Why do glib arrays use uninitalized memory? - c

Question

I've written a toy function using pointer arrays from GLib:
#include <glib-2.0/glib.h>
#include <stdio.h>
void append_string(GArray* arr) {
g_array_append_vals(arr, "foo", 1);
}
int main() {
GPtrArray *arr = g_ptr_array_new();
append_string((GArray*)arr);
g_ptr_array_free(arr, TRUE);
return 0;
}
However, running this with valgrind gives:
==5876== Memcheck, a memory error detector
==5876== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==5876== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==5876== Command: ./demo
==5876==
==5876== Conditional jump or move depends on uninitialised value(s)
==5876== at 0x4E532E9: ??? (in /usr/lib/libglib-2.0.so.0.4800.0)
==5876== by 0x4E536A7: g_array_append_vals (in /usr/lib/libglib-2.0.so.0.4800.0)
==5876== by 0x400727: append_string (main_arrays.c:5)
==5876== by 0x400747: main (main_arrays.c:11)
==5876==
==5876== Conditional jump or move depends on uninitialised value(s)
==5876== at 0x4E532EE: ??? (in /usr/lib/libglib-2.0.so.0.4800.0)
==5876== by 0x4E536A7: g_array_append_vals (in /usr/lib/libglib-2.0.so.0.4800.0)
==5876== by 0x400727: append_string (main_arrays.c:5)
==5876== by 0x400747: main (main_arrays.c:11)
==5876==
==5876== Conditional jump or move depends on uninitialised value(s)
==5876== at 0x4E536C9: g_array_append_vals (in /usr/lib/libglib-2.0.so.0.4800.0)
==5876== by 0x400727: append_string (main_arrays.c:5)
==5876== by 0x400747: main (main_arrays.c:11)
==5876==
==5876==
==5876== HEAP SUMMARY:
==5876== in use at exit: 18,604 bytes in 6 blocks
==5876== total heap usage: 8 allocs, 2 frees, 18,652 bytes allocated
==5876==
==5876== LEAK SUMMARY:
==5876== definitely lost: 0 bytes in 0 blocks
==5876== indirectly lost: 0 bytes in 0 blocks
==5876== possibly lost: 0 bytes in 0 blocks
==5876== still reachable: 18,604 bytes in 6 blocks
==5876== suppressed: 0 bytes in 0 blocks
==5876== Rerun with --leak-check=full to see details of leaked memory
==5876==
==5876== For counts of detected and suppressed errors, rerun with: -v
==5876== Use --track-origins=yes to see where uninitialised values come from
==5876== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 0 from 0)
Am I misusing the API, is this a bug in GLib, or is valgrind wrong? If it's a valgrind limitation, can I silence this valgrind warning whilst still having valgrind warn on genuine issues in my code?

Answer 1

GArray and GPtrArray are not equivalent. There's no subtyping here, even if they have same public fields.
Use g_ptr_array_add instead:
void append_string(GPtrArray* arr) {
g_ptr_array_add(arr, "foo");
}