gvpr (1)
NAME
gvpr  graph pattern scanning and processing languageSYNOPSIS
gvpr [icnqV?] [ o outfile ] [ a args ] [ 'prog'  f progfile ] [ files ]DESCRIPTION
gvpr (previously known as gpr) is a graph stream editor inspired by awk. It copies input graphs to its output, possibly transforming their structure and attributes, creating new graphs, or printing arbitrary information. The graph model is that provided by libcgraph(3). In particular, gvpr reads and writes graphs using the dot language.Basically, gvpr traverses each input graph, denoted by $G, visiting each node and edge, matching it with the predicateaction 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.
OPTIONS
The following options are supported: a args
 The string args is split into whitespaceseparated tokens, with the individual tokens available as strings in the gvpr program as ARGV[0],...,ARGV[ARGC1]. 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 nodeinduced 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 GVPRPATH to look for the file. If f is not given, gvpr will use the first nonoption argument as the program.
 q
 Turns off warning messages.
 n
 Turns off graph readahead. By default, the variable $NG is set to the next graph to be processed. This requires a read of the next graph before processing the current graph, which may block if the next graph is only generated in response to some action pertaining to the processing of the current graph.
 V
 Causes the program to print version information and exit.
 ?
 Causes the program to print usage information and exit.
OPERANDS
The following operand is supported: 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.
PROGRAMS
A gvpr program consists of a list of predicateaction clauses, having one of the forms: 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, END_G and END clauses. There can be any number of BEG_G, N and E statements, the first applied to graphs, the second to nodes, the third to edges. These are separated into blocks, a block consisting of an optional BEG_G statement and all N and E statements up to the next BEG_G statement, if any. The toplevel semantics of a gvpr program are:

