qsort on multiple indexes ex. primary index then secondary index

What would be the best way to implement a qsort that does sort on multiple indexes. ex. first sort on primary index then secondary index.

Code:

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//before sort: {(0, 3),(1, 2),(0, 1)}
//after sort: {(0, 1),(0, 3),(1, 2)}

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Code:

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typedef struct{
boolean element1;//primary
int element2;//secondary
}foo

int my_cmp(){
//?? what would be the most efficient implementation
}
int main(){
foo a[20];
//assume the array gets filled here

qsort(&a[0], sizeof(a),sizeof(a[0]), my_cmp);

return 0;
}

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Thank you!

It basically goes like this

Code:

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if ( a.e1 < b.e1 ) return -1;
if ( a.e1 > b.e1 ) return +1;
// e1 is equal, now go onto e2
if ( a.e2 < b.e2 ) return -1;
if ( a.e2 > b.e2 ) return +1;
// all the keys are equal
return 0;

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