File included output.txt is stdout.
Input:

#include <stdio.h>
#include <agraph.h>
int main (int argc, char** argv)
{
	Agraph_t *g;
	Agsym_t *symnlabel;
	g = agopen("Graph",Agundirected, 0);
	symnlabel = agattr(g,AGNODE,"label", "");

	char *tmp = "10.0.0.0 label";
	char *tmp2 = agstrdup_html(g, tmp);
	Agnode_t *n = agnode(g, "10.0.0.0",TRUE);
	agxset(n,symnlabel,tmp2);
	agwrite(g, stdout);

	tmp = "120.0.0.0 label";
	tmp2 = agstrdup_html(g, tmp);
	Agnode_t *n2 = agnode(g, "120.0.0.0",TRUE);
	agxset(n2,symnlabel,tmp2);

	agwrite(g, stdout);
}