How to access pointer inside structure in C

It is not a big deal to access pointer inside a structure in c but still, there are a lot of people who make mistakes. In this article, I will write a method to describe the way to how to access a pointer from a structure.

Here I am assuming you have already knowledge of structure and pointers. If you want a quick revision, please read the below articles.

Let’s take an example to understand the way to how to access pointer from a structure in C.

Suppose StudentInfo is a structure, this structure contains all the information of students like their name, age, roll number, address. Our requirement here to create a method to write the information into the structure. Before reading this example if you don’t know the dynamic memory allocation, please read the below articles.

See below example code,

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

//Calculate size of structure
#define SIZE_STRUCTURE(x) sizeof(x)

//Size of the student name
#define NAME_SIZE    40

//Size of the address
#define ADDRESS_SIZE  80


//structure template
typedef struct
{
    int age;
    int roll_number;
    char *name;
    char *address;

} sStudentInfo;


//To clear input buffer
void ClearNewLines(void)
{
    int c;
    do
    {
        c = getchar();
    }
    while (c != '\n' && c != EOF);
}

//Function to write information into the structure
sStudentInfo* writeStudentInfo(void)
{
    sStudentInfo *writeInfo = NULL; //declare pointer to structure

    int studentAge = 0; //declare var to store student age

    int studentRollNumber = 0; //declare var to store student roll number

    char studentName[NAME_SIZE] = { 0 }; //declare array to store student name

    char studentAddress[ADDRESS_SIZE] = { 0 }; //declare array to store student Address


    writeInfo = malloc(SIZE_STRUCTURE(sStudentInfo));//Allocate memory for structure pointer
    if (writeInfo == NULL)
        return NULL;

    printf("Enter the Age: "); //Enter Age of the student
    scanf("%d", &studentAge);

    ClearNewLines(); //Clear new Line

    writeInfo->age = studentAge; // Write age


    printf("\nEnter the roll number: ");//Enter roll number of the student
    scanf("%d", &studentRollNumber);

    ClearNewLines(); //Clear new Line

    writeInfo->roll_number = studentRollNumber;



    printf("\nEnter the name: ");//Enter name of the student
    fgets(studentName, NAME_SIZE, stdin);


    writeInfo->name = malloc(NAME_SIZE);
    if (writeInfo->name == NULL)
        return NULL;

    strncpy(writeInfo->name, studentName, NAME_SIZE);


    printf("\nEnter the address: "); //Enter addressof the student
    fgets(studentAddress, ADDRESS_SIZE, stdin);


    writeInfo->address = malloc(ADDRESS_SIZE);
    if (writeInfo->address == NULL)
        return NULL;

    strncpy(writeInfo->address, studentAddress, ADDRESS_SIZE);


    return writeInfo;
}

//Main function
int main(int argc, char *argv[])
{
    sStudentInfo *pTomInfo = NULL;

    pTomInfo = writeStudentInfo();
    if (pTomInfo == NULL)
    {
        printf("Fail to write student info\n");
        return 0;
    }

    printf("\n\n\n\n\n\n<!****  Read Information of student  ****!>\n\n\n\n\n\n");

    printf("Age: %d\n", pTomInfo->age); //Print Age of the student

    printf("Roll number: %d\n", pTomInfo->roll_number);//Print roll number of the student

    printf("Name: %s\n", pTomInfo->name);//Print name of the student

    printf("Address: %s\n", pTomInfo->address);//Print address of the student

    free(pTomInfo->name); //free allocated memory for name
    pTomInfo->name = NULL; //Avoid to make dangling pointer

    free(pTomInfo->address); //free allocated memory for address
    pTomInfo->address = NULL; //Avoid to make dangling pointer

    free(pTomInfo); //free allocated memory for structure
    pTomInfo = NULL; //Avoid to make dangling pointer

    return 0;
}

Output:

For efficiency, a pointer to the structures is generally passed to functions. The members of structures that are passed within the functions are accessed to perform dereferencing a structure pointer and selecting a member using the dot operator ( . ). It is very difficult to dereference the structure pointer every time.

Therefore, C provides a special pointer operator, (called arrow) to access a member of a structure pointed to by a pointer variable. The operator is a combination of minus symbol, -, followed by a greater-than symbol, >.

Arrow operator is exactly equivalent to a dereference operation followed by the dot ( . ) operator as shown below:

(*pTomInfo).age;

Remember, this is the same as:

pTomInfo->age;

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Some Important Observations of the above code:

1. The below two statements are very necessary, generally, people make mistakes here and forget to allocate separate memory for the pointers within the structure.

Like in above example, I have allocated separate memory for the student name and address.

writeInfo->name = malloc(sizeof(studentName));
if(writeInfo->name == NULL)
return NULL;

writeInfo->address = malloc(sizeof(studentAddress));
if(writeInfo->address == NULL)
return NULL;

2. Generally, people deallocates the memory allocated for structure pointers before deallocating the memory of structure members.

3. Use the fgets in place of the scanf and gets. It prevents the overflow.

How to access pointer inside structure in C

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Some Important Observations of the above code:

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