GVPR(1)GVPR(1)NAMEgvpr - graph pattern scanning and processing language
( previously known as gpr )
SYNOPSISgvpr [-icV?] [ -o outfile ] [ -a args ] [ 'prog' | -f progfile ] [
files ]
DESCRIPTIONgvpr 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 libagraph(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 predicate‐action rules sup‐
plied in the input program. The rules are evaluated in order. For
each predicate evaluating to true, the corresponding action is per‐
formed. 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, ini‐
tially 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 whitespace‐separated tokens, with
the individual tokens available as strings in the gvpr program
as ARGV[0],...,ARGV[ARGC-1]. Whitespace characters within sin‐
gle 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.
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 predicate‐action 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, 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 predi‐
cate 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 associ‐
ated 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 pred‐
icates and actions is that the former must have a type that may inter‐
preted as either true or false. Here the usual C convention is fol‐
lowed, in which a non‐zero value is considered true. This would include
non‐empty strings and non‐empty references to nodes, edges, etc. How‐
ever, 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 pat‐
terns 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 '#' charac‐
ter, 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 sup‐
plied, objects of all types can be used as subscripts. As in C, vari‐
ables and arrays must be declared. In particular, an undeclared vari‐
able 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 ]
Items in brackets are optional.
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 associ‐
ated 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 result‐
ing 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
BUILT‐IN 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 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.
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 non-zero if node n is in (sub)graph sg, or zero other‐
wise.
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 undi‐
rected, the distinction between head and tail nodes is unimpor‐
tant. 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 non-zero if edge e is in (sub)graph sg, or zero other‐
wise.
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.
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 inte‐
ger 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. If x is a graph, this indi‐
cates 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 implic‐
itly cloned. If a graph is cloned, all nodes, edges and sub‐
graphs 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 use‐
ful for those cases when name conflicts with one of the keywords
such as "head" or "root". Returns NULL on failure or if the
attribute is not defined.
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.
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. Returns NULL on failure or if
the attribute is not defined.
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, non‐
zero on failure. See setDflt 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.
kindOf(obj : obj_t) : string
returns an indication of what kind of graph object is the argu‐
ment. For nodes, edges, and graphs, it returns should be "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) for‐
mat 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 left‐
most copy of string t can be found, or -1 if t is not a sub‐
string of s.
match(s : string, p : string) : int
returns the index of the character in string s where the left‐
most 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 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 suc‐
cessfully scanned.
I/O
print(...) : void
print( expr, ... ) prints a string representation of each argu‐
ment 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 success‐
fully scanned.
openF(s : string, t : string) : int
opens the file s as an I/O stream. The string argument t speci‐
fies 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 char‐
acter 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.
Miscellaneous
exit() : void
exit(v : int) : void
causes gvpr to exit with the exit code v. v defaults to 0 if
omitted.
rand() : double
returns a pseudo‐random double between 0 and 1.
srand() : int
srand(v : int) : int
sets a seed for the random number generator. The optional argu‐
ment 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.
BUILT‐IN VARIABLES
gvpr provides certain special, built‐in 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.
$O : graph_t
denotes the output graph. Before graph traversal, it is initial‐
ized to the target graph. After traversal and any END_G actions,
if it refers to a non‐empty 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)
depth‐first traversal of the graph (cf. $tvtype below). The
default value is NULL for each input graph.
$tvtype : tvtype_t
indicates how gvpr traverses a graph. At present, it can only
take one of six values: TV_flat, TV_dfs, TV_fwd, TV_ref, TV_bfs,
TV_ne, and TV_en. TV_flat is the default. The meaning of these
values is discussed below.
ARGC : int
denotes the number of arguments specified by the -a args com‐
mand‐line argument.
ARGV : string array
denotes the array of arguments specified by the -a args command‐
line argument. The ith argument is given by ARGV[i].
BUILT‐IN 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 seem‐
ingly 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
a traversal of the graph using a depth‐first 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 tra‐
versal will stop. Note that using TV_dfs and $tvroot, it is pos‐
sible to create an infinite loop.
TV_fwd : tvtype_t
a traversal of the graph using a depth‐first search on the graph
following only forward arcs. In
TV_bfs : tvtype_t
a traversal of the graph using a bread‐first search on the graph
ignoring edge directions. See the item on TV_dfs above for the
role of $tvroot. libagraph(3), edges in undirected graphs are
given an arbitrary direction, which is used for this traversal.
The choice of roots for the traversal is the same as described
for TV_dfs above.
TV_rev : tvtype_t
a traversal of the graph using a depth‐first search on the graph
following only reverse arcs. In libagraph(3), edges in undi‐
rected graphs are given an arbitrary direction, which is used
for this traversal. The choice of roots for the traversal is the
same as described for TV_dfs above.
EXAMPLESgvpr-i 'N[color=="blue"]' file.dot
Generate the node‐induced subgraph of all nodes with color blue.
gvpr-c 'N[color=="blue"]{color = "red"}' file.dot
Make all blue nodes red.
BEGIN { int n, e; int tot_n = 0; int tot_e = 0; }
BEG_G {
n = nNodes($G);
e = nEdges($G);
printf ("%d nodes %d edges %s0, n, e, $G.name);
tot_n += n;
tot_e += e;
}
END { printf ("%d nodes %d edges total0, tot_n, tot_e) }
Version of the program gc.
gvpr-c ""
Equivalent to nop.
BEG_G { graph_t g = graph ("merge", "S"); }
E {
node_t h = clone(g,$.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] = %d0, n.name, deg[n]);
}
}
Computes the degrees of nodes with edges.
ENVIRONMENT
GPRPATH
Colon‐separated list of directories to be searched to find the
file specified by the -f option.
BUGS AND WARNINGS
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.
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 InxtnodeP 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 prefer‐
able 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 '=='.
AUTHOR
Emden R. Gansner <erg@research.att.com>
SEE ALSOawk(1), gc(1), dot(1), nop(1), libexpr(3), libagraph(3)
24 April 2008 GVPR(1)
