memcpy(msg->data+msg->dsize, ..)

**securelinprog** · 03-03-2010

memcpy function is used for copy to dst from src with size:

Code:

memcpy(dst, src, size)

If msg structure was defined as follow:

Code:

struct msg{
       char data[100];
       int dsize;
}

and buf defined as follow:

Code:

struct buf{
       char *data;
       int len;
}

Now if i run below command:

Code:

memcpy(msg->data+msg->dsize, buf->data, buf->len)

Does copy buf->data to msg->data? If yes, what is effect of msg->dsize?

I saw this in a C project.

**sganesh** · 03-03-2010

I tried what you are coming to say.

memcpy will copy the bytes from source to destination.

If buf->data is copied to msg->data then msg->dsize should be zero. otherwise it will point (buf->data+msg->dsize) to some other memory location. So it may give segmentation fault.

Try the following code to understand about memcpy.

Code:

#include<stdio.h>
#include<string.h>
#include<malloc.h>

struct msg{
       char data[100];
       int dsize;
};

struct buf{
       char *data;
       int len;
};

int main()
{
        char str1[]="hello";
        char str2[10];
        struct msg *m1;
        struct buf *b1;
        b1=(struct buf *)malloc(sizeof(struct buf));
        m1=(struct msg *)malloc(sizeof(struct msg));
        b1->data="hello";
        b1->len=strlen(b1->data);
        m1->dsize=0;
        memcpy(str2,str1,10);
        memcpy(m1->data+m1->dsize, b1->data, b1->len);
        printf("str1:%s str2:%s\n",str1,str2);
        printf("str1:%s str2:%s\n",b1->data,m1->data);
        return 0;
}

**thillai_selvan** · 03-03-2010

In this code it means that copying the data in the source to the destination's specific offset.
Usually if you simply used msg->data as a destination in the memcpy function it will be copied to the beginning.
Since array name is the address of the starting address. But we are now incrementing the pointer to some other offset using msg->dsize

**Alexander jack** · 03-03-2010

Since structure members occupies the memory in a sequential order ,
msg->data+msg->dsize this statement defined the location which is a bound of the structure
msg.

so msg->data and msg->dsize has their value unedited.

**securelinprog** · 03-03-2010

Originally Posted by sganesh

I tried...
...

Thank you.

if change size of msg->data from 100 to 2 and then try to copy buffer to msg, msg->dsize will be
invalid. I don't know what msg->dsize will has, I only know it will be invalid.

If change your code to it,

Code:

#include<stdio.h>
#include<string.h>
#include<malloc.h>

struct msg{
       char data[2];
       int dsize;
};

struct buf{
       char *data;
       int len;
};

int main()
{
        char str1[]="asdasd";
        char str2[5];
        struct msg *m1;
        struct buf *b1;
        b1=(struct buf *)malloc(sizeof(struct buf));
        m1=(struct msg *)malloc(sizeof(struct msg));
        b1->data="asdasd";
        b1->len=strlen(b1->data);
        m1->dsize=0;
        memcpy(str2,str1,5);
        memcpy(m1->data+m1->dsize, b1->data, b1->len);
        printf("str1:%s str2:%s\n",str1,str2);
        printf("b1:%s m1:%s\n",b1->data,m1->data);
        printf("m1->dsize:%d\n",m1->dsize);
        return 0;
}

and test it in two way, first with:

Code:

 memcpy(m1->data+m1->dsize, b1->data, b1->len);

second replace it with:

Code:

 memcpy(m1->data/*+m1->dsize*/, b1->data, b1->len);

In both, msg->dsize will be invalid.

My question was that what is effect of adding m1->dsize to first parameter of memcpy?

Isn't better at first check buf->len and msg->dsize then copy it?

Code:

memcpy(msg->data+msg->dsize, buf->data, buf->len)

Why using msg->dsize?

**sganesh** · 03-03-2010

I just wrote a program for copying the strings in the structure,

If you want to copy the whole structure you can use the size of the structure in the memcpy.
It will copy that many number of bytes from one structure to another.

