Hello @phoenix27, Sorry to be straightforward, but you are doing all wrong ! ;-)
If I'm not mistaken, the seq object looks like this (<https://github.com/nim-lang/Nim/blob/646bd99d461469f08e656f92ae278d6695b35778/lib/system/seqs_v2.nim#L20>) : type NimSeqPayload[T] = object cap: int data: UncheckedArray[T] NimSeqV2*[T] = object # \ # if you change this implementation, also change seqs_v2_reimpl.nim! len: int p: ptr NimSeqPayload[T] Run So if you cast a seq to a pointer, you will not get at all the buffer ! In fact, you get a pointer that points to something totally different (and you risk a SIGESEGV) The prefered way to use a seq with c is : * Initialize it with a length `var myList = newSeq[T](1024)` * Get the address of the underlying buffer `var bufAddr = addr(myList[0])` * Pass it to a c function along with your size (to avoid index out of bound) You can also use an array. Contrary to a seq, it doesn't encapsulate a buffer: var arr: array[64, int] doAssert(addr arr == addr arr[0]) var list = newSeq[int](64) doAssert(addr list != addr list[0]) Run If you want to allocate, or grow/shrink a hap buffer in c, you won't be able to use seq effiently or easily (Unfortunatly, seq API don't provide a way to transform a heap buffer into a seq without copy. ).