Evaluate the BEGIN clause, if any. For each input graph G { For each block { 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.
The blocks are evaluated in the order in which they occur. Within a block, the N clauses (E clauses, respectively) are evaluated in the order in which the occur. Note, though, that within a block, N or E clauses may be interlaced, depending on the traversal order.
Predicates and actions are sequences of statements in the C dialect supported by the expr(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 nonzero value is considered true. This would include nonempty strings and nonempty 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 graphbased 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 '...'. gvpr accepts C++ comments as well as cpptype 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 [ type0 ]
where type0 is optional. If it is supplied, the parser will enforce that all array subscripts have the specified type. If it is not supplied, objects of all types can be used as subscripts. 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 forr( array [ var ]) statement while( expression ) statement switch( expression ) case statements break [ expression ] continue [ expression ] return [ expression ]
In the second form of the for statement and the forr statement, the variable var is set to each value used as an index in the specified array and then the associated statement is evaluated. For numeric and string indices, the indices are returned in increasing (decreasing) numeric or lexicographic order for for (forr, respectively). This can be used for sorting.
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 for the purpose of regular expression matching. Patterns use ksh(1) file match pattern syntax. (For simple string equality, use the strcmp function.
gvpr will attempt to use an expression as a string or numeric value as appropriate. Both Clike casts and function templates will cause conversions to be performed, if possible.
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 libcgraph(3) model, attributes are stringvalued. In addition, gvpr supports certain pseudoattributes of graph objects, not necessarily stringvalued. 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 "<tailname><edgeop><headname>[<key>]", where <edgeop> is ">" or "" depending on whether the graph is directed or not. The bracket part [<key>] only appears if the edge has a nontrivial 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 (nonzero) if the graph is directed
 strict : int
 true (nonzero) if the graph is strict
BUILTIN FUNCTIONS
The following functions are built into gvpr. Those functions returning references to graph objects return NULL in case of failure.
Graphs and subgraph
 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 nonstrict, respectively. If t is empty, a directed, nonstrict 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.
Nodes
 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 in the root graph, or NULL.
 nxtnode_sg(sg : graph_t, n : node_t) : node_t
 returns the next node after n in sg, or NULL.
 isNode(sg : graph_t, s : string) : node_t
 looks for a node in (sub)graph sg of name s. If such a node exists, it is returned. Otherwise, NULL is returned.
 isSubnode(sg : graph_t, n : node_t) : int
 returns nonzero if node n is in (sub)graph sg, or zero otherwise.
 indegreeOf(sg : graph_t, n : node_t) : int
 returns the indegree of node n in (sub)graph sg.
 outdegreeOf(sg : graph_t, n : node_t) : int
 returns the outdegree of node n in (sub)graph sg.
 degreeOf(sg : graph_t, n : node_t) : int
 returns the degree of node n in (sub)graph sg.
Edges
 edge(t : node_t, h : node_t, s : string) : edge_t
 creates an edge with tail node t, head node h and name s in the root graph. If the graph is undirected, the distinction between head and tail nodes is unimportant. If such an edge already exists, it is returned.
 edge_sg(sg : graph_t, t : node_t, h : node_t, s : string) : edge_t
 creates an edge with tail node t, head node h and name s in (sub)graph sg (and all parent graphs). 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.
 isEdge_sg(sg : graph_t, t : node_t, h : node_t, s : string) : edge_t
 looks for an edge with tail node t, head node h and name s in (sub)graph sg. 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.
 isSubedge(g : graph_t, e : edge_t) : int
 returns nonzero if edge e is in (sub)graph sg, or zero otherwise.
 fstout(n : node_t) : edge_t
 returns the first outedge of node n in the root graph.
 fstout_sg(sg : graph_t, n : node_t) : edge_t
 returns the first outedge of node n in (sub)graph sg.
 nxtout(e : edge_t) : edge_t
 returns the next outedge after e in the root graph.
 nxtout_sg(sg : graph_t, e : edge_t) : edge_t
 returns the next outedge after e in graph sg.
 fstin(n : node_t) : edge_t
 returns the first inedge of node n in the root graph.
 fstin_sg(sg : graph_t, n : node_t) : edge_t
 returns the first inedge of node n in graph sg.
 nxtin(e : edge_t) : edge_t
 returns the next inedge after e in the root graph.
 nxtin_sg(sg : graph_t, e : edge_t) : edge_t
 returns the next inedge after e in graph sg.
 fstedge(n : node_t) : edge_t
 returns the first edge of node n in the root graph.
 fstedge_sg(sg : graph_t, n : node_t) : edge_t
 returns the first edge of node n in graph sg.
 nxtedge(e : edge_t, node_t) : edge_t
 returns the next edge after e in the root graph.
 nxtedge_sg(sg : graph_t, e : edge_t, node_t) : edge_t
 returns the next edge after e in the graph sg.
 opp(e : edge_t, node_t) : node_t
 returns the node on the edge e not equal to n. Returns NULL if n is not a node of e. This can be useful when using fstedge and nxtedge to enumerate the neighbors of n.
Graph I/O
 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.
Graph miscellany
 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.
 cloneG(g : graph_t, s : string) : graph_t
 creates a clone of graph g with name of s. If s is "", the created graph has the same name as g.
 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. In this case, the call is equivalent to cloneG(x,"").
 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 nodeinduced subgraph extension in its root graph.
 hasAttr(src : obj_t, name : string) : int
 returns nonzero if object src has an attribute whose name is name. It returns 0 otherwise.
 isAttr(g : graph_t, kind : string, name : string) : int
 returns nonzero if an attribute name has been defined in g for objects of the given kind. For nodes, edges, and graphs, kind should be "N", "E", and "G", respectively. It returns 0 otherwise.
 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". If the attribute has not been declared in the graph, the function will initialize it with a default value of "". To avoid this, one should use the hasAttr or isAttr function to check that the attribute exists.
 aset(src : obj_t, name : string, value : string) : int
 sets the value of attribute name in object src to value. Returns 0 on success, nonzero on failure. See aget above.
 getDflt(g : graph_t, kind : string, name : string) : string
 returns the default value of attribute name in objects in g of the given kind. For nodes, edges, and graphs, kind should be "N", "E", and "G", respectively. If the attribute has not been declared in the graph, the function will initialize it with a default value of "". To avoid this, one should use the isAttr function to check that the attribute exists.
 setDflt(g : graph_t, kind : string, name : string, value : string) : int
 sets the default value of attribute name to value in objects in g of the given kind. For nodes, edges, and graphs, kind should be "N", "E", and "G", respectively. Returns 0 on success, nonzero on failure. See getDflt above.
 fstAttr(g : graph_t, kind : string) : string
 returns the name of the first attribute of objects in g of the given kind. For nodes, edges, and graphs, kind should be "N", "E", and "G", respectively. If there are no attributes, the string "" is returned.
 nxtAttr(g : graph_t, kind : string, name : string) : string
 returns the name of the next attribute of objects in g of the given kind after the attribute name. The argument name must be the name of an existing attribute; it will typically be the return value of an previous call to fstAttr or nxtAttr. For nodes, edges, and graphs, kind should be "N", "E", and "G", respectively. If there are no attributes left, the string "" is returned.
 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.
 kindOf(obj : obj_t) : string
 returns an indication of the type of obj. For nodes, edges, and graphs, it returns "N", "E", and "G", respectively.
 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.
Strings
 sprintf(fmt : string, ...) : string
 returns the string resulting from formatting the values of the expressions occurring after fmt according to the printf(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.
 strcmp(s1 : string, s2 : string) : int
 provides the standard C function strcmp(3).
 length(s : string) : int
 returns the length of string s.
 index(s : string, t : string) : int
 rindex(s : string, t : string) : int
 returns the index of the character in string s where the leftmost (rightmost) 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.
 toupper(s : string) : string
 returns a version of s with the alphabetic characters converted to uppercase.
 tolower(s : string) : string
 returns a version of s with the alphabetic characters converted to lowercase.
 canon(s : string) : string
 returns a version of s appropriate to be used as an identifier in a dot file.
 html(g : graph_t, s : string) : string
 returns a ``magic'' version of s as an HTML string. This will typically be used to attach an HTMLlike label to a graph object. Note that the returned string lives in g. In particular, it will be freed when g is closed, and to act as an HTML string, it has to be used with an object of g. In addition, note that the angle bracket quotes should not be part of s. These will be added if g is written in concrete DOT format.
 ishtml(s : string) : int
 returns nonzero if and only if s is an HTML string.
 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 sscanf(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.
 split(s : string, arr : array, seps : string) : int
 split(s : string, arr : array) : int
 tokens(s : string, arr : array, seps : string) : int
 tokens(s : string, arr : array) : int

