Got segfault accessing array of pointers to struct when using pointers instead of references

Question

The function below takes a pointer to an array of pointers to API_AddParType structs and lists some of the fields of each array element. Everything works just like expected when accessing each element with (*ppParams)[ii]:

void ParamsListWrapper::ListParams2(API_AddParType** ppParams)
{
    unsigned long ii, nParams;
    nParams = BMGetHandleSize( (GSHandle)ppParams ) / sizeof(API_AddParType);

    // list params
    for ( ii = 0; ii < nParams; ii++ ) {
        WriteReport( "Param name = \"%s\", double = %f", (*ppParams)[ii].name, (*ppParams)[ii].value.real );
    }
}

Well, let's use some reference variable like API_AddParType& param:

void ParamsListWrapper::ListParams1(API_AddParType** ppParams)
{
    unsigned long ii, nParams;
    nParams = BMGetHandleSize( (GSHandle)ppParams ) / sizeof(API_AddParType);

    // list params
    for ( ii = 0; ii < nParams; ii++ ) {
        API_AddParType& param = (*ppParams)[ii];
        WriteReport( "Param name = \"%s\", double = %f", param.name, param.value.real );
    }
}

Now trying the same code with pointer to API_AddParType:

void ParamsListWrapper::ListParams3(API_AddParType** ppParams)
{
    unsigned long ii, nParams;
    nParams = BMGetHandleSize( (GSHandle)ppParams ) / sizeof(API_AddParType);

    // list params
    for ( ii = 0; ii < nParams; ii++ ) {
        API_AddParType* pParam = ppParams[ii];
        if (pParam != NULL) {
            WriteReport( "Param name = \"%s\", double = %f", pParam->name, pParam->value.real );
        } else {
            WriteReport( "Param is NULL" );
        }
    }
}

Got segmentation fault. How could it be? Why it fails when accessing data by pointers while references work without any problems?

Did you try to enable all warnings and debugging info (e.g. g++ -Wall -g on Linux) and to use valgrind.org and the gdb debugger? — Basile Starynkevitch
– Basile Starynkevitch, Commented Oct 4, 2012 at 12:01

Daniel Fischer · Accepted Answer · 2012-10-04 14:05:59Z

The function below takes a pointer to an array of pointers to API_AddParType structs

void ParamsListWrapper::ListParams2(API_AddParType** ppParams)

The function's argument is a pointer-to-pointer-to-API_AddParType, that could be a pointer to the first element of an array of pointers to API_AddParType,

ppParams
   |
   v
pParam0|pParam1|pParam2|...
   |       |       |    ...
   v       v       v
  xyz     abc     mno   ...

or it could be a pointer to (a pointer to the first element of an array of API_AddParType),

ppParams
   |
   v
 pParams
   |
   v
param0|param1|param2|...

where I named the anonymous memory location ppParams points to pParams (pointer to params), and the reason the function receives a pointer to pParams instead of pParams itself is probably that the pointer pParams itself may be changed by the function.

Now, in the first case, (*ppParams)[ii] = pParam0[ii] = the object ii units after the start of xyz, so you never look at what pParam1, pParam2 etc. point to (unless these pointers are set up to point inside the array whose first element pParam1 points to).

In the second case, (*ppParams)[ii] = pParams[ii] = the object ii units after the start of param0, and you look at all the neatly arranged paramN in sequence. Looks good.

With

API_AddParType* pParam = ppParams[ii]

in the first situation, you'd look at xyz, abc, mno, ... in sequence, that looks good.

But in the second situation, pParam would become the pointer ii units after pParams, but that is not the first element of an array of pointers to API_AddParType of size larger than one, so accessing ppParams[ii] is undefined behaviour for ii > 0, and since it is unlikely that there happen to be valid pointers to API_AddParType just behind pParams, the access to pParam->name resp. pParam->value.real probably causes a segfault.

The fact that you got a segmentation fault with ppParams[ii], but not with (*ppParams)[ii] strongly hints that the situation is actually the second, and not the first.

Thanks a lot! That was the second situation. Exactly as you proposed!

Zoneur · Accepted Answer · 2012-10-04 12:24:52Z

1

Try changing API_AddParType* pParam = ppParams[ii]; to API_AddParType* pParam = *ppParams; and accessing pParam->name like this pParam[ii].name.

By accessing (*ppParams)[ii] you are manipulating a single API_AddParType, but accessing pParam[ii] you are manipulating an array of API_AddParType.

answered Oct 4, 2012 at 12:24

Zoneur

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Comments

Sivachandran · Accepted Answer · 2012-10-04 12:32:26Z

1

Try changing the line

API_AddParType* pParam = ppParams[ii];

to

API_AddParType* pParam = (*ppParams) + ii;

answered Oct 4, 2012 at 12:32

Sivachandran

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molbdnilo · Accepted Answer · 2012-10-04 14:05:09Z

1

In the last case, you're treating ppParams as an array of pointers, not as a pointer to an array.

The ii:th element of the array is (*ppParams)[ii] in the pointer case as well as in the other cases.
You're looking for its address, which is

API_AddParType* pParam = &(*ppParams)[ii];

or

API_AddParType* pParam = (*ppParams) + ii;

answered Oct 4, 2012 at 14:05

molbdnilo

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