Re: Accessing waveform record values in C

["Hu, Yong via Tech-talk" <tech-talk at aps.anl.gov>]

Another way without any C codes: use “subArray” record(s) to extract individual element(s) from a waveform record.

record(subArray, "second_element")

{

              field(INP, "waveform_record CP")

              field(FTVL, "CHAR")

              field(MALM, "250")

              field(NELM, 1)

              field(INDX, 1)

}

 

See this for details (or search other website): https://wiki-ext.aps.anl.gov/epics/index.php/RRM_3-14_SubArray

 

HTH,

Yong Hu

 

From: Tech-talk <tech-talk-bounces at aps.anl.gov> on behalf of "tech-talk at aps.anl.gov" <tech-talk at aps.anl.gov>
Reply-To: Andy Foster <ajf at observatorysciences.co.uk>
Date: Thursday, February 18, 2021 at 3:22 AM
To: "Wang, Andrew" <wang126 at llnl.gov>
Cc: "tech-talk at aps.anl.gov" <tech-talk at aps.anl.gov>
Subject: Re: Accessing waveform record values in C

 

Hi Andy,

 

Have you set the field type of field "A" in the aSub record to CHAR? FTA set to CHAR.

 

Cheers, Andy

 

On Thu, 18 Feb 2021, 05:23 Wang, Andrew via Tech-talk, <tech-talk at aps.anl.gov> wrote:

All,

 

Basically I have a waveform record that contains ASCII values of chars shown below.

 

record(waveform, "waveform_record")

{

              field(DESC, "")

              field(SCAN, "Passive")

              field(FTVL, "CHAR")

              field(NELM, 250)

}

 

When I input caget waveform_record, I get:

 

waveform_record 250 72 101 108 108 111 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.

 

My goal is to access 72, 101, 108, 108, and 111 in a C code for an asub record.

 

record(aSub, "asub_record")

{

              field(DESC, "")

              field(SCAN, "Passive")

              field(SNAM, "c_code")

              field(INPA, "waveform_record")

              field(NOA, 250)

              …

}

 

In my C code, I attempt to access the waveform record.

 

char *waveform_record = (char *)prec->a;

printf("%s\n", waveform_record);

 

The terminal only prints out 72, the first element. But when I try to do

 

char *waveform_record = (char *)prec->a;

printf("%s\n", *(waveform_record + 4));

 

I get a segmentation fault rather than 111. Any advice on how I could potentially access the other elements in the waveform record? Feel like I am missing something very basic here.

 

Thanks,

Andy