The split function breaks the string s into fields, while the tokens function
breaks the string into tokens.
A field consists of all nonseparator characters between two separator characters or the beginning or
end of the string. Thus, a field may be the empty string. A
token is a maximal, nonempty substring not containing a separator character.
The separator characters are those given in the seps argument.
If seps is not provided, the default value is " \t\n".
The functions return the number of fields or tokens.
The fields and tokens are stored in the argument array. The array must be stringvalued and, if an index type is specified, it must be int. The entries are indexed by consecutive integers, starting at 0. Any values already stored in the array will be either overwritten, or still be present after the function returns.
I/O
 print(...) : void
 print( expr, ... ) prints a string representation of each argument in turn onto stdout, followed by a newline.
 printf(fmt : string, ...) : int
 printf(fd : int, fmt : string, ...) : int
 prints the string resulting from formatting the values of the expressions following fmt according to the printf(3) format fmt. Returns 0 on success. By default, it prints on stdout. If the optional integer fd is given, output is written on the open stream associated with fd.
 scanf(fmt : string, ...) : int
 scanf(fd : int, fmt : string, ...) : int
 scans in values from an input stream according to the scanf(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. By default, it reads from stdin. If the optional integer fd is given, input is read from the open stream associated with fd. Returns the number of items successfully scanned.
 openF(s : string, t : string) : int

