Basically, gvpr traverses each input graph, denoted by $G, visiting each node and edge, matching it with the predicate-action rules supplied in the input program. The rules are evaluated in order. For each predicate evaluating to true, the corresponding action is performed. During the traversal, the current node or edge being visited is denoted by $.

For each input graph, there is a target subgraph, denoted by $T, initially empty and used to accumulate chosen entities, and an output graph, $O, used for final processing and then written to output. By default, the output graph is the target graph. The output graph can be set in the program or, in a limited sense, on the command line.

-a args

The string args is split into whitespace-separated tokens, with the individual tokens available as strings in the gvpr program as ARGV[0],...,ARGV[ARGC-1]. Whitespace characters within single or double quoted substrings, or preceded by a backslash, are ignored as separators. In general, a backslash character turns off any special meaning of the following character. Note that the tokens derived from multiple -a flags are concatenated.

-c

Use the source graph as the output graph.

-i

Derive the node-induced subgraph extension of the output graph in the context of its root graph.

-o outfile

Causes the output stream to be written to the specified file; by default, output is written to stdout.

-f progfile

Use the contents of the specified file as the program to execute on the input. If progfile contains a slash character, the name is taken as the pathname of the file. Otherwise, gvpr will use the directories specified in the environment variable GPRPATH to look for the file. If -f is not given, gvpr will use the first non-option argument as the program.

-V

Causes the program to print version information and exit.

-?

Causes the program to print usage information and exit.

files

Names of files containing 1 or more graphs in the dot language. If no -f option is given, the first name is removed from the list and used as the input program. If the list of files is empty, stdin will be used.

BEGIN { action }

BEG_G { action }

N [ predicate ] { action }

E [ predicate ] { action }

END_G { action }

END { action }

A program can contain at most one of each of the BEGIN, BEG_G, END_G and END clauses. There can be any number of N and E statements, the first applied to nodes, the second to edges. The top-level semantics of a gvpr program are:

Evaluate the BEGIN clause, if any.
For each input graph G {
    Set G as the current graph and current object.
    Evaluate the BEG_G clause, if any.
    For each node and edge in G {
      Set the node or edge as the current object.
      Evaluate the N or E clauses, as appropriate.
    }
    Set G as the current object.
    Evaluate the END_G clause, if any.
}
Evaluate the END clause, if any.

The actions of the BEGIN, BEG_G, END_G and END clauses are performed when the clauses are evaluated. For N or E clauses, either the predicate or action may be omitted. If there is no predicate with an action, the action is performed on every node or edge, as appropriate. If there is no action and the predicate evaluates to true, the associated node or edge is added to the target graph.

Predicates and actions are sequences of statements in the C dialect supported by the libexpr(3) library. The only difference between predicates and actions is that the former must have a type that may interpreted as either true or false. Here the usual C convention is followed, in which a non-zero value is considered true. This would include non-empty strings and non-empty references to nodes, edges, etc. However, if a string can be converted to an integer, this value is used.

In addition to the usual C base types (void, int, char, float, long, unsigned and double), gvpr provides string as a synonym for char*, and the graph-based types node_t, edge_t, graph_t and obj_t. The obj_t type can be viewed as a supertype of the other 3 concrete types; the correct base type is maintained dynamically. Besides these base types, the only other supported type expressions are (associative) arrays.

Constants follow C syntax, but strings may be quoted with either "..." or '...'. In certain contexts, string values are interpreted as patterns for the purpose of regular expression matching. Patterns use ksh(1) file match pattern syntax. gvpr accepts C++ comments as well as cpp-type comments. For the latter, if a line begins with a '#' character, the rest of the line is ignored.

A statement can be a declaration of a function, a variable or an array, or an executable statement. For declarations, there is a single scope. Array declarations have the form:

 type array [ var ]

where the var is optional. As in C, variables and arrays must be declared. In particular, an undeclared variable will be interpreted as the name of an attribute of a node, edge or graph, depending on the context.

Executable statements can be one of the following:

{ [ statement ... ] }
expression               // commonly var = expression
if( expression ) statement [ else statement ]
for( expression ; expression ; expression ) statement
for( array [ var ]) statement
while( expression ) statement
switch( expression ) case statements
break [ expression ]
continue [ expression ]
return [ expression ]

In the second form of the for statement, the variable var is set to each value used as an index in the specified array and then the associated statement is evaluated. Function definitions can only appear in the BEGIN clause.

Expressions include the usual C expressions. String comparisons using == and != treat the right hand operand as a pattern. gvpr will attempt to use an expression as a string or numeric value as appropriate.

