cgraph | My graph library in C

just share my experience. I have no permission to post a comment, thus I post an answer for your reference. I am using ubuntu 20.04.

In the beginning,

I asked the same question directly in graphviz support forum and got as answer, that it's a known issue with Cmake and linked me to this thread https://gitlab.com/graphviz/graphviz/-/issues/1477

Tl:dr Graphviz is currently not buildable as as subdirectory in a Cmake project.

You need to define the attribute label before you can assign it to a node. From the CGraph documentation included in the Graphviz distribution (p. 11):

Setting attributes is a bit more complex. Before attaching an attribute to a graph component, the code must first set up the default case. This is accomplished by a call to agattr. It takes a graph, an object type (AGRAPH, AGNODE, AGEDGE), and two strings as arguments, and return a representation of the attribute. The first string gives the name of the attribute; the second supplies the default value. The graph must be the root graph.

This is true even for attributes normally defined by Graphviz, since the CGraph library is an independent component. In many use cases (including the ones in the Graphviz documentation), the graph is initialised from a base graph. Reading and parsing the base graph with agread will normally populate the standard attributes, so it may not always be necessary to use agattr. But in this case, you're defining the graph from scratch, so you need to define all of your attributes, even label.

I made a few changes to your code; see the comments below for an explanation.

File descriptors and file pointers are not the same thing. It's confusing, and made even more frustrating by the fact that FILE * is really hard to Google because of the symbol.

You need to fdopen the file descriptor (pipe.fileHandleForWriting.fileDescriptor), to receive a FILE * (UnsafeMutablePointer in Swift). This is what you then pass to agwrite.

It's important to fclose the file pointer when you're done writing to it, otherwise .readDataToEndOfFile() will never terminate. I made a helper function to ensure the fclose can't be forgetten. It's possible that agwrite closes the file pointer itself, internally. If that's the case, you should delete this code and just give it the result of fdopen, plain and simple.