What does pragma keylist keyword do?

Question

While reading about various IoT messaging protocols I came across a structure defined as below:

enum TempScale {
   CELSIUM,
   KELVIN,
   FARENHEIT
};

struct TempSensorType {
   short id;
   float temp;
   float hum;
   TempScale scale;
};
#pragma keylist TempSensorType id

My question is: What does this #pragma keylist keyword do and where can I find some documentation about using #pragma preprocessor directives (I believe it is such directive..).

Thanks.

Any use of `#pragma` is **strictly implementation defined**. Indeed that is the *purpose* of `#pragma`. So the **only** place any `#pragma` *could* be documented is the compiler manual. (That `#pragma` is strictly non-portable should be obvious.) — DevSolar, Jul 22 '15 at 11:16
Thanks DevSolar. This made me look into clang and gcc documentation and search there for pragmas. — lewiatan, Jul 22 '15 at 21:30

score 4 · Accepted Answer · answered Jul 21 '15 at 21:09

The #pragma you are looking at is the PrismTech method for defining a key value within an OMG-DDS (Data Distribution Service for Real-Time Systems) Type structure. In this case, it is defining the short 'id' as a key value. The comparable RTI definition would be

struct TempSensorType {
    short id; //@key
    float temp;
    float hum;
    TempScale scale;
}

For interoperability between vendors' implementations, you can safely do

struct TempSensorType {
    short id; //@key
    float temp;
    float hum;
    TempScale scale;
}
#pragma keylist TempSensorType id

because the RTI compiler ignores the pragmas, and the PT compiler ignores the //@key.

This will change with future versions of the specification for Extensible Types, which will define a standard method for all vendors to support.

Note that if you were looking at a generic list of IoT messaging protocols, the concept of a "key" value may not exist in the other messaging protocols you were looking at.

score 1 · Answer 2 · answered Jul 23 '15 at 03:07

Note that for DDS implementations that comply with the recently-adopted OMG DDS-XTYPES specification (http://www.omg.org/spec/DDS-XTypes/) the standard portable way to specify keys is either:

struct SensorType {
    @key short id;
    float temp;
    float hum;
    TempScale scale;
}

Or alternatively (to avoid breaking IDL compilers that do not understand the IDL annotations):

struct SensorType {
    short id; //@key
    float temp;
    float hum;
    TempScale scale;
}

Hans van 't Hag · Answer 3 · 2015-07-22T11:10:02.747

0

Note that the explicit key-list specification via the pragma allows to define a ordering in the keys which - depending on the use-case - could have noticable performance impact on maintaining (filling/reading/querying/filtering) the 'multi-dimensional-storage' for a dataReader (and/or durability-service)

edited Jul 22 '15 at 11:10

answered Jul 22 '15 at 10:41

Hans van 't Hag

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4

What does pragma keylist keyword do?

3 Answers3