CODEWITHSUNDEEP
ccasting
mannan
mannan

Reputation: 11

Typecasting Char to a structure type

I was working on a prob and as a part of it came across problem of the follwing type...

For the foll code 

#include<stdio.h>
#include<stdlib.h>
typedef struct sample
{
  int a;
  char b;
}sample;

int main()
{
  char *buf;

  buf = (char *)malloc(sizeof(char)*sizeof(sample));

  buf = ((sample *) buf);
  buf->a = 10;
  buf->b = 'm';
  printf("%d\n",(buf)->a);
  printf("%c\n",(buf)->b);

  return 0;
}

Compile time error was:

example.c:16: error: request for member a' in something not a structure or union example.c:17: error: request for memberb' in something not a structure or union

Is this because I cannot typecast a lower data type to higher type..??

I tried using Calloc in place of malloc so that a contiguos block can be allocated but to no avail..

Help me find the logical flaw in this conversion...

Thanks in advance

Upvotes: 1

Views: 2244

Answers (4)

freespace
freespace

Reputation: 16701

buf's type is still char *. Casting does not change the type of the receiving variable. The casts you are doing here is essentially pointless - the value of a pointer is not influenced by what it points to. Memory is memory.

To fix this, change the type of buf to sample *.

To fix it even further, remove the cast from malloc and the "recasting" of buf after it.

Upvotes: 0

Daniel Gehriger
Daniel Gehriger

Reputation: 7468

The line buf = ((sample *) buf); doesn't do anything. The variable buf still is of type char*.

sample* sample = (sample*)malloc(sizeof(sample));
sample->a = 10;
sample->b = 'm';
printf("%d\n",(sample)->a);
printf("%c\n",(sample)->b);

Is more like what you want.

Upvotes: 2

stacker
stacker

Reputation: 68962

Yopu should change

char *buf;    
buf = (char *)malloc(sizeof(char)*sizeof(sample));

to 

sample *buf;    
buf = (sample *)malloc(sizeof(sample));

because declaring buf as char * prevents the compiler from understand your intention (to point to a sample structure).

Upvotes: 4

wich
wich

Reputation: 17127

the type of buf does not change with the cast, you need to add an extra variable:

char* buf;
sample* s;

buf = malloc(sizeof(sample));
sample = (sample*)buf;

the sizeof(char) is completely superfluous by the way, it is 1 by definition.

Upvotes: 0

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