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Astl programming language for C


This is an implementation of the Astl programming language for C. Astl is an easy to learn scripting language that is specialized on attributed abstract syntax trees and control flow graphs.

Astl-C starts with one or more C sources, constructs an abstract syntax tree for it, and executes then your script. Astl scripts in their simplest form just provide tree expressions and associated actions which are executed for each matching subtree.

Take as example the Misra rule 14.10 which states that in Misra-C all if ... else if constructs shall be terminated with an else clause. If you want to check your sources for conformance, you can put the violating tree pattern in a tree expression:

attribution rules {
   ("if" cond then ("if" cond2 then2)) as if_stmt -> {
      println("if-else-chain without final else at ", location(if_stmt));

A so-called attribution rule consists of a tree expression followed by a block which is executed for each matching tree expression. Any number of attribution rules can be specified within an attribution rules section. The tree expressions of the attribution rules are considered during an implicitly executed traverse of the syntax tree in depth-first order (by default in preorder but this can be changed per individual rule).

Tree expressions are set in parentheses. In their simplest form, they specify an operator as string ("if" in the example above), followed by tree expressions for the direct subnodes. An "if" syntax node has two or three subnodes, the first represents the condition, the second represents the statement to be executed when the condition is true and the optional third subnode represents the else-part. Variables can be introduced and bound to subnodes. In this case, cond and then are bound to the first two subnodes of the outer "if" node, and if_stmt to the "if" node itself. These bound variables can be used within the following block. In this example, if_stmt is passed to the built-in function location which returns a string specifying the source location of the entire if-statement.

Add a hash-bang-header to it, i.e. '#!', followed by the path to astl-c, make it executable, and run it with all the source files:

$ misra-1410.ast example.c
if-else-chain without final else at example.c:6.4-10.4

A more elaborate example

Occasionally, the initialization of a for loop variable is missing. The gcc compiler delivers no warning and if the loop variable is of an unsigned integer type such an error may go unnoticed in cases as following where the index variable is guaranteed to be within the array index range within the body of the for loop:

for (std::size_t i; i < sizeof(a)/sizeof(a[0]); ++i) {
   a[i] = i;

As i is left uninitialized, it may be 0, positive but within range, or out of range. In the latter two cases the array is left partially or entirely uninitialized.

Astl-C represents a for loop with the exception of the trivial case for(;;) always as a syntax tree with the operator "for" and four operands representing the initialization, the condition, the increment, and the body of the loop. Following attribution rules would match all for loops:

   ("for" stmt) -> {
      // trivial case
   ("for" init condition increment stmt) -> {
      // regular case

We are now focusing on cases where a loop variable is declared within a for loop. The init subtree of the for loop is either an expression or a declaration. We are just interested in cases where we have a declaration which in turn consists of the declaration specifiers (like std::size_t in the example above) and an init declarator list which can declare one or more variables. Each of the variables within an init declarator list is introduced by an init declarator which consists of a declarator and optionally an initializer. We are interested here in cases where we get an init declarator without an initializer within the init part of a for loop. This can be expressed as follows:

         ) condition increment stmt) -> {
      println("for loop variable w/o initialization at ",

Note that we specify the to be matched syntax subtree only to the level of detail which is necessary. We insist, for example, on a declaration subnode within a for node. However, we do not specify how the first subnode of the declaration looks like. Instead we let it be bound to the variable specifiers. Next comes the init declarator list along with an init declarator with just one declarator without an initializer. Here we do not go into further detail regarding the declarator as we just let the subnode be bound to a variable of that name. Within the action for matching subtrees we can access all bound variables that are named within the tree expression.

As above, the predefined function location delivers the source location range of the corresponding syntax tree.

Unfortunately this tree expression does not work in the general case as more than one variable can be declared within a for loop:

for (std::size_t i, j = 0; i < sizeof(a)/sizeof(a[0]); ++i, j += i) {
   a[i] = j;

Here we would like to issue a warning regarding i but not for j. To catch the individual cases it is best to reorganize our tree expression where we want to catch cases of an init declarator without initializer within the initialization part of a for loop. This is possible using contextual tree expressions:

   ("init_declarator" declarator) in
         ("for" here condition increment stmt) -> {
      println("for loop variable w/o initialization at ",

Using the keyword in we can restrict matching to cases where an init declarator with just a declarator appears within a for node. But we must be careful here as this could also match declarations within the body of the for loop. Hence, we must make sure that contextual matches are restricted to the initialization part of the for loop. This is done using the keyword here which tells where the subnode has to appear to match the contextual condition.

