VHDL const string array with different length

Question

I want to make a list of strings in a Testbench to load different files for example.

I tried:

type tastring ARRAY(iADCCount_C-1 downto 0) of string;
constant Filenames     : tastring := ("file.txt",
                                      "anotherfile.txt",
                                      "jetanotherfile.txt");

Its not possible to have variable length strings in an array.

Also:

type tpstring is access string;
type tpastring is ARRAY(iADCCount_C-1 downto 0) of tpstring;
constant Filenames     : tpastring := (new string'("file.txt"),
                                       new string'(anotherfile.txt"),
                                       new string'(jetanotherfile.txt"));

Does not work! You cannot make an access type constant. Do I miss something? Is there a way to make a list of Strings without padding them to the same size?

See also the answer to this question: VHDL Port declaration with different sizes — Paebbels
– Paebbels, Commented Jan 12, 2018 at 19:25

Paebbels · Accepted Answer · 2018-01-13 09:33:41Z

2

You are almost correct :)

The second code snippet must use a variable, because access types can only be used for objects of kind variable.

type line_vector is array(iADCCount_C-1 downto 0) of line;
variable Filenames : line_vector := (
  new string'("file.txt"),
  new string'("anotherfile.txt"),
  new string'("jetanotherfile.txt")
);

Note 1: added missing " characters.
Note 2: type line is already defined in VHDL.
Note 3: type line_vector will be defined by VHDL-2017.

As an alternative, you can fill all string with character NUL. You might want to implement two functions for sizing the string to the constant's size and to trim the string (remove trailing NUL characters.

edited Jan 13, 2018 at 9:33

answered Jan 12, 2018 at 19:22

Paebbels

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2 Comments

user1155120 Over a year ago

Both the OP's second snippet and your answer are invalid VHDL, missing double quotes delimiting the beginning of the string literals in the initial value aggregate's second and third association element expressions. From IEEE Std 1076 (the LRM) revision -2000 onward a shared variable is required to be a protected type. A shared variable would allow access through protected type methods to Filenames by multiple process statements in the OP's testbench.

Paebbels Over a year ago

@user1155120 Fixed the missing quotes. It was a copy of the OP's code snippet. He has not expressed to use his code in an architecture. So if it's used in a process, it would be valid - but unusual, right?

DLnd · Accepted Answer · 2018-01-12 10:25:34Z

1

See this answer. AFAIK it is impossible to have arrays of variable-length strings.

If you implement custom trim functions as suggested in the answer, try using a fixed spacing character that cannot exist in file names(like ? for windows), as it will also ensure no problems with NUL-characters or other non-prinatbles that can cause hiccoughs with different synthesis tools.

answered Jan 12, 2018 at 10:25

DLnd

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1 Comment

Paebbels Over a year ago

Yes, but his second example tries to use pointers to strings of different lengths. He just needs to use a variable instead of a constant.

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