structure initialization

Question

The output of the code has to be zero, but I am having an error, please guide. I can initialize the structure with its type variable but how can I do with the pointer to initialize the whole structure?

#include<stdio.h>
typedef struct student
{
 int roll_id[10];
 int name_id[10];
 int postn;
} student;
int main()
{
 student p; /// here i can do student p={ {0} };
 student *pptr=&p;
 pptr= { {0} }; /// but can i initialize in this way ?? //error: expected 
 expression before ‘{’ token
 pptr->roll_id[9]=1;
 printf (" %d\n", pptr->postn); 
 return 0;
}

• Qaz: You're assigning { {0} } to a pointer.

Answer 1

As stated by chris on a comment, you are assigning "something not an integer" to a pointer. You should assign { {0} } instead to the content of that pointer (*pointer):

*pptr= { {0} }; //bad!

Anyway this is not the correct way to populate that structure and you should use:

*pptr= (student){ {0}, {0}, 0 };

as an example

edited to solve the error pointed by unwind

• Manuel Miranda: I think it's valif IF he can do it with the variable. "student p; /// here i can do student p={ {0} };", just wanted to explain the mechanics.

Answer 2

If all you want is a 0-initialized structure, you might as well do:

struct student *pptr = calloc(1, sizeof(struct student));

alternatively, following the code you posted:

struct student p;
struct student *pptr = &p;
memset(pptr, 0, sizeof(struct student));

of course, these approaches will only work if you want a 0-initialized struct. for anything more facy you'll have to write it out, or use some memcpy magic.

• sr01853: Is it correct to use memset without preallocating (malloc) the actual structure? , for the second case ofcourse in your solution.
• Andreas Grapentin: @Sibi you don't have to use malloc in the second example, because pptr points to the memory location of p, which resides in full size on the stack.
• sr01853: You mean in the stack frame of main() ?
• Andreas Grapentin: @Sibi I'd guess so, since it's a local variable. do you disagree?
• sr01853: Got you. . And the sizeof(struct student) in memset cleared the doubt I had. Thx a lot.

Answer 3

If you're using C99 or later, you can use compound literals. Otherwise you cannot, and must initialize each field separately.

With literals:

*pptr = (student) { 0, 0, 0 };

without:

pptr->roll_id[0] = 0;
pptr->name_id[0] = 0;
pptr->postn = 0;

UPDATE Edited, for some reason I thought that the roll_id and name_id fields were strings, but they're not.

It might be easier to just use memset() for this, to initialize all the values to zero:

memset(pptr, 0, sizeof *pptr);

Notice, as always, that this is a fantastic opportunity to use sizeof with the pointer, since reduces the typing and is easier to read and verify (for reasonable variable names such as we have here).

• Mike Kwan: I believe you're trying to assign a string literal as an element of an int array.
• Swetha.P: @unwind: pptr->roll_id[0-9]=0; // can i do this without loop
• acraig5075: This is wrong! There are no character strings in the question, and braced-enclosed initializer lists are not for assignment.
• unwind: @MikeKwan Thanks, must have mis-read the OP's code in some way.
• unwind: @acraig5075 Fixed the string mis-match, must have been confused yesterday. But yes, compound literals ("brace-enclosed initializer lists") are fine from C99, as I say.

Answer 4

use as below

int main()
{
 student p[] = { {0, 0, 0, 0, 0, 0, 0, 0, 0, 0}};
 student *pptr = p;
 pptr->roll_id[9] = 1;
 printf (" %d\n", pptr->postn); 
 return 0;
}

if you want to initialize entire array that is a member of structure , you have to do as like above.