Memset function in C initialised all the arrays

Question

I was trying to initialise a array made by pointer:

the code I used was:

    int c = 15;
    Struct *Pointer[c] = {NULL};
    memset( pointer, 0, c *sizeof(pointer) );

It worked, But this memset() function not only initialised my Pointer array, but also initialise all my other arrays...

is there any way to fix it?

I can not use for(){} or while function as it will increase my Time complixcity...

Cheers'

A more interesting adventure is what compiler you're using that allows an initial value assignment to a VLA. None of mine do (clang and msvc), so if Struct *Pointer[c] = {NULL}; "works" I'm curious as to how. I assume the Pointer vs pointer different is just a typo (that I also don't understand, as there is literally no "typing" to introduce a "typo" when copy/pasting what is supposed to be functioning code). — WhozCraig
– WhozCraig, Commented Apr 25, 2021 at 19:01

MikeCAT · Accepted Answer · 2021-04-25 19:05:27Z

1

sizeof(pointer) is the size of the entire array pointer. Multiplying integers larger than 1 to that for size to memset() will cause out-of-range access.

Remove the harmful multiplication.

int c = 15;
Struct *Pointer[c] /* = {NULL} */; /* VLA cannot be initialized */

/* some other code that uses Pointer */

memset(Pointer, 0, sizeof(Pointer));

edited Apr 25, 2021 at 19:05

answered Apr 25, 2021 at 18:52

MikeCAT

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3 Comments

leo Over a year ago

Hey Mike - Thank you for your answer - You are right - I miss typed sizeof(pointer) to sizeof(Struct)...ah...I am so dumb!!!!!!! I found that I can not use Struct *Pointer[c] = {NULL} as the C is a variable - the compiler won't let me do it.

leo Over a year ago

@0___________ Sorry Mate it was my bad - I should't have {NULL}, in C it doesn't work that way

MikeCAT Over a year ago

@0___________ This can be valid.

0___________ · Accepted Answer · 2021-04-25 19:02:31Z

1

memset(Pointer, 0, sizeof(Pointer));

or

memset(Pointer, 0, c * sizeof(*Pointer));

Spot the difference.

You can answer your question yourself if you put some research effort and printed out the sizeof(Pointer) and calculate why c * sizeof(Pointer) is wrong.

answered Apr 25, 2021 at 19:02

0___________

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