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<a name="Characters-and-Strings"></a>
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<hr size="1">
<a name="Characters-and-Strings-1"></a>
<h2 class="section">3.2 Characters and Strings</h2>

<a name="index-character-strings-4"></a>
<p>If a C program is to interact with a virtual code application by
exchanging text, it uses the representation for characters described in
<a href="Character-Table.html#Character-Table">Character Table</a>.  This convention would be inconvenient without a
suitable API, so the functions in this section address the need. These
functions are declared in the header file &lsquo;<tt>chrcodes.h</tt>&rsquo;.
</p>
<p>Some of these functions have two forms, with one of them having the
word <code>standard</code> as part of its name. The reason is to cope with
multiple character encodings. Versions of <code>avram</code> prior to 0.1.0
<a name="index-character-encodings"></a>
<a name="index-multiple-character-encodings"></a>
used a different character encoding than the one documented in
<a href="Character-Table.html#Character-Table">Character Table</a>. The functions described in <a href="Version-Management.html#Version-Management">Version Management</a> can be used to select backward compatible operation with
the older character encoding. The normal forms of the functions in
this section will use the older character set if a backward
compatibility mode is indicated, whereas the standard forms will use
the character encoding documented in <a href="Character-Table.html#Character-Table">Character Table</a> regardless.
</p>
<p>Standard encodings should always be assumed for library and function
<a name="index-standard-character-encoding"></a>
names associated with the <code>library</code> combinator (<a href="Calling-existing-library-functions.html#Calling-existing-library-functions">Calling existing library functions</a>), and for values of lists defined by
<code>avm_list_of_value</code> (<a href="Primitive-types.html#Primitive-types">Primitive types</a>), but version
dependent encodings should be used for all other purposes such as
error messages. Alternatively, the normal version dependent forms of
the functions below can be used safely in any case if backward
<a name="index-backward-compatability"></a>
compatibility is not an issue. This distinction is viewed as a
transitional feature of the API that will be discontinued eventually
when support for the old character set is withdrawn and the
<code>standard</code> forms are be removed.
</p>
<dl>
<dt><a name="index-avm_005fcharacter_005frepresentation"></a><u>Function:</u> list <b>avm_character_representation</b><i> (int <var>character</var>)</i></dt>
</dl>
<dl>
<dt><a name="index-avm_005fstandard_005fcharacter_005frepresentation"></a><u>Function:</u> list <b>avm_standard_character_representation</b><i> (int <var>character</var>)</i></dt>
<dd><p>This function takes an integer character code and returns a copy of
the list representing it, as per the table in <a href="Character-Table.html#Character-Table">Character Table</a>. Because the copy is shared, no memory is allocated by this
function so there is no possibility of overflow. Nevertheless, it is
the responsibility of the caller dispose of the list when it is no
longer needed by <code>avm_dispose</code>, just as if the copy were not
shared (<a href="Simple-Operations.html#Simple-Operations">Simple Operations</a>). For performance reasons, this
function is implemented as a macro. If the argument is outside the
range of zero to 255, it is masked into that range.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fcharacter_005fcode"></a><u>Function:</u> int <b>avm_character_code</b><i> (list <var>operand</var>)</i></dt>
</dl>
<dl>
<dt><a name="index-avm_005fstandard_005fcharacter_005fcode"></a><u>Function:</u> int <b>avm_standard_character_code</b><i> (list <var>operand</var>)</i></dt>
<dd><p>This function takes a list as an argument and returns the corresponding
character code, as per <a href="Character-Table.html#Character-Table">Character Table</a>. If the argument does not
represent any character, a value of <code>-1</code> is returned.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fstrung"></a><u>Function:</u> list <b>avm_strung</b><i> (char *<var>string</var>)</i></dt>
</dl>
<dl>
<dt><a name="index-avm_005fstandard_005fstrung"></a><u>Function:</u> list <b>avm_standard_strung</b><i> (char *<var>string</var>)</i></dt>
<dd><p>This function takes a pointer to a null terminated character string and
returns the list obtained by translating each character into its list
representation and enqueuing them together. Memory needs to be allocated
for the result, and if there isn&rsquo;t enough available, an error message is
written to standard error and the process is terminated. This function
is useful to initialize lists from hard coded strings at the beginning
of a run, as in this example.
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">hello_string = avm_strung(&quot;hello&quot;);
</pre></td></tr></table>

<p>This form initializes a single string, but to initialize a one line
message suitable for writing to a file, it would have to be a list of
strings, as in this example.
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">hello_message = avm_join(avm_strung(&quot;hello&quot;),NULL);
</pre></td></tr></table>

<p>The latter form is used internally by the library for initializing
most of the various error messages that can be returned by other functions.
</p></dd></dl>

<dl>
<dt><a name="index-avm_005frecoverable_005fstrung"></a><u>Function:</u> list <b>avm_recoverable_strung</b><i> (char *<var>string</var>, int *<var>fault</var>);</i></dt>
</dl>
<dl>
<dt><a name="index-avm_005frecoverable_005fstandard_005fstrung"></a><u>Function:</u> list <b>avm_recoverable_standard_strung</b><i> (char *<var>string</var>, int *<var>fault</var>);</i></dt>
<dd><p>This function is like <code>avm_strung</code> except that if it runs out of memory
it sets the integer referenced by <var>fault</var> to a non-zero value and returns
instead of terminating the process.
</p></dd></dl>

