_context_free_attr_values() to free value arrays

The existing XML generation functions are extended with an include_values parameter. When enabled, attribute values are refreshed from devices and serialized into the XML alongside their names.

How it works

1. When iio_context_get_xml_with_values() is called, it invokes iio_context_generate_xml(ctx, true)
2. iio_context_generate_xml(ctx, true) first calls iio_context_refresh_attr_values() to read current attribute values from the hardware
3. The refresh function iterates through each device and:
   • Calls iio_refresh_device_attr_values() to read device and debug attributes, allocating storage arrays on first use
   • For each channel, calls iio_refresh_channel_attr_values() to read channel attributes, allocating storage arrays on first use
   • For each buffer, calls iio_refresh_buffer_attr_values() to read buffer attributes, allocating storage arrays on first use
4. After values are refreshed, iio_snprintf_context_xml() generates the XML with the include_values flag, serializing stored
   values alongside attribute names
5. Before returning, iio_context_generate_xml() frees all attribute values via iio_context_free_attr_values()
6. The complete XML string with values is returned to the user

Usage

iio_genxml -e

PR Type

• Bug fix (a change that fixes an issue)
• New feature (a change that adds new functionality)
• Breaking change (a change that affects other repos or cause CIs to fail)

PR Checklist

• I have conducted a self-review of my own code changes
• I have commented new code, particularly complex or unclear areas
• I have checked that I did not introduce new warnings or errors (CI output)
• I have checked that components that use libiio did not get broken
• I have updated the documentation accordingly (GitHub Pages, READMEs, etc)

[999Marius]

Feedback Request

I wanted to get feedback on the -v short flag for --values. While it's distinct from -V (version), it may be confused with it.

Alternative options I thought of:

• -a, --values (for "attribute values")
• -w, --values (for "with values")
• --values only

Open to other suggestions for better naming as well.

Note: I discovered that buffer attributes changed (thanks to the pre-allocated buffer changes in main). This means buffer attribute values could and will be added to XML generation as a follow-up push.

[999Marius]

Update V2:

• Added buffer attribute value support (all attribute types are now supported)
• Ready for review

[rgetz]

does -e --emulator make more sense?

[999Marius]

does -e --emulator make more sense?

Good point, -e, --emulator is more accurate since this is intended for iio-emu integration.
Will update.

[999Marius]

Update V3:

• Renamed flag from -v (--values) to -e (--emulator)
• Rebased on latest main