Behavior of array

Question

I have a code as follows:

typedef struct Details {
    char a[32];
    char b[32];
    char c[32];
} Details_t;

char *xyz(Details_t *pdetails) {
    if ((NULL == pdetails->a) && (NULL == pdetails->b)) {
        return NULL;
    }
    int len = 0;
    char *newString = NULL;
    len = strlen(a) + strlen(b);
    newString = (char *)calloc(1, len + 3);
    strcpy(newString, a);
    strcat(newString, ";");
    strcat(newString, b);
    strcat(newString, ";");

    return newString;
}

Now I am passing the address of this structure from main().

main() {
    char *ret = NULL;
    Details_t var;
    memset((void *)&var, '\0', sizeof(Details_t));
    strcpy(var.b, "EXAMPLE");
    ret = xyz(&var);
    printf("OUTPUT==%s\n", ret);
}

My problem is: I am not copying any value in member a and I Have memset() structure details with NULL so all the members which are not copied should be NULL. But in xyz function,the condition below gets failed.

if ((NULL == pdetails->a) && (NULL == pdetails->b))

and output which I get is below:

OUTPUT==;EXAMPLE;

Why this condition gets failed?

I hope a,b and c are to show the general case and not what's in you code. Also, main() should be, at least, some variation of returnType main(){...} if not main(void) generally int main(void){...} 'cause why not? — ChiefTwoPencils
– ChiefTwoPencils, Commented Jul 2, 2013 at 10:40

quetzalcoatl · Accepted Answer · 2013-07-02 11:40:29Z

1

when this Details_t var; is executed, var.a and var.b own address. So var.a == NULL will return false.

edited Jul 2, 2013 at 11:40

quetzalcoatl

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answered Jul 2, 2013 at 11:06

Scy

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1 Comment

quetzalcoatl Over a year ago

followup: the point is that the Details_t::a are ARRAYS not POINTERS. There's a real difference. Since the array is embedded inside the structure, it has an address almost always, and actually the IF condition is always true in your case. The memset will indeed make the whole memory zeroed out, but there are no pointers in the structure. After memset you will effectively have a structure with three arrays inside and the contents of those arrays will consist of 32 zeroed characters. You may exploit that! Hence, instead of pdetails->a != NULL just check pdetails->a[0] == NULL.

David Ranieri · Accepted Answer · 2013-07-02 12:00:13Z

all the members which are not copied should be NULL

Thats not true, test this piece of code:

#include <stdio.h>
#include <string.h>

typedef struct Details{
    char a[32];
    char b[32];
    char c[32];
} Details_t;

int main(void)
{
    Details_t var, *pt;

    memset((void *)&var, '\0', sizeof(Details_t));
    printf("%s\n", var.a == NULL ? "null" : "not null"); /* output = not null */
    /* Also for a pointer */
    pt = &var;
    printf("%s\n", pt->a == NULL ? "null" : "not null"); /* output = not null */
    /* Maybe you want: */
    printf("%s\n", pt->a[0] == '\0' ? "empty" : "not empty"); /* output = empty */
    return 0;
}

1 Comment

jinal shah Over a year ago

that was my writing mistake

Ayse · Accepted Answer · 2013-07-02 11:05:00Z

Check out if this can help you :)

#include "stdafx.h"
#include<stdio.h>
typedef struct Details{
char a[32];
char b[32];
char c[32];
}Details_t;

char *xyz(Details_t *pdetails)
{
 if((strlen(pdetails->a)==0) && strlen(pdetails->a)==0)
 {
  return NULL;
 }
 int len = 0;
 char *newString = NULL;
  len = strlen(pdetails->a) + strlen(pdetails->a);
 newString  = (char *)calloc(1,len +3);
 strcpy(newString,pdetails->a);
 strcat(newString,";");
 strcat(newString,pdetails->a);
 strcat(newString,";");

 return newString;
}
//Now I am passing the address of this structure from main().

void main()
{
 char *ret = NULL;
 Details_t var;
 memset((void *)&var,'\0',sizeof(Details_t));
 strcpy(var.b,"EXAMPLE");
 ret = xyz(&var);
 printf("OUTPUT==%s\n",ret);

}

kotlomoy · Accepted Answer · 2013-07-02 12:06:38Z

0

Why this condition gets failed ?

Because a and b are not pointers. They are arrays. You can not assign NULL to them. You can not assign anything to array at all.

This condition should work for you:

if( 0 == pdetails->a[0] + pdetails->b[0] )

edited Jul 2, 2013 at 12:06

answered Jul 2, 2013 at 11:58

kotlomoy

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