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Malloc a struct

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Join Date: Oct 2008

Posts: 5

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dusse

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Newbie Poster

Malloc a struct

Nov 21st, 2008

Hello guys..
I´m trying to do something like the code below ..

 Help with Code Tags 
 c Syntax (Toggle Plain Text) 
struct node{
       int a;
       char *s;
};        
 
void main()
{
    node *x;
    int sMAX = 3;
    int nodeMAX = 10;
 
    x = (node*)malloc((sizeof(int)+sizeof(char)*sMAX)*nodeMAX);
...
}
struct node{
       int a;
       char *s;
};        

void main()
{
    node *x;
    int sMAX = 3;
    int nodeMAX = 10;
      
    x = (node*)malloc((sizeof(int)+sizeof(char)*sMAX)*nodeMAX);
...
}

in other words I want to alloc dynamically the number of nodes and also the length of s in node struct
I tried a lot of things like:

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 c Syntax (Toggle Plain Text) 
x = (node*)malloc((sizeof(node)*nodeMAX);
x = (node*)malloc((sizeof(struct node)*nodeMAX);
x = (node*)malloc((sizeof(node)+sizeof(char)*sMAX)*nodeMAX);
x = (node*)malloc((sizeof(node)*nodeMAX);
x = (node*)malloc((sizeof(struct node)*nodeMAX);
x = (node*)malloc((sizeof(node)+sizeof(char)*sMAX)*nodeMAX);

and my code do not run
Is it possible to do what I want?
thanks.

Last edited by Narue; Nov 21st, 2008 at 11:20 am. Reason: added code tags

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Narue

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Code Goddess

Re: Malloc a struct

Nov 21st, 2008

>I want to alloc dynamically the number of nodes and also the length of s in node struct
Do it separately:

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 c Syntax (Toggle Plain Text) 
struct node *x = malloc ( nodeMAX * sizeof *x );
int i;
 
for ( i = 0; i < nodeMAX; i++ )
  x->s = malloc ( sMAX );
struct node *x = malloc ( nodeMAX * sizeof *x );
int i;

for ( i = 0; i < nodeMAX; i++ )
  x->s = malloc ( sMAX );

>Is it possible to do what I want?
In C89, not without invoking undefined behavior. In C99 the "struct hack" has been blessed and is directly supported as a flexible array member:

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 c Syntax (Toggle Plain Text) 
struct node{
  int a;
  char s[];
};
 
struct node *x = malloc ( nodeMAX * ( sizeof *x + sMAX ) );
struct node{
  int a;
  char s[];
};

struct node *x = malloc ( nodeMAX * ( sizeof *x + sMAX ) );