Code:

#include<stdio.h>
#include<string.h>
#include<malloc.h>

struct msg{
       char data[100];
       int dsize;
};

struct buf{
       char data[100];
       int len;
};

int main()
{
        char str1[]="hello";
        char str2[10];
        struct msg *m1;
        struct buf *b1;
        b1=(struct buf *)malloc(sizeof(struct buf));
        m1=(struct msg *)malloc(sizeof(struct msg));
        strcpy(b1->data,"hello");
        b1->len=strlen(b1->data);
//      m1->dsize=0;
        memcpy(str2,str1,10);
        memcpy(m1->data, b1->data, sizeof(struct buf));
        printf("str1:%s str2:%s\n",str1,str2);
        printf("str1:%s size:%d str2:%s size:%d \n",b1->data,b1->len,m1->data,m1->dsize);
        return 0;
}

**securelinprog** · 03-03-2010

Originally Posted by sganesh

I just wrote a ..
...

you don't ask answer of my question. I ask why using msg->dsize in first parameter of memcpy?

your written code only copy all one structure to another one.
however, i thank you for it.

**sganesh** · 03-03-2010

Originally Posted by securelinprog

memcpy function is used for copy to dst from src with size:

Code:

memcpy(dst, src, size)

If msg structure was defined as follow:

Code:

struct msg{
       char data[100];
       int dsize;
}

and buf defined as follow:

Code:

struct buf{
       char *data;
       int len;
}

Now if i run below command:

Code:

memcpy(msg->data+msg->dsize, buf->data, buf->len)

Does copy buf->data to msg->data? If yes, what is effect of msg->dsize?

I saw this in a C project.

Actually, I didn't use the msg->dsize as a first parameter of memcpy() I think you only used in your question.

**securelinprog** · 03-03-2010

Ok, right.

I understand why using msg->dsize in the memcpy function.

By using it, can copy all buf structure to msg->data and msg->dsize simultaneously.

Thanks to all specially sganesh.

**_Mike** · 03-03-2010

Originally Posted by securelinprog

I understand why using msg->dsize in the memcpy function.

By using it, can copy all buf structure to msg->data and msg->dsize simultaneously.

No. msg->dsize is used as an offset from the start of msg->data.
Imagine having the following buffers

Code:

msg->data : { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 } ( 10 zeroes)
buf->data : { 1, 1, 1 } (3 ones)

And then you have this code

Code:

memcpy(msg->data+msg->dsize, buf->data, buf->len)

If dsize is 0 then the result is
msg->data : { 1, 1, 1, 0, 0, 0, 0, 0, 0, 0 }

If dsize is 4 then the result is
msg->data : { 0, 0, 0, 0, 1, 1, 1, 0, 0, 0 }

**securelinprog** · 03-03-2010

Originally Posted by _Mike

No. msg->dsize is used as an offset from the start of msg->data.
Imagine having the following buffers

Code:

msg->data : { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 } ( 10 zeroes)
buf->data : { 1, 1, 1 } (3 ones)

And then you have this code

Code:

memcpy(msg->data+msg->dsize, buf->data, buf->len)

If dsize is 0 then the result is
msg->data : { 1, 1, 1, 0, 0, 0, 0, 0, 0, 0 }

If dsize is 4 then the result is
msg->data : { 0, 0, 0, 0, 1, 1, 1, 0, 0, 0 }

Thank you _Mike.

Which parameter determine dsize was 4 or 0? Does size of msg->data and buf->data is effective?

Thank you very much.

**_Mike** · 03-03-2010

Originally Posted by securelinprog

Which parameter determine dsize was 4 or 0?

dsize is whatever you set it to be in your code.. the 0 and 4 were just random numbers I used as an example.

Does size of msg->data and buf->data is effective?

the size of msg->data has to be at least the size of buf->data + msg->dsize or you would get a buffer overrun.

**securelinprog** · 03-06-2010

Thank you _Mike. I understood well.

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