How to write new line to file in VHDL? - vhdl

Question

I would like to separate my data with new line character in an output file, but the following codes result in error "can't resolve overload for procedure call":
write(out_line, "\n");
write(out_line, "");
write(out_line, '');
An example code how I want to use it:
ENTITY writer IS
PORT ( clk : IN STD_LOGIC := '0'; start : IN STD_LOGIC := '0');
END ENTITY;
ARCHITECTURE arch OF writer IS
SIGNAL vect : STD_LOGIC_VECTOR (2 downto 0) := "000";
TYPE state_type IS (init, write_file);
SIGNAL state : state_type := init;
BEGIN
PROCESS (clk, start)
FILE out_file : text;
VARIABLE out_line : line;
BEGIN
IF rising_edge(clk) THEN
CASE state IS
WHEN init =>
IF start = '1' THEN
state <= write_file;
ELSE
state <= init;
END IF;
WHEN write_file =>
state => init;
FOR i IN 0 TO 10 LOOP
write(out_line, vect);
writeline(out_file, out_line);
-- write(out_line, "\n"); <--
-- write(out_line, ""); <--
-- write(out_line, ''); <-- None of these work
writeline(out_file, out_line);
END LOOP;
END CASE;
END IF;
END PROCESS;
END ARCHITECTURE;
So I would like to know, is it possible in VHDL? If yes, how?

Answer 1

The following will consistently give you a single blank line:
write(out_line, string'(""));
writeline(out_file, out_line);
I suspect what #Dani posted may be tool dependent. For example while on one popular simulator, the following produces one line feed:
write(out_line, LF);
writeline(out_file, out_line);
However when I add a space after the LF, I get two lines:
write(out_line, LF & ' ');
writeline(out_file, out_line);

Answer 2

Creating a minimal, complete and verifiable example from the question's incomplete sample code:
library ieee; -- ADDED
use ieee.std_logic_1164.all; -- ADDED
use std.textio.all; -- ADDED
-- use ieee.std_logic_textio.all; -- ADDED for revisions earlier than -2008
ENTITY writer IS
-- PORT ( clk : IN STD_LOGIC := '0'; start : IN STD_LOGIC := '0');
END ENTITY;
ARCHITECTURE arch OF writer IS
SIGNAL vect : STD_LOGIC_VECTOR (2 downto 0) := "000";
-- TYPE state_type IS (init, write_file);
-- SIGNAL state : state_type := init;
BEGIN
PROCESS -- (clk, start)
FILE out_file : text;
VARIABLE out_line : line;
BEGIN
file_open(out_file, "some_file", WRITE_MODE); -- ADDED
-- IF rising_edge(clk) THEN
-- CASE state IS
-- WHEN init =>
-- IF start = '1' THEN
-- state <= write_file;
-- ELSE
-- state <= init;
-- END IF;
-- WHEN write_file =>
-- state => init;
FOR i IN 0 TO 10 LOOP
write(out_line, vect);
writeline(out_file, out_line);
-- write(out_line, "\n"); <--
-- write(out_line, ""); <--
-- write(out_line, ''); <-- None of these work
writeline(out_file, out_line);
END LOOP;
-- END CASE;
-- END IF;
wait; -- ADDED
END PROCESS;
END ARCHITECTURE;
demonstrates a way to get a blank line in the output:
some_file contents:
000
000
000
000
000
000
000
000
000
000
000
The second writeline procedure call produces an empty line without an intervening write procedure call.
Why is seen in IEEE Std 1076-2008 16.4 Package TEXTIO:
Procedures READLINE, WRITELINE, and TEE declared in package TEXTIO read and write entire lines of a file of type TEXT. Procedure READLINE causes the next line to be read from the file and returns as the value of parameter L an access value that designates an object representing that line. If parameter L contains a non-null access value at the start of the call, the procedure may deallocate the object designated by that value. The representation of the line does not contain the representation of the end of the line. It is an error if the file specified in a call to READLINE is not open or, if open, the file has an access mode other than read-only (see 5.5.2). Procedures WRITELINE and TEE each cause the current line designated by parameter L to be written to the file and returns with the value of parameter L designating a null string. Procedure TEE additionally causes the current line to be written to the file OUTPUT. If parameter L contains a null access value at the start of the call, then a null string is written to the file or files. If parameter L contains a non-null access value at the start of the call, the procedures may deallocate the object designated by that value. It is an error if the file specified in a call to WRITELINE or TEE is not open or, if open, the file has an access mode other than write-only.
The language does not define the representation of the end of a line. An implementation shall allow all possible values of types CHARACTER and STRING to be written to a file. However, as an implementation is permitted to use certain values of types CHARACTER and STRING as line delimiters, it might not be possible to read these values from a TEXT file.
A line feed (LF) format effector occurring as an element of a string written to a file of type TEXT, either using procedure WRITELINE or TEE, or using the WRITE operation implicitly defined for the type TEXT, is interpreted by the implementation as signifying the end of a line. The implementation shall transform the LF into the implementation-defined representation of the end of a line.
...
For each WRITE, OWRITE, and HWRITE procedure, after data is appended to the string value designated by the parameter L, L designates the entire line. The procedure may modify the value of the object designated by the parameter L at the start of the call or may deallocate the object.
If deallocation occurs out_line will have a value of null after a writeline call and a null string is written in the immediately following writeline call which also provides an end of line.
If the object value accessed by out_line is a null array (having no elements, 5.3.2.2 Index constraints and discrete ranges, a null string) the immediately following writeline call will result in an end of line being written to the file.
In essence your code example already contains one of these methods for writing a blank line, which depends on whether deallocation is used conditionally (may).
Variants of allocate() and free() can be relatively expensive in terms of execution time and when the size of the allocated object and it's element size is known a smaller 'allocated' object can be written to the same object space saving simulation time. The simulation kernel representation of an array object can have bounds separate from the array value, deallocation and re-allocation can be reserved for when the object size is larger than the previously allocated size or an explicit deallocate call occurs.
There's also a requirement that an implementation translate an LF character to an end of line in a write to a file. This is the other mechanism that allows you to write an LF character as the last or only character to a line and get a following blank line.
You could also explicitly write a null string to out_line
write(out_line, string'(""));
prior to the second writeline call. The qualified expression provides the type of the string literal unlike the attempt commented out in the original question where the type of the string literal can't be determined. See 9.3.2 Literals "... The type of a string or bit string literal shall be determinable solely from the context in which the literal appears, excluding the literal itself but using the fact that the type of the literal shall be a one-dimensional array of a character type. ...". The procedure write would be ambiguous in this context, failing overload resolution (12.5 The context of overload resolution).

Answer 3

Finally after a lot of searching and trying I found that the following code works:
write(out_line, lf);
writeline(out_file, out_line);
I found that write(out_line, cr); does the same thing, and write(out_line, nul); adds ' ' character between the outputs.