<dl>
<dt><a name="index-_002aavm_005funstrung"></a><u>Function:</u> char <b>*avm_unstrung</b><i> (list <var>string</var>, list *<var>message</var>, int *<var>fault</var>)</i></dt>
</dl>
<dl>
<dt><a name="index-_002aavm_005fstandard_005funstrung"></a><u>Function:</u> char <b>*avm_standard_unstrung</b><i> (list <var>string</var>, list *<var>message</var>, int *<var>fault</var>)</i></dt>
<dd><p>This function performs an inverse operation to
<code>avm_recoverable_strung</code>, taking a list representing a character
string to the character string in ASCII null terminated form as per
the standard C representation. Memory is allocated for the result by
this function which should be freed by the caller.
</p>
<p>In the event of an exception, the integer referenced by <code>fault</code>
is assigned a non-zero value and an error message represented as a
list is assigned to the list referenced by <code>message</code>. The error
message should be reclaimed by the caller with <code>avm_dispose</code>
(<a href="Simple-Operations.html#Simple-Operations">Simple Operations</a> if it is non-empty. Possible error messages
are <code>&lt;'memory overflow'&gt;</code>, <code>&lt;'counter overflow'&gt;</code>, and
<code>&lt;'invalid text format'&gt;</code>.
</p></dd></dl>

<dl>
<dt><a name="index-avm_005fscanned_005flist"></a><u>Function:</u> list <b>avm_scanned_list</b><i> (char *<var>string</var>)</i></dt>
<dd><p>An application that makes use of virtual code snippets or data that are
known at compile time can use this function to initialize them. The
argument is a string in the format described in <a href="Concrete-Syntax.html#Concrete-Syntax">Concrete Syntax</a>,
and the result is the list representing it. For example, the program
discussed in <a href="Example-Script.html#Example-Script">Example Script</a> could be hard coded into a C program
by pasting the data from its virtual code file into an expression of
this form.
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">cat_program = avm_scanned_list(&quot;sKYQNTP\\&quot;);
</pre></td></tr></table>

<p>Note that the backslash character in the original data has to be
preceded by an extra backslash in the C source, because backslashes
usually mean something in C character constants.
</p>
<p>The <code>avm_scanned_list</code> function needs to allocate memory. If there
isn&rsquo;t enough memory available, it writes a message to standard error and
causes the process to exit.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fmultiscanned"></a><u>Function:</u> list <b>avm_multiscanned</b><i> (char **<var>strings</var>)</i></dt>
<dd><p>Sometimes it may be useful to initialize very large lists from
strings, but some C compilers impose limitations on the maximum length
of a string constant, and the ISO standard for C requires only 512
bytes.  This function serves a similar purpose to
<code>avm_scanned_list</code>, but allows the argument to be a pointer to a
null terminated array of strings instead of one long string, thereby
circumventing this limitation in the compiler.
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">char *code[] = {&quot;sKYQ&quot;,&quot;NTP\\&quot;,NULL};
...
cat_program = avm_multiscanned(code);
</pre></td></tr></table>

<p>If there is insufficient memory to allocate the list this function needs
to create, it causes an error message to be written to standard error,
and then kills the process.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fprompt"></a><u>Function:</u> char* <b>avm_prompt</b><i> (list <var>prompt_strings</var>)</i></dt>
<dd><p>This function takes a list representing a list of character strings, and
returns its translation to a character string with the sequence 13 10
used as a separator. For example, given a tree of this form
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">some_message = avm_join(
   avm_strung(&quot;hay&quot;),
   avm_join(
      avm_strung(&quot;you&quot;),
      NULL));
</pre></td></tr></table>

<p>the result returned by <code>prompt_strings(some_message)</code> would be a
pointer to a null terminated character string equivalent to the C constant
<code>&quot;hay\13\10you&quot;</code>.
</p>
<p>Error messages are printed and the process terminated in the event of
either a memory overflow or an invalid character representation.
</p>
<p>This function is used by <code>avram</code> in the evaluation of interactive
<a name="index-interactive-applications-2"></a>
virtual code applications, whose output has to be compared to the output
from a shell command in this format. The separator is chosen to be
compatible with the <code>expect</code> library.
</p></dd></dl>

<dl>
<dt><a name="index-avm_005frecoverable_005fprompt"></a><u>Function:</u> char* <b>avm_recoverable_prompt</b><i> (list <var>prompt_strings</var>, list *<var>message</var>, int *<var>fault</var>)</i></dt>
<dd><p>This function performs the same operation as <code>avm_prompt</code> but
allows the caller to handle exceptional conditions. If an exception
such as a memory overflow occurs, the integer referenced by
<code>fault</code> is assigned a non-zero value and a representation of the
error message as a list of strings is assigned to the list referenced
by <code>message</code>.
</p>
<p>This function is used to by <code>avram</code> to evaluate the
<code>interact</code> combinator (<a href="Interaction-combinator.html#Interaction-combinator">Interaction combinator</a>), when
terminating in the event of an error would be inappropriate.
</p></dd></dl>

<dl>
<dt><a name="index-avm_005finitialize_005fchrcodes"></a><u>Function:</u> void <b>avm_initialize_chrcodes</b><i> ()</i></dt>
<dd><p>This function has to be called before any of the other character
conversion functions in this section, or else their results are
undefined. It performs the initialization of various internal data
structures.
</p></dd></dl>
<dl>
<dt><a name="index-avm_005fcount_005fchrcodes"></a><u>Function:</u> void <b>avm_count_chrcodes</b><i> ()</i></dt>
<dd><p>This function can be called at the end of a run, after the last call to
any of the other functions in this section, but before
<code>avm_count_lists</code> if that function is also being used. The purpose
of this function is to detect and report memory leaks. If any memory
associated with any of these functions has not been reclaimed by the
client program, a message giving the number of unreclaimed lists will be
written to standard error.
</p></dd></dl>

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