Once again, here I am writing C without really knowing what I'm doing...

I've slapped together a simple function that I can call from a C# program that takes a DOT string, an output format, and a file name and renders a graph using Graphviz.

#include "types.h"
#include "graph.h"
#include "gvc.h"

#define FUNC_EXPORT __declspec(dllexport)

// Return codes
#define GVUTIL_SUCCESS          0
#define GVUTIL_ERROR_GVC        1
#define GVUTIL_ERROR_DOT        2
#define GVUTIL_ERROR_LAYOUT     3
#define GVUTIL_ERROR_RENDER     4

FUNC_EXPORT int RenderDot(char * dotData, const char * format,
        const char * fileName) {
    Agraph_t * g;    // The graph
    GVC_t * gvc;     // The Graphviz context
    int result;      // Result of layout and render operations

    // Create a new graphviz context
    gvc = gvContext();
    if (!gvc) return GVUTIL_ERROR_GVC;

    // Read the DOT data into the graph
    g = agmemread(dotData);
    if (!g) return GVUTIL_ERROR_DOT;

    // Layout the graph
    result = gvLayout(gvc, g, "dot");
    if (result) return GVUTIL_ERROR_LAYOUT;

    // Render the graph
    result = gvRenderFilename(gvc, g, format, fileName);
    if (result) return GVUTIL_ERROR_RENDER;

    // Free the layout
    gvFreeLayout(gvc, g);

    // Close the graph
    agclose(g);

    // Free the graphviz context
    gvFreeContext(gvc);

    return GVUTIL_SUCCESS;
}

It compiles fine, but when I call it, I get GVUTIL_ERROR_LAYOUT. At first, I thought it might have been how I was declaring my P/Invoke signature, so I tested it from a C program instead, but it still failed in the same way.

RenderDot("digraph graphname { a -> b -> c; }", "png", "C:\testgraph.png");

Did I miss something?

EDIT

If there's a chance it has to do with how I'm compiling the code, here's the command I'm using:

cl gvutil.c /I "C:\Program Files (x86)\Graphviz2.26\include\graphviz"
    /LD /link /LIBPATH:"C:\Program Files (x86)\Graphviz2.26\lib\release"
    gvc.lib graph.lib cdt.lib pathplan.lib

I've been following this tutorial that explains how to use Graphviz as a library, so I linked to the .lib files that it listed.