opens the file s as an I/O stream. The string argument t
specifies how the file is opened. The arguments are the same as for
the C function
fopen(3).
It returns an integer denoting the stream, or 1 on error.
As usual, streams 0, 1 and 2 are already open as stdin, stdout, and stderr, respectively. Since gvpr may use stdin to read the input graphs, the user should avoid using this stream.
 closeF(fd : int) : int
 closes the open stream denoted by the integer fd. Streams 0, 1 and 2 cannot be closed. Returns 0 on success.
 readL(fd : int) : string
 returns the next line read from the input stream fd. It returns the empty string "" on end of file. Note that the newline character is left in the returned string.
Math
 exp(d : double) : double
 returns e to the dth power.
 log(d : double) : double
 returns the natural log of d.
 sqrt(d : double) : double
 returns the square root of the double d.
 pow(d : double, x : double) : double
 returns d raised to the xth power.
 cos(d : double) : double
 returns the cosine of d.
 sin(d : double) : double
 returns the sine of d.
 atan2(y : double, x : double) : double
 returns the arctangent of y/x in the range pi to pi.
 MIN(y : double, x : double) : double
 returns the minimum of y and x.
 MAX(y : double, x : double) : double
 returns the maximum of y and x.
Associative Arrays
 # arr : int
 returns the number of elements in the array arr.
 idx in arr : int
 returns 1 if a value has been set for index idx in the array arr. It returns 0 otherwise.
 unset(v : array, idx) : int
 removes the item indexed by idx. It returns 1 if the item existed, 0 otherwise.
 unset(v : array) : void
 reinitializes the array.
Miscellaneous
 exit(v : int) : void
 causes gvpr to exit with the exit code v.
 system(cmd : string) : int
 provides the standard C function system(3). It executes cmd in the user's shell environment, and returns the exit status of the shell.
 rand() : double
 returns a pseudorandom double between 0 and 1.
 srand() : int
 srand(v : int) : int
 sets a seed for the random number generator. The optional argument gives the seed; if it is omitted, the current time is used. The previous seed value is returned. srand should be called before any calls to rand.
 colorx(color : string, fmt : string) : string
 translates a color from one format to another. The color argument should be a color in one of the recognized string representations. The fmt value should be one of "RGB", "RGBA", "HSV", or "HSVA". An empty string is returned on error.
BUILTIN VARIABLES
gvpr provides certain special, builtin variables, whose values are set automatically by gvpr depending on the context. Except as noted, the user cannot modify their values.
 $ : obj_t
 denotes the current object (node, edge, graph) depending on the context. It is not available in BEGIN or END clauses.
 $F : string
 is the name of the current input file.
 $G : graph_t
 denotes the current graph being processed. It is not available in BEGIN or END clauses.
 $NG : graph_t
 denotes the next graph to be processed. If $NG is NULL, the current graph $G is the last graph. Note that if the input comes from stdin, the last graph cannot be determined until the input pipe is closed. It is not available in BEGIN or END clauses, or if the n flag is used.
 $O : graph_t
 denotes the output graph. Before graph traversal, it is initialized to the target graph. After traversal and any END_G actions, if it refers to a nonempty graph, that graph is printed onto the output stream. It is only valid in N, E and END_G clauses. The output graph may be set by the user.
 $T : graph_t
 denotes the current target graph. It is a subgraph of $G and is available only in N, E and END_G clauses.
 $tgtname : string
 denotes the name of the target graph. By default, it is set to "gvpr_result". If used multiple times during the execution of gvpr, the name will be appended with an integer. This variable may be set by the user.
 $tvroot : node_t
 indicates the starting node for a (directed or undirected) depthfirst or breadthfirst traversal of the graph (cf. $tvtype below). The default value is NULL for each input graph. After the traversal at the given root, if the value of $tvroot has changed, a new traversal will begin with the new value of $tvroot. Also, set $tvnext below.
 $tvnext : node_t
 indicates the next starting node for a (directed or undirected) depthfirst or breadthfirst traversal of the graph (cf. $tvtype below). If a traversal finishes and the $tvroot has not been reset but the $tvnext has been set but not used, this node will be used as the next choice for $tvroot. The default value is NULL for each input graph.
 $tvedge : edge_t
 For BFS and DFS traversals, this is set to the edge used to arrive at the current node or edge. At the beginning of a traversal, or for other traversal types, the value is NULL.
 $tvtype : tvtype_t
 indicates how gvpr traverses a graph. It can only take one of the constant values with the previx "TV_" described below. TV_flat is the default.
 In the underlying graph library cgraph(3), edges in undirected graphs are given an arbitrary direction. This is used for traversals, such as TV_fwd, requiring directed edges.
 ARGC : int
 denotes the number of arguments specified by the a args commandline argument.
 ARGV : string array
 denotes the array of arguments specified by the a args commandline argument. The ith argument is given by ARGV[i].
BUILTIN CONSTANTS
There are several symbolic constants defined by gvpr.
 NULL : obj_t
 a null object reference, equivalent to 0.
 TV_flat : tvtype_t
 a simple, flat traversal, with graph objects visited in seemingly arbitrary order.
 TV_ne : tvtype_t
 a traversal which first visits all of the nodes, then all of the edges.
 TV_en : tvtype_t
 a traversal which first visits all of the edges, then all of the nodes.
 TV_dfs : tvtype_t

 TV_postdfs : tvtype_t
 TV_prepostdfs : tvtype_t
 a traversal of the graph using a depthfirst search on the underlying undirected graph. To do the traversal, gvpr will check the value of $tvroot. If this has the same value that it had previously (at the start, the previous value is initialized to NULL.), gvpr will simply look for some unvisited node and traverse its connected component. On the other hand, if $tvroot has changed, its connected component will be toured, assuming it has not been previously visited or, if $tvroot is NULL, the traversal will stop. Note that using TV_dfs and $tvroot, it is possible to create an infinite loop.
 By default, the traversal is done in preorder. That is, a node is visited before all of its unvisited edges. For TV_postdfs, all of a node's unvisited edges are visited before the node. For TV_prepostdfs, a node is visited twice, before and after all of its unvisited edges.
 TV_fwd : tvtype_t

 TV_postfwd : tvtype_t
 TV_prepostfwd : tvtype_t
 A traversal of the graph using a depthfirst search on the graph following only forward arcs. The choice of roots for the traversal is the same as described for TV_dfs above. The different order of visitation specified by TV_fwd, TV_postfwd and TV_prepostfwd are the same as those specified by the analogous traversals TV_dfs, TV_postdfs and TV_prepostdfs.
 TV_rev : tvtype_t

