[Haskell-beginners] FFI to POSIX libc; strerror (errnum); unfreed memory

[Sylvain Henry]

Hi,

I get exactly this valgrind profile with the following C code:

#include<stdio.h>
#include<string.h>

int main() {
     for (int i=0; i<=450; i++) {
         printf("%s\n", strerror(i));
     }
}

So there isn't anything Haskell specific here. Looking at glibc sources, 
there is a "strerror_l_buf" buffer kept in the TLS 
(https://elixir.bootlin.com/glibc/latest/source/string/strerror_l.c#L45) 
so that's expected.

Use strerror_r instead to use a buffer you manage explicitly (e.g. with 
allocaBytes).

Sylvain

PS: are you running valgrind on your Haskell program? A valgrind profile 
for a Haskell program has many more entries. E.g. for a program compiled 
with GHC 9.6.4:

==36067== 2,097,152 bytes in 1 blocks are still reachable in loss record 
21 of 21
==36067==    at 0x4843788: malloc (vg_replace_malloc.c:442)
==36067==    by 0x2AD470: stgMallocBytes (in 
/home/hsyl20/projects/ghc/scratch/strerror/Test)
==36067==    by 0x2B96D8: initEventLogging (in 
/home/hsyl20/projects/ghc/scratch/strerror/Test)
==36067==    by 0x2B7CFE: initTracing (in 
/home/hsyl20/projects/ghc/scratch/strerror/Test)
==36067==    by 0x2A60AE: hs_init_ghc (in 
/home/hsyl20/projects/ghc/scratch/strerror/Test)
==36067==    by 0x2A56F0: hs_main (in 
/home/hsyl20/projects/ghc/scratch/strerror/Test)
==36067==    by 0x22D725: main (in 
/home/hsyl20/projects/ghc/scratch/strerror/Test)


On 12/04/2024 07:58, Folsk Pratima wrote:
> 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.
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