union serve esempio più semplice

[RedsAnDev]

per spiegare la union

[RedsAnDev]

A simple example about union and blob.

#include <stdio.h>
#include <string.h>
#define WORD_SIZE 256

struct template{
      char send_id[6];
      char dest_id[6];
      char type[3];
      char mess[240];
};

union payload{
    struct template dataset;
    char blob[WORD_SIZE];
};

void printField(char[],int);

int main(){
 union payload tmp;
 strcpy(tmp.dataset.send_id,"123ABCD");
 strcpy(tmp.dataset.dest_id,"456DEF");
 strcpy(tmp.dataset.type,"200");
 strcpy(tmp.dataset.mess,"Messaggio ricevuto");
 printf("\nSTART\n");
 printf("SENDER\t\t %s\nRECEIVER\t %s\nTYPE OF MESSAGE\t %s \nMESSAGE\t\t %s \nBLOB\t\t %s\n",tmp.dataset.send_id,tmp.dataset.dest_id,tmp.dataset.type,tmp.dataset.mess,tmp.blob);
 printf("\nRemember EOF and %%s\n");
 printf("\nSENDER\t\t");
 printField(tmp.dataset.send_id,6);
 printf("\nRECEIVER\t");
 printField(tmp.dataset.dest_id,6);
 printf("\nTYPE OF MESSAGE\t");
 printField(tmp.dataset.type,3);
 printf("\nMESSAGE\t\t");
 printField(tmp.dataset.mess,240);

printf("\nEND\n");
return 0;
}

void printField(char elem[],int size){
 char tmp[size];
 for(int i=0;i<size;i++)
    tmp[i]=elem[i];
 printf("%s",tmp);
}

Output

START SENDER 123ABC456DEF200Messaggio ricevuto RECEIVER 456DEF200Messaggio ricevuto TYPE OF MESSAGE 200Messaggio ricevuto MESSAGE Messaggio ricevuto BLOB 123ABC456DEF200Messaggio ricevuto

Remember EOF and %s

SENDER 123ABC RECEIVER 456DEF TYPE OF MESSAGE 200DEF MESSAGE Messaggio ricevuto END

Use union as a quickly way to set up structure variable (some net-tool works this way to manipulate package, e.g scapy).

[Koldar]

The example you've put is really tricky (especially since valgrind might give you an invalid read).

Use union as a quickly way to set up structure variable Why? Can't you use struct?

Why use a struct within a union? As for a simple example for union I mean something VERY easy, like:

typedef union values {
    int int_val;
    float float_val;
    char char_val;
} values;

int main() {
    values union_variable;
    union_variable.int_val = 512;
    printf("the size of the union is the size of the field having the longest size required: %ld\n", sizeof(union_variable));
    printf("the memory for each field is the same. int value is %d but char value is \"%c\" since the first byte of the int value is 0 for amost sure.\n");
}