 TV_postrev : tvtype_t
 TV_prepostrev : tvtype_t
 A traversal of the graph using a depthfirst search on the graph following only reverse arcs. The choice of roots for the traversal is the same as described for TV_dfs above. The different order of visitation specified by TV_rev, TV_postrev and TV_prepostrev are the same as those specified by the analogous traversals TV_dfs, TV_postdfs and TV_prepostdfs.
 TV_bfs : tvtype_t
 A traversal of the graph using a breadthfirst search on the graph ignoring edge directions. See the item on TV_dfs above for the role of $tvroot.
EXAMPLES

gvpr i 'N[color=="blue"]' file.gv
Generate the nodeinduced subgraph of all nodes with color blue.

gvpr c 'N[color=="blue"]{color = "red"}' file.gv
Make all blue nodes red.

BEGIN { int n, e; int tot_n = 0; int tot_e = 0; } BEG_G { n = nNodes($G); e = nEdges($G); printf ("%d nodes %d edges %s\n", n, e, $G.name); tot_n += n; tot_e += e; } END { printf ("%d nodes %d edges total\n", tot_n, tot_e) }
Version of the program gc.

gvpr c ""
Equivalent to nop.

BEG_G { graph_t g = graph ("merge", "S"); } E { node_t h = clone(g,$.head); node_t t = clone(g,$.tail); edge_t e = edge(t,h,""); e.weight = e.weight + 1; } END_G { $O = g; }
Produces a strict version of the input graph, where the weight attribute of an edge indicates how many edges from the input graph the edge represents.

BEGIN {node_t n; int deg[]} E{deg[head]++; deg[tail]++; } END_G { for (deg[n]) { printf ("deg[%s] = %d\n", n.name, deg[n]); } }
Computes the degrees of nodes with edges.

BEGIN { int i, indent; int seen[string]; void prInd (int cnt) { for (i = 0; i < cnt; i++) printf (" "); } } BEG_G { $tvtype = TV_prepostfwd; $tvroot = node($,ARGV[0]); } N { if (seen[$.name]) indent; else { prInd(indent); print ($.name); seen[$.name] = 1; indent++; } }
Prints the depthfirst traversal of the graph, starting with the node whose name is ARGV[0], as an indented list.
ENVIRONMENT
 GVPRPATH
 Colonseparated list of directories to be searched to find the file specified by the f option. gvpr has a default list built in. If GVPRPATH is not defined, the default list is used. If GVPRPATH starts with colon, the list is formed by appending GVPRPATH to the default list. If GVPRPATH ends with colon, the list is formed by appending the default list to GVPRPATH. Otherwise, GVPRPATH is used for the list. On Windows systems, replace ``colon'' with ``semicolon'' in the previous paragraph.
BUGS AND WARNINGS
Scripts should be careful deleting nodes during N{} and E{} blocks using BFS and DFS traversals as these rely on stacks and queues of nodes.When the program is given as a command line argument, the usual shell interpretation takes place, which may affect some of the special names in gvpr. To avoid this, it is best to wrap the program in single quotes.
If string constants contain pattern metacharacters that you want to escape to avoid pattern matching, two backslashes will probably be necessary, as a single backslash will be lost when the string is originally scanned. Usually, it is simpler to use strcmp to avoid pattern matching.
As of 24 April 2008, gvpr switched to using a new, underlying graph library, which uses the simpler model that there is only one copy of a node, not one copy for each subgraph logically containing it. This means that iterators such as nxtnode cannot traverse a subgraph using just a node argument. For this reason, subgraph traversal requires new functions ending in "_sg", which also take a subgraph argument. The versions without that suffix will always traverse the root graph.
There is a single global scope, except for formal function parameters, and even these can interfere with the type system. Also, the extent of all variables is the entire life of the program. It might be preferable for scope to reflect the natural nesting of the clauses, or for the program to at least reset locally declared variables. For now, it is advisable to use distinct names for all variables.
If a function ends with a complex statement, such as an IF statement, with each branch doing a return, type checking may fail. Functions should use a return at the end.
The expr library does not support string values of (char*)0. This means we can't distinguish between "" and (char*)0 edge keys. For the purposes of looking up and creating edges, we translate "" to be (char*)0, since this latter value is necessary in order to look up any edge with a matching head and tail.
Related to this, strings converted to integers act like char pointers, getting the value 0 or 1 depending on whether the string consists solely of zeroes or not. Thus, the ((int)"2") evaluates to 1.
The language inherits the usual C problems such as dangling references and the confusion between '=' and '=='.