How Can I Generate Call Graphs for C Code to Trace Execution Paths?

Generating Call Graphs for C Code to Trace Execution Paths

In C programming, identifying all possible execution paths leading to a particular function can be a time-consuming manual task, especially when dealing with complex codebases. This is where call graphs come in, providing a graphical representation of function call relationships.

CodeViz and Doxygen Limitations

While tools like CodeViz and Doxygen can generate call graphs, they often focus on display only callees of the target function. This can be insufficient for situations where you need to trace back the entire calling hierarchy.

An Effective Method Using LLVM and Graphviz

A robust approach involves leveraging the LLVM compiler infrastructure and Graphviz for call graph generation:

1. Compile the code to LLVM bitcode using clang -S -emit-llvm.
2. Analyze the bitcode to identify call relationships using opt -analyze.
3. Generate a DOT representation of the call graph using -dot-callgraph.
4. Visualize the graph using dot -Tpng.

Here's an example:

$ clang++ -S -emit-llvm main1.cpp -o - | opt -analyze -dot-callgraph
$ dot -Tpng -ocallgraph.png callgraph.dot

This will create a call graph PNG image (callgraph.png) depicting the execution paths leading to the target function.

Handling External Linkage

External linkage can introduce complexities, as main is assumed to be called externally. To address this:

1. Use -std-link-opts in the analyzer to consider external functions.
2. Use c filt to unmangle function names and class names.
3. Filter out the external node from the DOT representation.

By incorporating these steps, you can generate comprehensive call graphs that effectively trace execution paths in C code, simplifying code comprehension and debugging.

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