C variable initialization valgrind complains

Question

I have a simple question here. I have some variable declarations as follows:

char long_name_VARA[]="TEST -- Gridded 450m daily Evapotranspiration (ET)";
int16 fill_PET_8day=32767;

Given the above valgrind complains for the char declaration as follows:

Invalid write of size 8
==21902==    at 0x408166: main (main.c:253)
==21902==  Location 0x7fe677840 is 0 bytes inside long_name_VARA[0]

and for the int16 declaration as follows:

==21902== Invalid write of size 2
==21902==    at 0x408178: main (main.c:226)
Location 0x7fe677420 is 0 bytes inside local var "fill_PET_8day"

What am i doing wrong in my declarations here?

Also can I not declare a char array like this:

char temp_year[5]={0}

Answer 1

When I run the following

/* test.c */
#include <stdio.h>

int main(void)
{
    char s[8000000];
    int x;

    s[0] = '\0';
    x=5;
    printf("%s %d\n",s,x);
    return 0;
}

with Valgrind I get

$ gcc test.c
$ valgrind ./a.out
==1828== Memcheck, a memory error detector
==1828== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==1828== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==1828== Command: ./a.out
==1828== 
==1828== Warning: client switching stacks?  SP change: 0xfff0002e0 --> 0xffe85f0d0
==1828==          to suppress, use: --max-stackframe=8000016 or greater
==1828== Invalid write of size 1
==1828==    at 0x400541: main (in /home/m/a.out)
==1828==  Address 0xffe85f0d0 is on thread 1's stack
==1828==  in frame #0, created by main (???)
==1828== 
==1828== Invalid write of size 8
==1828==    at 0x400566: main (in /home/m/a.out)
==1828==  Address 0xffe85f0c8 is on thread 1's stack
==1828==  in frame #0, created by main (???)
==1828== 
==1828== Invalid read of size 1
==1828==    at 0x4E81ED3: vfprintf (vfprintf.c:1642)
==1828==    by 0x4E88038: printf (printf.c:33)
==1828==    by 0x40056A: main (in /home/m/a.out)
==1828==  Address 0xffe85f0d0 is on thread 1's stack
==1828==  in frame #2, created by main (???)
==1828== 
 5
==1828== Invalid read of size 8
==1828==    at 0x4E88040: printf (printf.c:37)
==1828==    by 0x40056A: main (in /home/m/a.out)
==1828==  Address 0xffe85f0c8 is on thread 1's stack
==1828==  in frame #0, created by printf (printf.c:28)
==1828== 
==1828== Warning: client switching stacks?  SP change: 0xffe85f0d0 --> 0xfff0002e0
==1828==          to suppress, use: --max-stackframe=8000016 or greater
==1828== 
==1828== HEAP SUMMARY:
==1828==     in use at exit: 0 bytes in 0 blocks
==1828==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==1828== 
==1828== All heap blocks were freed -- no leaks are possible
==1828== 
==1828== For counts of detected and suppressed errors, rerun with: -v
==1828== ERROR SUMMARY: 4 errors from 4 contexts (suppressed: 0 from 0)

If I give the option Valgrind warns with I get

$ valgrind --max-stackframe=10000000 ./a.out
==1845== Memcheck, a memory error detector
==1845== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==1845== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==1845== Command: ./a.out
==1845== 
 5
==1845== 
==1845== HEAP SUMMARY:
==1845==     in use at exit: 0 bytes in 0 blocks
==1845==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==1845== 
==1845== All heap blocks were freed -- no leaks are possible
==1845== 
==1845== For counts of detected and suppressed errors, rerun with: -v
==1845== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

So the "Invalid reads/writes" are due to the large stack variable without the proper --max-stackframe=... option.

Answer 2

As mentioned, the declarations aren't the issue.

Although FYI, it may be better to declare your constant string as const...

const char* long_name_VARA = "TEST -- Gridded 450m daily Evapotranspiration (ET)";

or even...

const char* const long_name_VARA = "TEST -- Gridded 450m daily Evapotranspiration (ET)";

This prevents the string from being modified in code, (and the pointer).

Answer 3

The warning messages you quoted show invalid memory accesses, which happen to hit memory areas belonging to the above two variables. The variables in question are victims of the error, not perpetrators. The variables are not to blame here. Nothing is wrong with the above declarations. Most likely these declarations are not in any way relevant here.

The perpetrators are lines at main.c:253 and main.c:226, which you haven't quoted yet. That's where your problem occurs.

A wild guess would be that you have another object declared after fill_PET_8day (an array?). When working with that other object, you overrun its memory boundary by ~10 bytes, thus clobbering fill_PET_8day and first 8 bytes of long_name_VARA. This is what valgrind is warning you about.