Expressions of graphical type (i.e., graph_t, node_t, edge_t, obj_t) may be followed by a field reference in the form of .name. The resulting value is the value of the attribute named name of the given object. In addition, in certain contexts an undeclared, unmodified identifier is taken to be an attribute name. Specifically, such identifiers denote attributes of the current node or edge, respectively, in N and E clauses, and the current graph in BEG_G and END_G clauses.

As usual in the libagraph(3) model, attributes are string-valued. In addition, gvpr supports certain pseudo-attributes of graph objects, not necessarily string-valued. These reflect intrinsic properties of the graph objects and cannot be set by the user.

head : node_t

the head of an edge.

tail : node_t

the tail of an edge.

name : string

the name of an edge, node or graph. The name of an edge has the form "<tail-name><edge-op><head-name>[ <key>]", where <edge-op> is "->" or "--" depending on whether the graph is directed or not. The bracket part [<key>] only appears if the edge has a non-trivial key.

indegree : int

the indegree of a node.

outdegree : int

the outdegree of a node.

degree : int

the degree of a node.

root : graph_t

the root graph of an object. The root of a root graph is itself.

parent : graph_t

the parent graph of a subgraph. The parent of a root graph is NULL

n_edges : int

the number of edges in the graph

n_nodes : int

the number of nodes in the graph

directed : int

true (non-zero) if the graph is directed

strict : int

true (non-zero) if the graph is strict

The following functions are built into gvpr. Those functions returning references to graph objects return NULL in case of failure.

graph(s : string, t : string) : graph_t

creates a graph whose name is s and whose type is specified by the string t. Ignoring case, the characters U, D, S, N have the interpretation undirected, directed, strict, and non-strict, respectively. If t is empty, a directed, non-strict graph is generated.

subg(g : graph_t, s : string) : graph_t

creates a subgraph in graph g with name s. If the subgraph already exists, it is returned.

isSubg(g : graph_t, s : string) : graph_t

returns the subgraph in graph g with name s, if it exists, or NULL otherwise.

fstsubg(g : graph_t) : graph_t

returns the first subgraph in graph g, or NULL if none exists.

nxtsubg(sg : graph_t) : graph_t

returns the next subgraph after sg, or NULL.

isDirect(g : graph_t) : int

returns true if and only if g is directed.

isStrict(g : graph_t) : int

returns true if and only if g is strict.

nNodes(g : graph_t) : int

returns the number of nodes in g.

nEdges(g : graph_t) : int

returns the number of edges in g.

node(sg : graph_t, s : string) : node_t

creates a node in graph g of name s. If such a node already exists, it is returned.

subnode(sg : graph_t, n : node_t) : node_t

inserts the node n into the subgraph g. Returns the node.

fstnode(g : graph_t) : node_t

returns the first node in graph g, or NULL if none exists.

nxtnode(n : node_t) : node_t

returns the next node after n, or NULL.

isNode(sg : graph_t, s : string) : node_t

looks for a node in graph g of name s. If such a node exists, it is returned. Otherwise, NULL is returned.

edge(t : node_t, h : node_t, s : string) : edge_t

creates an edge with tail node t, head node h and name s. If the graph is undirected, the distinction between head and tail nodes is unimportant. If such an edge already exists, it is returned.

subedge(g : graph_t, e : edge_t) : edge_t

inserts the edge e into the subgraph g. Returns the edge.

isEdge(t : node_t, h : node_t, s : string) : edge_t

looks for an edge with tail node t, head node h and name s. If the graph is undirected, the distinction between head and tail nodes is unimportant. If such an edge exists, it is returned. Otherwise, NULL is returned.

fstout(n : node_t) : edge_t

returns the first out edge of node n.

nxtout(e : edge_t) : edge_t

returns the next out edge after e.

fstin(n : node_t) : edge_t

returns the first in edge of node n.

nxtin(e : edge_t) : edge_t

returns the next in edge after e.

fstedge(n : node_t) : edge_t

returns the first edge of node n.

nxtedge(e : edge_t, node_t) : edge_t

returns the next edge after e.

write(g : graph_t) : void

prints g in dot format onto the output stream.

writeG(g : graph_t, fname : string) : void

prints g in dot format into the file fname.

fwriteG(g : graph_t, fd : int) : void

prints g in dot format onto the open stream denoted by the integer fd.

readG(fname : string) : graph_t

returns a graph read from the file fname. The graph should be in dot format. If no graph can be read, NULL is returned.