The error messages for these warnings would be more helpful if the actual loop variable is named where an initialization is missing. We can do this by using a tree expression that starts even deeper at the direct declarator:

("direct_declarator" ("identifier" varname)) in
   ("init_declarator" declarator) and in
      ("for" here condition increment stmt) -> {
   println("for loop variable ", varname,
      " w/o initialization at ", location(declarator));

Cascading contextual tree expressions match if the first tree expression is embedded within a subnode matching the next tree expression and if the latter subnode is embedded within the next tree expression.

Syntax trees consist of regular operator nodes with optional subnodes or tokens. Tokens are always embedded within special subnodes which have no other role than to make matching easier. In C, all identifiers are embedded in subnodes of type identifier. Similarly, integer constants are embedded in subnodes of type integer constant, floating point constants in subnodes of type floating constant etc. In the example above the actual token is bound to a variable named varname which, like all variables bound to tokens, is of string type and can be printed directly.

Unfortunately, this rule is still not good enough as it provides warnings in perfectly legitimate cases. Sometimes the exceptions are not obvious right from the beginning but by testing a rule against real world programs. When testing the rule above I came across following case where the loop variable is initialized within the condition of the for loop:

for (char* line; (line = readline(fp)); free(line)) {
   // ...

This can no longer be easily handled by one single tree expression. But several rules can cooperate here through the use of attributions. The syntax trees can be arbitrarily attributed in Astl. Each variable that is bound to a node in the syntax tree can be used to refer to attributes associated with that particular node.

Here is a simple example that uses attributes:

   ("for" init condition increment stmt) as forloop -> pre {
      forloop.vars = {}; // candidates where we possibly issue a warning

Tree expressions in parentheses which describe a node of the syntax tree can be followed by the keyword as and a variable name, causing the variable to be bound to the matching node within the action. In the example above, the variable named forloop is bound to the for node.

Nodes within a syntax tree are visited depth-first and each node is visited twice, first in preorder, second in postorder after all the subnodes were visited. Actions may be marked pre or post, allowing them to be associated to the visit in preorder or postorder, respectively.

Within the action we are free to set any attributes related to a syntax tree node by qualifying attribute names by the variable name bound to the node. In the example above, we have an attribute named vars which is refered to as forloop.vars where forloop is bound to the corresponding node. This variable will be used in the following to keep track of all possible candidates, i.e. all for loop variables that are left uninitialized. We will then eliminate candidates whenever we see an assignment to them within the condition. Finally, we will print warnings for all surviving candidates when we visit the for node in postorder.

Following rule catches all for loop variables without initialization as before:

   ("direct_declarator" ("identifier" varname)) in
	 ("init_declarator" ("declarator" *)) and in
	 ("declaration" *) as decl and in
	 ("for" here condition increment stmt) as forloop -> post {
      forloop.vars{varname} = {
	 loc -> location(decl),
	 text -> gentext(decl)

As a warning at that point is premature we just keep record of its location and, to make error messages more readable, add the program text of the entire declaration. The predefined function gentext generates program text from the syntax tree using the print rules which can be imported using import printer; at the beginning of the Astl script. (Note that Astl does not deliver the original program text beyond the level of preserved tokens as syntax trees can be changed using mutation rules in Astl.) The syntax for dictionaries in Astl is similar to that of hashes in Perl, i.e. we can collect key/value pairs within {...} and keys are separated by the arrow symbol -> from the value (Perl uses => for this delimiter).

Next we would like to eliminate all candidates that are assigned to within the condition of the for loop:

   ("=" ("identifier" varname) rvalue) in
	 ("for" init here increment stmt) as forloop -> post {
      delete forloop.vars{varname}; // delete corresponding candidate

Note that delete has no effect if the key does not exist. Finally, we print warnings for the survivors:

   ("for" init condition increment stmt) as forloop -> post {
      /* print warnings for surviving candidates */
      foreach (varname, record) in (forloop.vars) {
	 println("for loop variable ", varname,
	    " is not initialized at ", record.loc, ": ", record.text);

Put together, this script delivers for

extern int f(int);
int main() {
   for (int i, j = 0, k; i = f(j); j += i) {
      // ...

following error message:

for loop variable k is not initialized at badfor.c:3.8-24: int i, j = 0, k;

In the examples directory this script is to be found at for-loop/check-for-loops.ast.


I worked on this project in 2009, suspended it, and resumed it for a short period in 2011. I am now consolidating it towards more recent versions of C++, bison and the libraries the project uses.

Downloading and building

If you want to clone it, you should do this recursively:

git clone --recursive

To build it, you need bison 3.x, a recent g++ with C++14 support, GMP, pcre2, and boost libraries:

(cd astl/astl && make)
(cd astl-c && make)

A reference manual of the Astl programming language is included in the astl/refman subdirectory.

Some examples are to be found in the examples subdirectory.


Astl programming language for C







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