Why memset of array of structure changing the program behaviour?

Question

#include <stdio.h>
#include <string.h>
#define PIPE "myPipeName"

typedef enum 
{
    ID1,
    ID2
}TEST_ID;

typedef struct
{
    double dCnt;
    TEST_ID id ;
}Response;

int main()
{
    char pipeName[256]=PIPE;
    Response res[2];
    printf("1. pipeName : %s , PIPE : %s\n",pipeName,PIPE);
    memset(res,0,2*sizeof(res));
    printf("2. pipeName : %s , PIPE : %s\n",pipeName,PIPE);

    return 0;
}

Actual o/p:

pipeName : myPipeName , PIPE :myPipeName

pipeName : , PIPE : myPipeName

Expected o/p:

pipeName : myPipeName , PIPE :myPipeName

pipeName : myPipeName , PIPE :myPipeName

Please let me know how can I solve this ?

Thanks . Today I lost my common sense it seems .. But I don't know why you guys down vote . — Bhavith C Acharya
– Bhavith C Acharya, Commented May 20, 2016 at 8:40

Destructor · Accepted Answer · 2016-05-20 08:28:33Z

4

You're running out of bound there, which invokes undefined behavior

Change

 memset(res,0,2*sizeof(res));
              ^^^^^^^^^^^^

to

memset(res,0,sizeof(res));

or, if you prefer the multiplied version (for better readability, maybe?), use

memset( res , 0 , 2 * sizeof(res[0]));

or

memset( res , 0 , 2 * sizeof(Response));

That said, uninitialized automatic variable value is indeterministic. Don't try to use them.

edited May 20, 2016 at 8:28

Destructor

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answered May 20, 2016 at 8:18

Sourav Ghosh

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2 Comments

songyuanyao Over a year ago

multiplied version is not a good idea. We have to maintain it if the dimension of res changed.

Sourav Ghosh Over a year ago

@songyuanyao That is why that's a second choice, but nevertheless, it's possible.

Biruk Abebe · Accepted Answer · 2016-05-20 08:27:39Z

1

Response res[2];//is an array of 2 Responses

sizeof(res);//get the size of the array in bytes

memset(res,0,2*sizeof(res));//the multiplication by the size of the array is not needed here and 
                            //the memset is writing 0 out of bound of the array and probably
                            //into pipeName which would be treated as null terminator character

Writing out of the array bound is undefined behavior, so change to:

memset(res,0,sizeof(res));

edited May 20, 2016 at 8:27

answered May 20, 2016 at 8:20

Biruk Abebe

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Comments

LPs · Accepted Answer · 2016-05-20 08:21:24Z

0

You are setting a wrong size value

memset(res,0,2*sizeof(res));

Should be

memset(res,0,sizeof(res));

Because of sizeof(res) return the size of array in bytes.

Or

memset(res,0,2*sizeof(Response));

Because of sizeof(Response) return the size of the Response typedef struct in bytes.

answered May 20, 2016 at 8:21

LPs

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