Program crash on memset? - c

Question

I am struggling with memset.
If I write in my array my program crashes. If I comment out the memset i have no problems.
My type struct:
typedef struct
{
char Frage [maxLEN_F_A];
char Antwort[maxLEN_F_A];
} Fragenfeld;
My declaration of the struct:
Fragenfeld QuizFragen[maxFragen];
Fragenfeld *ptrQuizFragen = QuizFragen;
The memset call:
memset(&ptrQuizFragen,0,maxFragen*sizeof(Fragenfeld));
My function, where I edit the value of the adress:
int Fragen_einlesen(Fragenfeld *Quizfragen)
{
....
strncpy(Quizfragen->Frage,sEingabe, maxLEN_F_A);
}

Answer 1

When you write
memset(&ptrQuizFragen,0,maxFragen*sizeof(Fragenfeld));
you're saying "please set a lot of of bytes, starting at the address of the pointer variable ptrQuizFragen, to zero." Notice that this is different than saying "please set a lot of bytes, starting at the beginning of the array pointed at by ptrQuizFragen, to zero." This means the bytes are getting written to the wrong place, which is what's causing your segfault.
Graphically, the setup looks something like this:
ptrQuizFragen
+-----------+
| |
+-----------+
|
v
+-----------+-----------+-----------+ ... +-----------+
| | | | | |
+-----------+-----------+-----------+ ... +-----------+
QuizFragen
The line you've written puts the bytes starting where ptrQuizFragen is located in memory, which does this:
ptrQuizFragen
+-----------+
| 00000000000000000000000000000000000 ... 000000000000 (oops!)
+-----------+
+-----------+-----------+-----------+ ... +-----------+
| | | | | |
+-----------+-----------+-----------+ ... +-----------+
QuizFragen
The line you want is
memset(ptrQuizFragen, 0, maxFragen * sizeof(Fragenfeld));
which says to put the bytes at the memory location pointed at by ptrQuizFragen. That would do this:
ptrQuizFragen
+-----------+
| |
+-----------+
|
v
+-----------+-----------+-----------+ ... +-----------+
| 000000000 | 000000000 | 000000000 | | 000000000 |
+-----------+-----------+-----------+ ... +-----------+
QuizFragen

Answer 2

This ...
memset(&ptrQuizFragen,0,maxFragen*sizeof(Fragenfeld));
... requests that the memory starting at the address of variable ptrQuizFragen be written, but what you want is that memory at the location to which the pointer points is written:
memset(ptrQuizFragen, 0, maxFragen*sizeof(Fragenfeld));
Alternatively, for the same reason that you can initialize ptrQuizFragen as you do, you could also do away with it altogether:
memset(QuizFragen, 0, maxFragen*sizeof(Fragenfeld));
Additionally, it would be clearer to express the wanted size directly in terms of the object being written to:
memset(QuizFragen, 0, sizeof(QuizFragen));
However, if this is for one-time initialization then I would not use memset at all:
Fragenfeld QuizFragen[maxFragen] = {0};
Although that's valid C with well-defined sematics, some compilers may warn. If yours does, and that makes you uncomfortable, then one alternative would be to expand a bit to
Fragenfeld QuizFragen[maxFragen] = { { {0}, {0} } };