Question

Hi I have problem with this:

tmpy = (int**)realloc(os,y*sizeof(int*));
os = tmpy;
tmpy=NULL;
for(i=0;i<y;i++) {
    tmpx = (int*)realloc(os[i],x*sizeof(int)); 
    os[i] = tmpx;
    tmpx=NULL;
}

Program runs good but valgrind gives this error:

==12513== Conditional jump or move depends on uninitialised value(s)
==12513==    at 0x402BF0D: realloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==12513==    by 0x8048760: main (main.c:55)
==12513==  Uninitialised value was created by a heap allocation
==12513==    at 0x402BF52: realloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==12513==    by 0x804871E: main (main.c:50)
==12513== 

What is wrong? Thank you for your help.

Answer 1

Assuming os is being realloc()d to a larger size, then the additional int* effectively added to os will be uninitialised pointers (accessed by os[i]). These unitialised pointers are eventually passed to realloc() within the for loop:

tmpx = (int*)realloc(os[i],x*sizeof(int));

The pointer passed to realloc() must be NULL or a value previously returned by malloc(), realloc() or calloc(). Set the additional int* to NULL before entering the for loop. Note the cast on return value of realloc() is unrequired.