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Passing Structures to Functions

You can pass a structure to a function the same way you pass any variable. There are three approaches:
  1. Pass by Value
  2. Pass by Reference (using pointer)
  3. Return a structure from a function

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1. Pass by Value

A copy of the structure is sent to the function. Changes made inside the function do not affect the original. 
#include <stdio.h>

struct Student {
    char name[20];
    int roll;
    float marks;
};

void display(struct Student s) {
    s.marks = 99.9f;   // modifies only the local copy
    printf("Inside : %s %d %.2f\n", s.name, s.roll, s.marks);
}

int main() {
    struct Student s1 = {"Vishal", 101, 75.5};
    display(s1);
    printf("Outside: %.2f\n", s1.marks);   // original unchanged
    return 0;
}
Output: 
Inside : Vishal 101 99.90
Outside: 75.50

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2. Pass by Reference (Using Pointer)

The address of the structure is sent. Changes made inside the function do affect the original. Members are accessed using the arrow operator (->). 
#include <stdio.h>

struct Student {
    char name[20];
    int roll;
    float marks;
};

void update(struct Student *s) {
    s->marks = 99.9f;   // modifies the original
}

int main() {
    struct Student s1 = {"Vishal", 101, 75.5};
    update(&s1);   // pass address
    printf("After update: %.2f\n", s1.marks);
    return 0;
}
Output: 
After update: 99.90

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3. Return a Structure from a Function

#include <stdio.h>

struct Student {
    char name[20];
    int roll;
    float marks;
};

struct Student getData() {
    struct Student s = {"Kumar", 102, 80.5};
    return s;
}

int main() {
    struct Student s1 = getData();
    printf("%s - %d - %.2f\n", s1.name, s1.roll, s1.marks);
    return 0;
}
// Output: Kumar - 102 - 80.50

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Comparison

MethodCopies DataModifies OriginalSpeed
Pass by ValueYesNoSlower
Pass by ReferenceNoYesFaster
Return structYes—Moderate
Best practice: For large structures, always prefer passing by reference using a pointer. Use const struct Type *ptr if you want to read without modifying.