In Nomenclature we mentioned that poke, the program, implements a domain specific programming language called Poke, with a big p. In the examples so far we have already used the Poke language, somewhat extensively, while interacting with the program using the REPL.
For example, in:
(poke) 10 + 2 12
We are giving poke a Poke expression 10 + 2 to be
evaluated. Once the expression is evaluated, the REPL prints the
resulting value for us.
Similarly, when we define a variable or a type with var and
type respectively, we are providing poke definitions to be
evaluated. When we assign a value to a variable we are actually
providing a Poke statement to be executed.
So the REPL accepts poke commands, some of which happen to be
Poke expressions, definitions or statements. But we also have used
dot-commands like .file or .info. These dot-commands
are not part of the Poke programming language.
Every time we insert a line in the REPL and hit enter, poke recognizes the nature of the line, and then does the right thing. If the line is recognized as a Poke expression, for example, the Poke compiler is used to compile the statement into a routine, that is executed by the Poke Virtual Machine. The resulting value is then printed for the benefit of the user.
Poke programs are basically a collection of definitions and statements, which most often than not are stored in files, which avoids the need to type them again and again. By convention, we use the .pk file extension when naming files containing Poke programs.
Remember how we defined the foreground and background pixels for
p.sbm in the REPL?
(poke) var bga = [255UB, 255UB, 255UB] (poke) var fga = [255UB, 99UB, 71UB]
Where bga is a white pixel and fga is a tomato colored
pixel. We could write these definitions in a file colors.pk
like this:
var white = [255UB, 255UB, 255UB]; var tomato = [255UB, 99UB, 71UB];
Note that variable definitions in Poke are terminated by a semicolon
(;) character, but we didn’t need to specify them when we
issued the definitions in the REPL. This is because poke adds the
trailing semicolon for us when it detects a Poke construction
requiring it is introduced in the REPL.
Another difference is that Poke constructions can span for multiple lines, like in most programming languages. For example, we could have the following variable definition in a file:
var matrix = [[10, 20, 30],
[40, 50, 60],
[70, 80, 90]];
Once we have written our colors.pk file, how can we make poke
aware of it? A possibility is to use the load construction:
(poke) load colors
Assuming a file named colors.pk exists in the current working directory, poke will load it and evaluate its contents. After this, we can use the colors:
(poke) tomato [0xffUB,0x63UB,0x47UB]
Before, we would need to define these colors every time we would like to poke SBM files or, in fact, any RGB24 data. Now we just have to load the file colors.pk and use the variables defined there.
If you try to load a file whose name contains a dash character
(-) you will get an error message:
(poke) load my-colors
<stdin>:1:8: error: syntax error, unexpected '-', expecting ';'
load my-colors;
^
This is because the argument to load is interpreted as a Poke
identifier, and dash characters are not allowed in identifiers. To
alleviate this problem, you can also specify the name of the file to
load encoded in a string, line in:
(poke) load "my-colors.pk"
If you use this form of load, however, you are required to
specify the complete name of the file, including the .pk file
extension.
Since loading files is such a common operation, poke provides a
dot-command .load that does auto-completion:
(poke) .load my-colors.pk
Which is equivalent to load "my-colors.pk".
Since load is part of the Poke language, it can also be used in
Poke programs stored in files. We will explore this later.
In the last section we defined a couple of RGB colors white and
tomato in a file called colors.pk. If we keep adding
colors to the file, we may end with a nice collection of colors that
are available to us at any time, by just loading the file.
Since there are many ways to understand the notion of “color”, and also many ways to implement these many notions, it would be better to be more precise and call our file rgb24.pk, since the notion of color we are using is of that RGB24. While are at it, let’s also rename the variables to reflect the fact they denote not just any kind of colors, but RGB24 colors:
var rgb24_white = [255UB, 255UB, 255UB]; var rgb24_tomato = [255UB, 99UB, 71UB];
At this point, we can also add the definitions of a couple of types to our rgb24.pk:
type RGB_Color_Beam = byte; type RGB24_Color = RGB_Color_Beam[3]; var rgb24_white = [255UB, 255UB, 255UB]; var rgb24_tomato = [255UB, 99UB, 71UB];
Any time we want to manipulate RGB24 colors we can just load the file rgb24.pk and use these types and variables.
