Issues with dynamically allocating a string array

Question

I am trying to dynamically allocate the array frags2 of size numberOfFrags and copy over the contents of the original array to it. I have tried numerous approaches and searching and do not understand what is going wrong here. sizeof on the new array returns 0 instead of what I thought I malloc'd.

Any help would be much appreciated!

 int main(int argc, const char* argv[]) {
     char* frags[MAX_FRAG_COUNT];  
     FILE* fp = fopen(argv[1], "r");
     int numberOfFrags = ReadAllFragments(fp, frags, MAX_FRAG_COUNT);
     fclose(fp);
     char** frags2 = (char**)malloc(numberOfFrags * sizeof(char*));
     for (int i = 0; i < numberOfFrags; i++) {
         frags2[i] = frags[i];
     }
     qsort(frags2, sizeof(frags2) / sizeof(char *), sizeof(char*), cstring_cmp);

See How to find the sizeof(a pointer pointing to an array)

Bo Persson
– Bo Persson

2012-07-09 09:27:51 +00:00
Commented Jul 9, 2012 at 9:27 — Bo Persson
– Bo Persson, Commented Jul 9, 2012 at 9:27

cnicutar · Accepted Answer · 2012-07-09 09:14:27Z

4

Sizeof on the new array returns 0 instead of what I thought I malloc'd

It's not an array, it's a pointer. In this context the operator sizeof yields the size of the pointer, not the amount you allocated.

So instead of that sizeof stuff, you can try

qsort(frags2, numberOfFrags, sizeof(char*), cstring_cmp);

answered Jul 9, 2012 at 9:14

cnicutar

184k26 gold badges378 silver badges398 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Jason Block Over a year ago

Thanks a lot, that worked. But what I don't understand is what makes the original array, frags, any different. Calling sizeof(frags) returns 80000 (MAX_FRAG_COUNT is 20000).

cnicutar Over a year ago

@JasonBlock You just said it: The original array. frags2 is not an array, it's a pointer to which you assign an address. Another (misleading) way to put it is that "sizeof works at compile-time".

Collectives™ on Stack Overflow

Issues with dynamically allocating a string array

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related