Declaring and freeing 2D dynamic array and array of structures

Question

I am new to using malloc in C. I was trying to declare a dynamic array of structures and later free it, similary a 2D aray and free it. I am using gcc to compile the code.

1. First question is regarding using array of structures,

   struct OPinfo {
   
           long NLocal;
   
           double ReFrame,ImFrame,lcl_ReFrame,lcl_ImFrame,lcl_SqFrame;
   
   };
   
   struct OPinfo *OPSTR;
   
   
   void declare_local_structure() {
           OPSTR = (struct OPinfo*) malloc(NBinsR * sizeof(struct OPinfo));
           int i =0;
           while(i<NBinsR) {
                   OPSTR[i].NLocal = 0;
                   OPSTR[i].ReFrame = 0.0;
                   OPSTR[i].ImFrame = 0.0;
                   OPSTR[i].lcl_ReFrame = 0.0;
                   OPSTR[i].lcl_ImFrame = 0.0;
                   OPSTR[i].lcl_SqFrame = 0.0;
                   i++;
           }
   
   }
   void free_local_structure() {
           fprintf(stderr,"%d %d\n",*OPSTR,OPSTR);
           free(OPSTR);
   }
   

The fprintf(..OPSTR) part is giving same address in declare_local_structure() and free_local_structure() indicating no pointer mishap in the mean time. If I comment out the free(OPSTR) part, the program works well. Otherwise, it runs and at the end, where free_local_structure() is called, it is giving the following error. * Error in `./a.out': free(): invalid size: 0x0000000002d2fd40 *

2. In the same program in another part I am using a 2D array of pointers, which I declared by,

   double **Gxy, **GxyRot;
   Gxy = (double**) malloc((NBinsXY) * sizeof(double *));
   GxyRot = (double**) malloc((NBinsXY) * sizeof(double *));
   
   for(j=0; j<NBinsXY; j++) {
           Gxy[j] = malloc(NBinsXY *   sizeof(double));
           GxyRot[j] =  malloc(NBinsXY *   sizeof(double));
   }
   

and free by,

for(l=0;l<NBinsXY;l++) {   
        free(Gxy[l]);
        free(GxyRot[l]);
}

free(Gxy);
free(GxyRot);

This again gives the same error above but if I free by,

free(*Gxy);
free(*GxyRot);
free(Gxy);
free(GxyRot);

the program runs without error.

Where are the errors? I tried "valgrind", but could not understand the output. Another point is the program gave error while compiling with C99. gcc -std=c99 *.c headers/*.h -lm

Answer 1

I agree with the comments above, about actively using a debugger as it will provide detailed information about the nature and location of your issues.

Regarding allocating memory in C, (not applicable to C++) a general rule, when using [m][c]alloc in C, is that (although still rigorously debated) it not necessary (or recommended) that you cast the return. For example, the following:

Gxy = (double**) malloc((NBinsXY) * sizeof(double *)); 

should be written:

Gxy = malloc((NBinsXY) * sizeof(double *));
if(Gxy)//and always test for success before using pointers created using malloc/calloc
{
   ... 

Regarding freeing, the rule is for every call to [m][c]alloc() there must be a corresponding equal number of calls to free(...). Because you are using a loop to allocate memory, it makes sense to use one during free as well:

for(j=0; j<NBinsXY; j++) {
    free(Gxy[j]);
    free(GxyRot[j]); 
}
free(Gxy);
free(GxyRot);