SegFault while using Malloc/Realloc on an uninitialized pointer

Question

I'm working on a "light drone simulator" (in C) and one of my task is to add a static target when I left click on the map. So far so good. The thing is that instead of creating a matrix with given sizes as a global variable I would like to start from an uninitialized pointer of pointer and as soon as I left click, I realloc new memory in order to always have enough memory available but not more. Also my "matrix" should have 4 columns for 3D coordinates and 1 for "activation".

Here is a part of my code:

/* Main program */

double **TargetsArray;

int main(int argc, char *argv[]) {

/* Call of model-specific mouse handling function */
   HandleSpecialMouseEvent(button, state, x, y, &ActualFlockingParams,
            &ActualVizParams, TargetPosition, TargetsArray, Modder);
}

void HandleSpecialMouseEvent(int button,
        int state,
        int x,
        int y,
        flocking_model_params_t * FlockingParams,
        vizmode_params_t * VizParams, 
        double * CoordTarg, double ** TargetsArray,
        const int Modifier) {

       static int cnt = 0;

       if (button == GLUT_LEFT && state == GLUT_DOWN && Modifier == GLUT_ACTIVE_ALT) {
            FillVect(CoordTarg, MouseCoordToReal_2D(x, VizParams->MapSizeXY,
                    VizParams->Resolution) + VizParams->CenterX, -MouseCoordToReal_2D(y, VizParams->MapSizeXY,
                    VizParams->Resolution) + VizParams->CenterY, 0);
            printf("%d\n", cnt);

            if (cnt == 0) {

                TargetsArray = malloc( sizeof *TargetsArray );
                TargetsArray[cnt] = malloc( sizeof **TargetsArray * 4);
                printf("Hello");
                TargetsArray[cnt][0] = CoordTarg[0];
                TargetsArray[cnt][1] = CoordTarg[1];
                TargetsArray[cnt][2] = 0;
                TargetsArray[cnt][3] = 1;

                cnt += 1;
            }
            else {
                printf("Youhou");
                TargetsArray = realloc(TargetsArray, sizeof *TargetsArray * (cnt + 1) );
                TargetsArray[cnt] = malloc( sizeof **TargetsArray * 4);
                TargetsArray[cnt][0] = CoordTarg[0];
                TargetsArray[cnt][1] = CoordTarg[1];
                TargetsArray[cnt][2] = 0;
                TargetsArray[cnt][3] = 1;
                TargetsArray[cnt - 1][3] = 0;

                cnt += 1;

            }

            // for (int j = 0; j < cnt; j++) {
            //     for (int i = 0; i < 4; i++) {
            //         printf("%f\t", TargetsArray[j][i]);
            //     }
            //     printf("\n");
            // }
        }


}

I usually get 2 types of errors:

0
Hello1
Youhou2
Erreur de segmentation (core dumped)

OR 

0
Hello1
free(): invalid next size (fast)
Abandon (core dumped)

I know there are simplier ways of doing it but I'd really like to understand why this is not working. I'm pretty sure it is due to the way I use malloc and realloc but I also don't understand why in the first "loop" the program doesn't go into the if statement whereas it does print "0" and then why it prints "Hello" before printing the counter (cnt)?

I hope I put enough code for you to understand my issue but please let me know if something is missing.

Thanks !!

Answer 1

The lines about malloc, and realloc, please check the memory you asked is available and could be allocated, both return a NULL in case of error.

Simply check if there are enough memory, and could be allocated, for both alloc calls, like following:

TargetsArray = malloc( sizeof *TargetsArray );
if(TargetsArray  == NULL){
//oh! not enough mem, or whatever...
}
//...
void* TargetsArray_ra = realloc(TargetsArray, sizeof(*TargetsArray) * (cnt + 1) );
if(TargetsArray_ra == NULL){
//oh! could not realloc
//if you wish to return, don't forget to free up the TargetsArray 
if(TargetsArray != NULL){free(TargetsArray);TargetsArray=NULL;}
//return;
}else{//when realloc was success
TargetsArray = TargetsArray_ra ;
}

Same check for TargetsArray[cnt] = malloc( sizeof **TargetsArray * 4); , make sure it takes a proper non- NULL ptr, before you access it

Also those if-else blocks are kind of the same, except the last statement, besides that when you debug the code, at which line you get segfault? You sure the passed CoordTarg is not NULL , and has proper length? What about FillVect func? it's working as expected?

Answer 2

You need to change your function's argument TargetsArray so that it points on the global variable and not only store the same value.

Main program:

double **TargetsArray;

int main(int argc, char *argv[]) {

/* Call of model-specific mouse handling function */
   HandleSpecialMouseEvent(button, state, x, y, &ActualFlockingParams,
            &ActualVizParams, TargetPosition, &TargetsArray, Modder);
}

void HandleSpecialMouseEvent(int button,
        int state,
        int x,
        int y,
        flocking_model_params_t * FlockingParams,
        vizmode_params_t * VizParams, 
        double * CoordTarg, double *** TargetsArray,
        const int Modifier) {

       static int cnt = 0;

       if (button == GLUT_LEFT && state == GLUT_DOWN && Modifier == GLUT_ACTIVE_ALT) {
            FillVect(CoordTarg, MouseCoordToReal_2D(x, VizParams->MapSizeXY,
                    VizParams->Resolution) + VizParams->CenterX, -MouseCoordToReal_2D(y, VizParams->MapSizeXY,
                    VizParams->Resolution) + VizParams->CenterY, 0);
            printf("%d\n", cnt);

            if (cnt == 0) {

                *TargetsArray = malloc( sizeof **TargetsArray );
                (*TargetsArray)[cnt] = malloc( sizeof ***TargetsArray * 4);
                printf("Hello\n");
                (*TargetsArray)[cnt][0] = CoordTarg[0];
                (*TargetsArray)[cnt][1] = CoordTarg[1];
                (*TargetsArray)[cnt][2] = 0;
                (*TargetsArray)[cnt][3] = 1;

                cnt += 1;
            }
            else {
                printf("Youhou");
                *TargetsArray = realloc(*TargetsArray, sizeof **TargetsArray * (cnt + 1) );
                (*TargetsArray)[cnt] = malloc( sizeof ***TargetsArray * 4);
                (*TargetsArray)[cnt][0] = CoordTarg[0];
                (*TargetsArray)[cnt][1] = CoordTarg[1];
                (*TargetsArray)[cnt][2] = 0;
                (*TargetsArray)[cnt][3] = 1;
                (*TargetsArray)[cnt - 1][3] = 0;

                cnt += 1;

            }

            // for (int j = 0; j < cnt; j++) {
            //     for (int i = 0; i < 4; i++) {
            //         printf("%f\t", *TargetsArray[j][i]);
            //     }
            //     printf("\n");
            // }
        }


}

NOTE: Parentheses are very important

In each time you reallocate memory, you are storing a new memory address in your TargetsArray . Since you're changing the value inside the function, you need to give your function the address of the variable TargetsArray . Other ways you will only store the reallocated address on the function's local TargetsArray .