Defining a recursive function inside another function in C

Question

I'm trying to write a recursive function (printPath) inside another function (dijkstraSSSP), but it gives me a compiler error.

When I run gcc dijkstra.c WGraph.c PQueue.c, it compiles just fine. However, when I run dcc -Wall -Werror -std=c11 -o dijkstra dijkstra.c WGraph.c PQueue.c. I got this error:

dijkstra.c:48:38: error: function definition is not allowed here
void printPath (int currentNode) {

Which I think it is because I define a function inside another function. If this is the case, how should I modify the code? Thanks a lot!

void dijkstraSSSP(Graph g, Vertex source) {
    int  dist[MAX_NODES];
    int  pred[MAX_NODES];
    bool vSet[MAX_NODES];  // vSet[v] = true <=> v has not been processed
    int s, t;

    PQueueInit();
    int nV = numOfVertices(g);
    for (s = 0; s < nV; s++) {
        joinPQueue(s);
        dist[s] = VERY_HIGH_VALUE;
        pred[s] = -1;
        vSet[s] = true;
    }

    dist[source] = 0;  

    int relaxWeight;
    for (s = 0; s < numOfVertices(g); s++) {
        s = leavePQueue(dist);
        //printf("iterating with s = %d\n", s);
        //printf("updating vSet[%d] = false\n", s);
        vSet[s] = false;
        for (t = 0; t < numOfVertices(g); t++) {
            //printf("iterating with t = %d\n", t);
            //printf("checking if s = %d is adj to t = %d and vSet[%d] = true\n", s, t, t);
            if (adjacent(g, s, t) != 0 && vSet[t] == true) {
                //printf("YES\n");
                relaxWeight = adjacent(g, s, t);
                if (dist[t] > dist[s] + relaxWeight) {
                    //printf("updating dist[%d] = dist[%d] + adjWeight[%d]\n", t, s, relaxWeight);
                    dist[t] = dist[s] + adjacent(g, s, t);
                    //printf("updating pred[%d] = %d\n", t, s);
                    pred[t] = s;
                }
            }
        }
    }
 
    void printPath (int currentNode) {
        if (pred[currentNode] == -1) {
            printf("%d", currentNode);
            return;
        } else {
            printPath (pred[currentNode]);
            printf("-%d", currentNode);
        }
    }

    for (int i = 0; i < numOfVertices(g); i++) {
        if (dist[i] == VERY_HIGH_VALUE) {
            printf("%d: no path\n", i);
        } else{
            printf("%d: distance = %d, shortest path: ", i, dist[i]);
            printPath(i);
            printf("\n");
        }
    }
}

Answer 1

Defining a function inside another function is not defined in the C Standard. gcc allows it as an extension, but most compilers don't.

You should move the definition of printPath outside the body of the dijkstraSSSP function, before it in the source code, and pass pred as an extra argument:

void printPath(const int *pred, int currentNode) {
    if (pred[currentNode] == -1) {
        printf("%d", currentNode);
    } else {
        printPath(pred, pred[currentNode]);
        printf("-%d", currentNode);
    }
}

void dijkstraSSSP(Graph g, Vertex source) {
    int  dist[MAX_NODES];
    int  pred[MAX_NODES];
    bool vSet[MAX_NODES];  // vSet[v] = true <=> v has not been processed
    int s, t;

    PQueueInit();
    int nV = numOfVertices(g);
    for (s = 0; s < nV; s++) {
        joinPQueue(s);
        dist[s] = VERY_HIGH_VALUE;
        pred[s] = -1;
        vSet[s] = true;
    }

    dist[source] = 0;  

    int relaxWeight;
    for (s = 0; s < numOfVertices(g); s++) {
        s = leavePQueue(dist);
        //printf("iterating with s = %d\n", s);
        //printf("updating vSet[%d] = false\n", s);
        vSet[s] = false;
        for (t = 0; t < numOfVertices(g); t++) {
            //printf("iterating with t = %d\n", t);
            //printf("checking if s = %d is adj to t = %d and vSet[%d] = true\n", s, t, t);
            if (adjacent(g, s, t) != 0 && vSet[t] == true) {
                //printf("YES\n");
                relaxWeight = adjacent(g, s, t);
                if (dist[t] > dist[s] + relaxWeight) {
                    //printf("updating dist[%d] = dist[%d] + adjWeight[%d]\n", t, s, relaxWeight);
                    dist[t] = dist[s] + adjacent(g, s, t);
                    //printf("updating pred[%d] = %d\n", t, s);
                    pred[t] = s;
                }
            }
        }
    }
 
    for (int i = 0; i < numOfVertices(g); i++) {
        if (dist[i] == VERY_HIGH_VALUE) {
            printf("%d: no path\n", i);
        } else{
            printf("%d: distance = %d, shortest path: ", i, dist[i]);
            printPath(pred, i);
            printf("\n");
        }
    }
}