I need to turn POSIX errno to the error string. StrError.hs import Foreign.C.String import Foreign.C.Types foreign import ccall unsafe "strerror" c_strerror :: CInt -> IO CString strError :: Int -> IO String strError i = do cstr <- c_strerror ci peekCAString cstr where ci = fromIntegral i main = mapM_ (>>= putStrLn) $ map strError [1 .. 450] $ ghc -g StrError.hs $ valgrind --leak-check=full --show-leak-kinds=all ./StrError ==2350== ==2350== HEAP SUMMARY: ==2350== in use at exit: 18 bytes in 1 blocks ==2350== total heap usage: 685 allocs, 684 frees, 97,998 bytes allocated ==2350== ==2350== 18 bytes in 1 blocks are still reachable in loss record 1 of 1 ==2350== at 0x48407B4: malloc (vg_replace_malloc.c:381) ==2350== by 0x496A427: __vasprintf_internal (vasprintf.c:71) ==2350== by 0x493DBD5: asprintf (asprintf.c:31) ==2350== by 0x498A9F0: strerror_l (strerror_l.c:45) ==2350== by 0x408BC0: ??? (StrError.hs:8) ==2350== ==2350== LEAK SUMMARY: ==2350== definitely lost: 0 bytes in 0 blocks ==2350== indirectly lost: 0 bytes in 0 blocks ==2350== possibly lost: 0 bytes in 0 blocks ==2350== still reachable: 18 bytes in 1 blocks ==2350== suppressed: 0 bytes in 0 blocks ==2350== ==2350== For lists of detected and suppressed errors, rerun with: -s ==2350== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0) I am already frustrated with having to use IO for this. I feel like it is better to manually create a pure solution by copying whatever the C code from libc does. But I still do not understand what causes this memory to remain reachable. Calling strerror from pure C code does not leave reachable memory regions. Can anyone explain? And maybe you have a suggestion as to how to implement this strError function properly? Just in case you are the type of a person to inquire as to *why* I need this, for fun. I need it for fun.