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Struct with variable length arrays of struct

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Struct with variable length arrays of struct

Jul 17th, 2007

I need to create a data structure of data structures, but I can know "how many structs I need" only at runtime.

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 c Syntax (Toggle Plain Text) 
typedef struct tagPredefinedStruct{
   int   iSize;
   int   iCount;
} PREDEFINEDSTRUCT;
 
int x;
x = GetNumberofStructs();  //x is known only at runtime.
 
typedef struct APIStruct {
   int                iNumber;
   PREDEFINEDSTRUCT   PreDefStruct[x];
   int                iDate;
} APISTRUCT;
typedef struct tagPredefinedStruct{
   int   iSize;
   int   iCount;
} PREDEFINEDSTRUCT;

int x;
x = GetNumberofStructs();  //x is known only at runtime.

typedef struct APIStruct {
   int                iNumber;
   PREDEFINEDSTRUCT   PreDefStruct[x];
   int                iDate;
} APISTRUCT;

Currently, the compiler gives an error C2057: expected constant expression for PreDefStruct[x].

I have to pass the address of this APISTRUCT structure to an API so I need all the memory allocated together in a chunk and can't use linked lists.

Thanks a lot for reading my post. I graciously await your response.

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Hamrick

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Re: Struct with variable length arrays of struct

Jul 17th, 2007

You can't set the size of an array at run time. You have to dynamically create the array from a pointer.

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
typedef struct tagPredefinedStruct{
   int   iSize;
   int   iCount;
} PREDEFINEDSTRUCT;
 
typedef struct APIStruct {
   int                iNumber;
   PREDEFINEDSTRUCT   *PreDefStruct;
   int                iDate;
} APISTRUCT;
 
APISTRUCT api;
int x;
 
x = GetNumberofStructs();  //x is known only at runtime.
api.PreDefStruct = malloc( x * sizeof( PREDEFINEDSTRUCT ) );
typedef struct tagPredefinedStruct{
   int   iSize;
   int   iCount;
} PREDEFINEDSTRUCT;

typedef struct APIStruct {
   int                iNumber;
   PREDEFINEDSTRUCT   *PreDefStruct;
   int                iDate;
} APISTRUCT;

APISTRUCT api;
int x;

x = GetNumberofStructs();  //x is known only at runtime.
api.PreDefStruct = malloc( x * sizeof( PREDEFINEDSTRUCT ) );

The truth does not change according to our ability to stomach it.

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Re: Struct with variable length arrays of struct

Jul 17th, 2007

Thanks for the help. It compiles fine now. However, it crashes when I try to assign a value to the data structure.

 Help with Code Tags 
 c Syntax (Toggle Plain Text) 
.
.
.
x = GetNumberofStructs();  //x = 3 in this case
api.PreDefStruct = malloc( x * sizeof( PREDEFINEDSTRUCT ) );
api.PreDefStruct[0].iSize = 4; //here it crashes
.
.
.
x = GetNumberofStructs();  //x = 3 in this case
api.PreDefStruct = malloc( x * sizeof( PREDEFINEDSTRUCT ) );
api.PreDefStruct[0].iSize = 4; //here it crashes

Please help. Thanks.
My friend said another way is to give it an initial size of the array then change that size later. I don't know how to implement it though.

$Ali$

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Hamrick

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Re: Struct with variable length arrays of struct

Jul 17th, 2007

•

However, it crashes when I try to assign a value to the data structure.

Check the pointer to see if it's 0. If it is then malloc failed to allocate the memory.

•

My friend said another way is to give it an initial size of the array then change that size later.

You can't. Array sizes have to be constant and they can't be changed. A variable sized array is what I showed you.

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Re: Struct with variable length arrays of struct

Jul 17th, 2007

The pointer is not zero. It's assigned a memory location of 0x6e615c31.

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Hamrick

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Re: Struct with variable length arrays of struct

Jul 17th, 2007

Does this work for you?

