Getting a segmentation fault when running Release version, but not Debug version in Code Blocks

Question

I am using Code Blocks:

Release 13.12 rev 9501 (2013-12-25 18:25:45) gcc 4.8.2 Linux/unicode - 64 bit

On the following system:

Linux Mint 17.3 Cinnamon (version 2.8.7) 64-bit
Intel Core i7-4790
NVIDIA GeForce GTX 750 Ti

When I set selected target to "Debug" and run my code, it works perfectly. I have run it many times without any problem. But when I select the "Release" target and run the program from the command prompt quite often there is a segmentation fault.

My C++ program is about a thousand lines, and I don't know what part of the program is causing the segmentation fault. So I don't know which code is relevant at this stage (I will post some code here when I have more information). It does make use of pointers and a dynamic data structure. I am guessing that something is not being initialised correctly, but I don't know how to go about debugging the "Release" version. I have not changed any of the compiler or linker settings from the default values. What could be different in the "Release" version to cause this problem?

(Update) Following Nathan's advice I was able to isolate the segmentation fault. There is a for loop, but under certain circumstances the upper limit (end index) of this loop was not being initialised, along the lines of:

void fnProc(bool var_val, int inp_val, int test_val, int st_idx, int lim_idx, int xarr[])
{
    int idx, end_idx;
    if (test_val > inp_val)
        end_idx = someFn(inp_val, lim_idx, xarr);
    if (!var_val)
        for (idx = st_idx; idx <= end_idx; idx++)
            xarr[idx] = 0;
}

I was able to fix the problem, by initialising the "end_idx" variable to zero ("st_idx" is always greater than one):

void fnProc(bool var_val, int inp_val, int test_val, int st_idx, int lim_idx, int xarr[])
{
    int idx, end_idx;
    if (test_val > inp_val)
        end_idx = someFn(inp_val, lim_idx, xarr);
    else
        end_idx = 0;
    if (!var_val)
        for (idx = st_idx; idx <= end_idx; idx++)
            xarr[idx] = 0;
}

Is it possible to get Code Blocks / GCC compiler to issue a warning when a variable has not been initialised like this? I see there is a GCC option: -Wuninitialized, but I don't see this in Code Blocks.

Answer 1

Is it possible to get Code Blocks / GCC compiler to issue a warning when a variable has not been initialised like this? I see there is a GCC option: -Wuninitialized, but I don't see this in Code Blocks

You can add any compiler options that are not available from the Compiler settings -> Compiler flags menu by navigating from the workspace tree-view Your Project -> Build options -> {Debug|Release} -> Compiler settings -> Other compiler options and listing them in the text-box.

However, it is unnecessary to add -Wuninitialized in this way because it is enabled by -Wall and is provoked by your code with the usual release build optimisation -O2 (or any higher than -O0):-

foo.cpp

extern int someFn(int, int, int[]);

void fnProc(bool var, int inp_val, int test_val, int st_idx, int lim_idx, int xarr[])
{
    int idx, end_idx;
    if (test_val > inp_val)
        end_idx = someFn(inp_val, lim_idx, xarr);
    if (!var)
        for (idx = st_idx; idx <= end_idx; idx++)
            xarr[idx] = 0;
}

With g++ 5.2:

$ g++ -O2 -Wall -c foo.cpp
foo.cpp: In function ‘void fnProc(bool, int, int, int, int, int*)’:
foo.cpp:5:14: warning: ‘end_idx’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     int idx, end_idx;
              ^

and this holds good back at least as far back as GCC 4.4.7

-Wall is enabled from the Code::Blocks Compiler settings -> Compiler flags menu by ticking item Warnings -> Enable all common compiler warnings.

In Code::Blocks 16.01 (which I currently have) even this setting is unnecessary because -Wall is enabled for both Debug and Release configurations by default, thus the warning duly appears in a default Code::Blocks console project release build of foo.cpp:-

-------------- Build file: Release in deleteme (compiler: GNU GCC Compiler 5.2)---------------

g++-5 -Wall -fexceptions -O2  -c /home/imk/develop/deleteme/foo.cpp -o obj/Release/foo.o
/home/imk/develop/deleteme/foo.cpp: In function ‘void fnProc(bool, int, int, int, int, int*)’:
/home/imk/develop/deleteme/foo.cpp:5:14: warning: ‘end_idx’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     int idx, end_idx;
              ^
Process terminated with status 0 (0 minute(s), 0 second(s))
0 error(s), 1 warning(s) (0 minute(s), 0 second(s))

As far as I recall, -Wall has always been a default GCC option in Code::Blocks for the 6 years or so that I've used it, but maybe I am mistaken.