Experimental Physics and
Industrial Control System

TZVETKOV Stephane via Tech-talk <tech-talk at aps.anl.gov> · Fri, 7 Jul 2023 09:49:21 +0000

Hi everyone,

I'd like to share my observation about the maximum size of a macro.

I was working on an IOC with a record similar to the one below:
```
record(waveform,"my:test:waveform") {
    field(FTVL, "CHAR")
    field(NELM, 1024)
    field(INP, { const: "${A_MACRO_CONTAINING_A_VERY_VERY_LONG_STRING}" } )
}
```

I somehow found out that the associated NORD field was always limited to 256, and that my string was always cropped to 256 characters. It took me some time to actually understand that the limitation was that an EPICS macro has a maximum size of 256 characters. E.g., the below record would not have such problem:
```
record(waveform,"my:test:waveform") {
    field(FTVL, "CHAR")
    field(NELM, 1024)
    field(INP, { const: "a plain text very long string, containing more than 256 characters, not in a macro..." } )
}
```

(by the way, I also discovered that the name of a macro is also limited to 256 characters)

In order to work around this limitation, I just recompiled the EPICS base with a minor modification in `modules/libcom/src/macLib/macLib.h`. E.g., changing the maximum size from 256 to 65535:
```
vi modules/libcom/src/macLib/macLib.h
        > ...
     ~ > #define MAC_SIZE 65535
        > ...
```

As far as I have tested, this modification didn't seem to cause any problem. But, I wonder if more experienced EPICS developers have comments about this. Maybe increasing macro size has side effects that I didn't anticipated. Maybe there is a cleaner/better way to do this.

I'd like to hear your feedbacks, if any.

I didn't found a lot of documentation/examples on the subject, so I hope that this tech-talk message will help future readers for troubleshooting this kind of issue.

Special thanks to Rémi NICOLE for his help.

Cheers,

Stéphane Tzvetkov
CEA Saclay – DRF / IRFU / DIS / LDISC

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System