In poke parlance we call files like the above, that contain definitions of conceptually related entities, pickles. Pickles can be very simple, like the rgb24.pk sketched above, or fairly complicated like dwarf.pk.
It is common for pickles to load other pickles. For example,
if we were to write a SBM pickle, we would load the RGB24 pickle from
it:
load rgb24; [... SBM definitions ...]
This way, when we load sbm from the repl, the dependencies
get loaded as well.
GNU poke includes several already written pickles for commonly used
file formats, and other domains. The load construction knows
where these pickles are installed, so in order to load the pickle to
manipulate ELF files, for example, all you have to do is to:
(poke) load elf
As we have seen, a pickle provides Poke variables, types and functions related to some definite domain. Let’s say we are interested in editing an ELF file. We know that GNU poke comes with a pre-installed pickle named elf.pk. We can load it like this:
(poke) load elf
By convention, the entities provided by a pickle foo.pk are usually prefixed like:
foo_.
Foo_.
Therefore, once the pickle is loaded we can use the .info
dot-command in order to get a glimpse of the functionality provided by
the pickle. See .info. Example:
(poke) .info variables elf Name Declared at elf_stb_names elf-common.pk:53 elf_stt_names elf-common.pk:72 elf_stv_names elf-common.pk:84
The above tells us that the elf.pk pickle provides these three variables. Types are way more interesting:
(poke) .info types Elf64 Name Declared at Elf64_Ehdr elf-64.pk:184 Elf64_Off elf-64.pk:26 Elf64_SectionFlags elf-64.pk:130 Elf64_Shdr elf-64.pk:152 Elf64_RelInfo elf-64.pk:36 Elf64_Chdr elf-64.pk:85 Elf64_Rela elf-64.pk:61 Elf64_Phdr elf-64.pk:169 Elf64_Sxword elf-64.pk:24 Elf64_Sym elf-64.pk:71 Elf64_Rel elf-64.pk:45 Elf64_File elf-64.pk:207 Elf64_Addr elf-64.pk:25 Elf64_Group elf-64.pk:121 Elf64_Dyn elf-64.pk:98 Elf64_Xword elf-64.pk:23
We immediately notice the Elf64_File. Looks like what we would
need to map in order to access the entire ELF data in some given IO
space (like a file.) We can ask for more details about it:
(poke) .info type Elf64_File Class: struct Name: Elf64_File Complete: no Fields: Name Type ehdr Elf64_Ehdr shdr Elf64_Shdr[] phdr Elf64_Phdr[] Methods: Name Type get_section_name (offset<Elf_Word,8>)string get_symbol_name (Elf64_Shdr,,offset<Elf_Word,8>)string get_sections_by_name (string)Elf64_Shdr[] get_sections_by_type (Elf_Word)Elf64_Shdr[] section_name_p (string)int get_string (offset<Elf_Word,8>)string get_group_signature (Elf64_Shdr)string get_group_signatures ()string[] get_section_group (string)Elf64_Shdr[]
We see that an Elf64_File has three fields, and the nature of
these files: a header ehdr, an array of section headers
shdr and an array of program headers phdr.
The type description above also lists the methods defined in the type.
Looking at the method name and its type is very usually revealing
enough to use it. For example, we see that
get_sections_by_name returns an array of section headers,
characterized by some given section name:
(poke) var elf = Elf64_File @ 0#B
(poke) elf.get_sections_by_name (".text")
[Elf64_Shdr {
sh_name=0x1bU#B,
sh_type=0x1U,
sh_flags=#<ALLOC,EXECINSTR>,
sh_addr=0x0UL#B,
sh_offset=0x40UL#B,
sh_size=0xbUL#B,
sh_link=0x0U,
sh_info=0x0U,
sh_addralign=0x1UL,
sh_entsize=0x0UL#b
}]
Ultimately, of course the best way to see what a pickle provides is to go read its source code.
When poke starts it loads the file .pokerc located in our home directory, if it exists. This initialization file contains poke commands, one per line.
If we wanted to get some Poke file loaded at startup, we could do it
by adding a load command to our .pokerc. For example:
# My poke configuration - jemarch [...] .load ~/.poke.d/mydefs.pk