freadG(fd : int) : graph_t

returns the next graph read from the open stream fd. Returns NULL at end of file.

delete(g : graph_t, x : obj_t) : void

deletes object x from graph g. If g is NULL, the function uses the root graph of x. If x is a graph or subgraph, it is closed unless x is locked.

isIn(g : graph_t, x : obj_t) : int

returns true if x is in subgraph g. If x is a graph, this indicates that g is the immediate parent graph of x.

clone(g : graph_t, x : obj_t) : obj_t

creates a clone of object x in graph g. In particular, the new object has the same name/value attributes and structure as the original object. If an object with the same key as x already exists, its attributes are overlaid by those of x and the object is returned. If an edge is cloned, both endpoints are implicitly cloned. If a graph is cloned, all nodes, edges and subgraphs are implicitly cloned. If x is a graph, g may be NULL, in which case the cloned object will be a new root graph.

copy(g : graph_t, x : obj_t) : obj_t

creates a copy of object x in graph g, where the new object has the same name/value attributes as the original object. If an object with the same key as x already exists, its attributes are overlaid by those of x and the object is returned. Note that this is a shallow copy. If x is a graph, none of its nodes, edges or subgraphs are copied into the new graph. If x is an edge, the endpoints are created if necessary, but they are not cloned. If x is a graph, g may be NULL, in which case the cloned object will be a new root graph.

copyA(src : obj_t, tgt : obj_t) : int

copies the attributes of object src to object tgt, overwriting any attribute values tgt may initially have.

induce(g : graph_t) : void

extends g to its node-induced subgraph extension in its root graph.

aget(src : obj_t, name : string) : string

returns the value of attribute name in object src. This is useful for those cases when name conflicts with one of the keywords such as "head" or "root".

aset(src : obj_t, name : string, value : string) : int

sets the value of attribute name in object src to value. Returns 0 on success, non-zero on failure. See aget above.

compOf(g : graph_t, n : node_t) : graph_t

returns the connected component of the graph g containing node n, as a subgraph of g. The subgraph only contains the nodes. One can use induce to add the edges. The function fails and returns NULL if n is not in g. Connectivity is based on the underlying undirected graph of g.

lock(g : graph_t, v : int) : int

implements graph locking on root graphs. If the integer v is positive, the graph is set so that future calls to delete have no immediate effect. If v is zero, the graph is unlocked. If there has been a call to delete the graph while it was locked, the graph is closed. If v is negative, nothing is done. In all cases, the previous lock value is returned.

sprintf(fmt : string, ...) : string

returns the string resulting from formatting the values of the expressions occurring after fmt according to the (3) format fmt

gsub(str : string, pat : string) : string

gsub(str : string, pat : string, repl : string) : string

returns str with all substrings matching pat deleted or replaced by repl, respectively.

sub(str : string, pat : string) : string

sub(str : string, pat : string, repl : string) : string

returns str with the leftmost substring matching pat deleted or replaced by repl, respectively. The characters '^' and '$' may be used at the beginning and end, respectively, of pat to anchor the pattern to the beginning or end of str.

substr(str : string, idx : int) : string

substr(str : string, idx : int, len : int) : string

returns the substring of str starting at position idx to the end of the string or of length len, respectively. Indexing starts at 0. If idx is negative or idx is greater than the length of str, a fatal error occurs. Similarly, in the second case, if len is negative or idx + len is greater than the length of str, a fatal error occurs.

length(s : string) : int

returns the length of the string s.

index(s : string, t : string) : int

returns the index of the character in string s where the leftmost copy of string t can be found, or -1 if t is not a substring of s.

match(s : string, p : string) : int

returns the index of the character in string s where the leftmost match of pattern p can be found, or -1 if no substring of s matches p.

canon(s : string) : string

returns a version of s appropriate to be used as an identifier in a dot file.

xOf(s : string) : string

returns the string "x" if s has the form "x,y", where both x and y are numeric.

yOf(s : string) : string

returns the string "y" if s has the form "x,y", where both x and y are numeric.

llOf(s : string) : string

returns the string "llx,lly" if s has the form "llx,lly,urx,ury", where all of llx, lly, urx, and ury are numeric.

urOf(s)

urOf(s : string) : string returns the string "urx,ury" if s has the form "llx,lly,urx,ury", where all of llx, lly, urx, and ury are numeric.

sscanf(s : string, fmt : string, ...) : int

scans the string s, extracting values according to the (3) format fmt. The values are stored in the addresses following fmt, addresses having the form &v, where v is some declared variable of the correct type. Returns the number of items successfully scanned.