Structure tag

What is a tag?

A tag is the optional name written after the keyword struct in a structure definition. It names the structure type, so that the type can be used again later.

struct student {        /* "student" is the tag */
    char name[20];
    int roll;
};

Must a tag be included in the structure definition?

No. A structure can be defined without a tag, but then variables must be declared at the same time (in the definition), and the type cannot be used again later because it has no name.

struct { int x, y; } p;     /* no tag, variable p declared here */

Must a tag be included in a variable declaration?

Yes, when the variable is declared separately from the definition, the tag is needed along with struct, as in struct student s1;. If the variable is declared together with the definition (struct {...} p;) no tag is needed. With typedef, the new type name can replace struct tag.

#include <stdio.h>

struct book {                /* "book" is the tag */
    char title[30];
    float price;
} b1;                        /* variable declared with the definition */

struct book b2;              /* variable declared later using the tag */

struct {                     /* no tag: variable must be declared here */
    int x, y;
} p;

int main(void)
{
    b1.price = 250;
    b2.price = 300;
    p.x = 1;
    printf("%.0f %.0f %d\n", b1.price, b2.price, p.x);
    return 0;
}

Output: 250 300 1