Suspect that segmentation fault on large arrays occurs because the list1[] and list2[] ran out of space. With the recursive calls, code is heavily using the stack space. Use malloc() and free() for large arrays instead of VLA[]

Memory allocation could be reduced. Via recursion, this takes > 2n (maybe 4n) memory space. At worst it should be 2n. At a minimum, allocate, populate and sort list1[], then do the same for list2[].

Use size_t rather than int for a integer type that can handle all array indexes.

// void mergesort (int* list, int len)
void mergesort (int* list, size_t len)

Cope with 0 length.

 // if(len == 1) return;
 if(len <= 1) return;

Suspect that segmentation fault on large arrays occurs because the list1[] and list2[] ran out of space. With the recursive calls, code is heavily using the stack space. Use malloc() and free() for large arrays instead of VLA[]

Memory allocation could be reduced. Via recursion, this takes > 2n (maybe 4n) memory space. At worst it should be 2n.

Use size_t rather than int for a integer type that can handle all array indexes.

// void mergesort (int* list, int len)
void mergesort (int* list, size_t len)

Cope with 0 length.

 // if(len == 1) return;
 if(len <= 1) return;