Question

I am mallocing an array of c strings. After releasing it, I get the following error:

Assembler(87536) malloc: *** error for object 0x108500840: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug

Why is that? I am pretty sure I am doing the malloc correctly. I’m pretty experienced with memory management, but I am not sure why this is giving me an error. The array is should hold three strings, each of which is 2 characters long.

Here is how I am mallocing the array:

char **reg_store;
reg_store = malloc(3 * (sizeof(char*)));
if (reg_store == NULL) {
     fprintf(Out, "Out of memory\n");
     exit(1);
}

for (int i = 0; i < 3; i++) {
    reg_store[i] = malloc(2 * sizeof(char));
    if (reg_store[i] == NULL) {
          fprintf(Out, "Out of memory\n");
          exit(1);
    }
}

Here is how I am freeing it:

for (int i = 0; i < 3; i++) {
    free(reg_store[i]);
}
free(reg_store);

Here is what I have in between:

// Keeps a reference to which register has been parsed for storage

int count = 0;
char *reg = NULL;
char *inst_ptr // POINTS TO SOME STRING. EXAMPLE: $t2, $t1, $a0

while (1) {

    // Parses the string in inst_ptr with dollar, comma and space as a delimiter.
    reg = parse_token(inst_ptr, " $,\n", &inst_ptr, NULL);

    if (reg == NULL || *reg == '#') {
        break;
    }

    reg_store[count] = reg;
    count++;
    free(reg);
}

I am printing out reg after I call parse_token and it does print out correctly. I am also printing out reg_store[count] and it does also print out correctly.

Answer 1

Your problem is here:

reg_store[count] = reg;
free(reg);

and later

free(reg_store[i]);

reg is already freed and you free it another time (not talking about the problems with using it later). to fix this replace

reg_store[count] = reg;

with

strcpy(reg_store[count], reg);

or as suggested in the comments, since you know its two charaters, its better to memcpy it:

memcpy(reg_store[count], reg, 2);