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
#include <stdio.h>
 
typedef struct tagPredefinedStruct{
  int   iSize;
  int   iCount;
} PREDEFINEDSTRUCT;
 
typedef struct APIStruct {
  int                iNumber;
  PREDEFINEDSTRUCT   *PreDefStruct;
  int                iDate;
} APISTRUCT;
 
int main( void ) {
  APISTRUCT api;
 
  api.PreDefStruct = malloc( 3 * sizeof( PREDEFINEDSTRUCT ) );
 
  api.PreDefStruct[0].iSize = 0;
  api.PreDefStruct[1].iSize = 1;
  api.PreDefStruct[2].iSize = 2;
 
  printf( "%d\n", api.PreDefStruct[0].iSize );
  printf( "%d\n", api.PreDefStruct[1].iSize );
  printf( "%d\n", api.PreDefStruct[2].iSize );
 
  free( api.PreDefStruct );
 
  return 0;
}
#include <stdio.h>

typedef struct tagPredefinedStruct{
  int   iSize;
  int   iCount;
} PREDEFINEDSTRUCT;

typedef struct APIStruct {
  int                iNumber;
  PREDEFINEDSTRUCT   *PreDefStruct;
  int                iDate;
} APISTRUCT;

int main( void ) {
  APISTRUCT api;

  api.PreDefStruct = malloc( 3 * sizeof( PREDEFINEDSTRUCT ) );

  api.PreDefStruct[0].iSize = 0;
  api.PreDefStruct[1].iSize = 1;
  api.PreDefStruct[2].iSize = 2;

  printf( "%d\n", api.PreDefStruct[0].iSize );
  printf( "%d\n", api.PreDefStruct[1].iSize );
  printf( "%d\n", api.PreDefStruct[2].iSize );

  free( api.PreDefStruct );

  return 0;
}

The truth does not change according to our ability to stomach it.

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Re: Struct with variable length arrays of struct

Jul 17th, 2007

No, it's not working. It crashes on line 19 of your proposed code: api.PreDefStruct[0].iSize = 0;. Thanks for continuing to help. And so promptly.

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Re: Struct with variable length arrays of struct

Jul 17th, 2007

> It's assigned a memory location of 0x6e615c31
Which looks a lot like a string fragment of "na\1"

The \ is telling, does it look like part of a filename perhaps?

I'm betting that you're copying a string into some other allocated memory, and this is where you observe the trashing of memory as a crash.

If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
--
If your code lacks code tags, you will be IGNORED

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Re: Struct with variable length arrays of struct

Jul 18th, 2007

Thanks for all your help, but I don't think my problem can be solved using a struct in this manner.

What I need is all of the data items of the struct allocated together in one chunk. What I'm seeing from my debugger is that the 1st data item is allocated at: 0x0426c814, the 2nd at: 0x6674725c & so on...

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Re: Struct with variable length arrays of struct

#10

Jul 18th, 2007

I read about a trick that makes everything together. Move the PreDefStruct member to be last, make it an array of 1, and then make the APISTRUCT variable dynamic. If you allocate the size of APISTRUCT plus the size of x PREDEFINEDSTRUCTs I think you get what you need.

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
#include <stdio.h>
 
typedef struct tagPredefinedStruct{
  int   iSize;
  int   iCount;
} PREDEFINEDSTRUCT;
 
typedef struct APIStruct {
  int                iNumber;
  int                iDate;
  PREDEFINEDSTRUCT   PreDefStruct[1];
} APISTRUCT;
 
int main( void ) {
  APISTRUCT *api = malloc( sizeof( APISTRUCT ) + 3 * sizeof( PREDEFINEDSTRUCT ) );
 
  api->PreDefStruct[0].iSize = 0;
  api->PreDefStruct[1].iSize = 1;
  api->PreDefStruct[2].iSize = 2;
 
  printf( "%d\n", api->PreDefStruct[0].iSize );
  printf( "%d\n", api->PreDefStruct[1].iSize );
  printf( "%d\n", api->PreDefStruct[2].iSize );
 
  free( api );
 
  return 0;
}
#include <stdio.h>

typedef struct tagPredefinedStruct{
  int   iSize;
  int   iCount;
} PREDEFINEDSTRUCT;

typedef struct APIStruct {
  int                iNumber;
  int                iDate;
  PREDEFINEDSTRUCT   PreDefStruct[1];
} APISTRUCT;

int main( void ) {
  APISTRUCT *api = malloc( sizeof( APISTRUCT ) + 3 * sizeof( PREDEFINEDSTRUCT ) );

  api->PreDefStruct[0].iSize = 0;
  api->PreDefStruct[1].iSize = 1;
  api->PreDefStruct[2].iSize = 2;

  printf( "%d\n", api->PreDefStruct[0].iSize );
  printf( "%d\n", api->PreDefStruct[1].iSize );
  printf( "%d\n", api->PreDefStruct[2].iSize );

  free( api );

  return 0;
}