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Full Discussion: C Programming.. Typecasting of structure pointers..
Top Forums Programming C Programming.. Typecasting of structure pointers.. Post 302694333 by Corona688 on Thursday 30th of August 2012 11:39:35 AM
Old 08-30-2012
Yes, what you usually do is something like this:

Code:
#include <stdio.h>

typedef struct type_float {
        int type;
        float payload;
} type_float;

typedef struct type_string {
        int type;
        char payload[64];
} type_string;

typedef union alltypes {
        /* An enum is like an integer, plus a bunch of #define's.
            TYPE_FLOAT becomes 0, TYPE_STRING becomes 1, etc, etc. */
        enum { TYPE_FLOAT, TYPE_STRING } type;
        type_float t_float;
        type_string t_string;
} alltypes;

int dosomething(alltypes *a)
{
        switch(a->type)
        {
        case TYPE_FLOAT:
                printf("TYPE_FLOAT:  %f\n", a->t_float.payload);
                break;
        case TYPE_STRING:
                printf("TYPE_STRING:  %s\n", a->t_string.payload);
                break;
        default:
                printf("Wrong type '%d'\n", a->type);
                return(-1);
                break;
        }

        return(0);
}

int main(int argc, char *argv[])
{
        type_float tf={TYPE_FLOAT, 3.14159 };
        type_string ts={TYPE_STRING, "HEY GUYS"};
        dosomething((alltypes *)&tf);
        dosomething((alltypes *)&ts);
        return(0);
}

Note that a union actually stores all elements in the same place, so its members t_float, t_string and type actually start at the same address. They all share their type variable in common. (Wherever the held-in-common variable is used, I highlight in red.) So you can pass different types of structures, and have the first value define which type it gets used as, without lots of different kinds of messy typecasts.
This User Gave Thanks to Corona688 For This Post:
Priya Amaresh
 

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LEARN ABOUT SUSE

msgb

msgb(9S)						    Data Structures for Drivers 						  msgb(9S)

NAME

       msgb, mblk - STREAMS message block structure

SYNOPSIS

       #include <sys/stream.h>

INTERFACE LEVEL

       Architecture independent level 1 (DDI/DKI)

DESCRIPTION

       A  STREAMS  message  is made up of one or more message blocks, referenced by a pointer to a msgb structure. The b_next and  b_prev pointers
       are used to link messages together on a QUEUE. The  b_cont pointer links message blocks together when a message consists of more  than  one
       block.

       Each   msgb  structure also includes a pointer to a  datab(9S) structure, the data block (which contains pointers to the actual data of the
       message), and the type of the message.

STRUCTURE MEMBERS

       struct msgb     *b_next; 	  /* next message on queue */
       struct msgb     *b_prev; 	  /* previous message on queue */
       struct msgb     *b_cont; 	  /* next message block */
       unsigned char   *b_rptr; 	  /* 1st unread data byte of buffer */
       unsigned char   *b_wptr; 	  /* 1st unwritten data byte of buffer */
       struct datab    *b_datap;	  /* pointer to data block */
       unsigned char   b_band;		  /* message priority  */
       unsigned short  b_flag;		  /* used by stream head  */

       Valid flags are as follows:

       MSGMARK	       Last byte of message is marked.

       MSGDELIM        Message is delimited.

       The msgb structure is defined as type mblk_t.

SEE ALSO

       datab(9S)

       Writing Device Drivers

       STREAMS Programming Guide

SunOS 5.10							    11 Apr 1991 							  msgb(9S)
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