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Oct 12th, 2009

why use of size 0 or 1 array in structure for URI'S etc

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Why do some code for eg in gstreamer etc use code which has code like

struct xxx
{
int foo;
...
..
char arrURI[1];
char data[0];
}

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ashok.g15

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Oct 12th, 2009

Re: why use of size 0 or 1 array in structure for URI'S etc

Array indexes start at 0.
char arrURI[1]; -- creates an array of size one
-- so there are two indexes 0 and 1
char data[0]; -- creates an initially empty array
Hope that is what you were looking for.

Last edited by Grn Xtrm; Oct 12th, 2009 at 10:56 am.

Grn Xtrm

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Oct 12th, 2009

Re: why use of size 0 or 1 array in structure for URI'S etc

If the array is the last one in the struct, you are probably seeing the struct hack. Prior to C99 it is an unportable trick for allocating arrays in a struct:

C Syntax (Toggle Plain Text)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
typedef struct
{
    char s[1];
} String;
 
int main()
{
    /* allocate 15 extra bytes for the string */
    String *s = malloc(sizeof *s + 15);
 
    strcpy(s->s, "test string");
    puts(s->s);
    free(s);
 
    return 0;
}
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct
{
    char s[1];
} String;

int main()
{
    /* allocate 15 extra bytes for the string */
    String *s = malloc(sizeof *s + 15);

    strcpy(s->s, "test string");
    puts(s->s);
    free(s);

    return 0;
}

The idea is that by tacking on extra memory and accessing it by overflowing the last array member, you can save an extra call to malloc:

C Syntax (Toggle Plain Text)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
typedef struct
{
    char *s;
} String;
 
int main()
{
    String *s = malloc(sizeof *s);
 
    s->s = malloc(15);
    strcpy(s->s, "test string");
    puts(s->s);
    free(s->s);
    free(s);
 
    return 0;
}
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct
{
    char *s;
} String;

int main()
{
    String *s = malloc(sizeof *s);

    s->s = malloc(15);
    strcpy(s->s, "test string");
    puts(s->s);
    free(s->s);
    free(s);

    return 0;
}

C99 makes the struct hack a legal feature of the language, but I do not think it is a good idea even then because it makes the code less clear.

Tom Gunn

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Oct 13th, 2009

Re: why use of size 0 or 1 array in structure for URI'S etc

Click to Expand / Collapse Quote originally posted by Grn Xtrm ...

Array indexes start at 0.
char arrURI[1]; -- creates an array of size one
-- so there are two indexes 0 and 1
char data[0]; -- creates an initially empty array
Hope that is what you were looking for.

Thank you very much for responding .

As I understand.
from the example I have given
char arrURI[1] ;
is same to declaring
char arrURI;
BOTH allocates only one byte.

But in real usage arrURI is assigned some string which is more than one byte when it is declared as char arrURI[1].

In char data[0];
data is a constant pointer which cannot be assigned another address and data has no size . So how is it equivalent as creating an empty array.

Hope my question is clear.

ashok.g15

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Oct 13th, 2009

Re: why use of size 0 or 1 array in structure for URI'S etc

Quote ...

char arrURI[1]; -- creates an array of size one
-- so there are two indexes 0 and 1

Actually there is only 1 index, which is 0 in char A[1];
A[0] = 'a'; //ok
A[1] = 'b'; //bug

firstPerson

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Oct 13th, 2009

Re: why use of size 0 or 1 array in structure for URI'S etc

In the case of zero sized array, it was generally used as an easy way to index into a variable sized buffer ...

C Syntax (Toggle Plain Text)
struct Header {
    DWORD dwMagic;
    DWORD dwSize;
    BYTE data[];
};
 
void MyFunction()
{
    Header *pHdr = (Header *)buffer;
 
    Read(buffer, MAX_SIZE);
 
    switch(pHdr->dwMagic)
   {
        case M_TAG('R','I','F','F') :
            HandleRIFF(pHdr->data,pHdr->dwSize);
            break;
        // ... etc
   }
 
}
struct Header {
    DWORD dwMagic;
    DWORD dwSize;
    BYTE data[];
};

void MyFunction()
{
    Header *pHdr = (Header *)buffer;
 
    Read(buffer, MAX_SIZE);

    switch(pHdr->dwMagic)
   {
        case M_TAG('R','I','F','F') :
            HandleRIFF(pHdr->data,pHdr->dwSize);
            break;
        // ... etc
   }

}

It lets the compiler calculate the offset so you don't have to.

Last edited by SVR; Oct 13th, 2009 at 12:49 pm.

SVR

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Oct 13th, 2009

Re: why use of size 0 or 1 array in structure for URI'S etc

Click to Expand / Collapse Quote originally posted by firstPerson ...

Actually there is only 1 index, which is 0 in char A[1];
A[0] = 'a'; //ok
A[1] = 'b'; //bug

Wow, yes you are right. I mistyped there. Thanks for the correction.
With only one element there is only one index, which is 0.

Grn Xtrm

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Oct 14th, 2009

Re: why use of size 0 or 1 array in structure for URI'S etc

Why can't we just use a pointer. It will do the same
eg : BYTE *byte;

ashok.g15

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Oct 14th, 2009

Re: why use of size 0 or 1 array in structure for URI'S etc

Quote ...

Why can't we just use a pointer.

In C99 because a pointer might not work while an empty array will. Prior to C99, I cannot think of any reason why an array is better than a pointer except for keeping the array size out of the struct size when using sizeof . I think the original struct hack uses an array size of 0, and it evolved to use 1 for compilers that do not allow an array size of 0.

Tom Gunn

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Oct 14th, 2009

Re: why use of size 0 or 1 array in structure for URI'S etc

Click to Expand / Collapse Quote originally posted by ashok.g15 ...

Why can't we just use a pointer. It will do the same
eg : BYTE *byte;

Let's try it with my previous example.

C Syntax (Toggle Plain Text)
struct Header {
    DWORD dwMagic;
    DWORD dwSize;
    BYTE *data;
};
 
void MyFunction()
{
    Header *pHdr = (Header *)buffer;
 
    Read(buffer, MAX_SIZE);
 
    switch(pHdr->dwMagic)
   {
        case M_TAG('R','I','F','F') :
            HandleRIFF(pHdr->data,pHdr->dwSize);
            break;
        // ... etc
   }
}
struct Header {
    DWORD dwMagic;
    DWORD dwSize;
    BYTE *data;
};

void MyFunction()
{
    Header *pHdr = (Header *)buffer;
 
    Read(buffer, MAX_SIZE);

    switch(pHdr->dwMagic)
   {
        case M_TAG('R','I','F','F') :
            HandleRIFF(pHdr->data,pHdr->dwSize);
            break;
        // ... etc
   }
}

Now this will crash. Why ?

SVR

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