C Library - fread() function

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The C library size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream) function reads data from the given stream into the array pointed to, by ptr.It is commonly used for reading binary files but can be used for text files as well.

Syntax

Following is the C library syntax of the fread() function −

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

Parameters

This function accepts three parameters −

• ptr: A pointer to a block of memory where the read data will be stored.
• size: The size, in bytes, of each element to be read.
• nmemb: The number of elements, each of size bytes, to be read.
• stream: A pointer to the FILE object that specifies an input stream.

Return Value

The function returns the number of elements successfully read, which may be less than nmemb if a read error or end-of-file (EOF) occurs. If size or nmemb is zero, fread returns zero and the contents of the memory pointed to by ptr are unchanged.

Example 1: Reading an Array of Integers from a Binary File

The program reads an array of 5 integers from a binary file and prints each integer.

Below is the illustration of C library fread() function.

#include <stdio.h>

int main() {
   FILE *file;
   int numbers[5];
   
   file = fopen("numbers.bin", "rb");
   if (file == NULL) {
       perror("Error opening file");
       return 1;
   }

   size_t result = fread(numbers, sizeof(int), 5, file);
   if (result != 5) {
       perror("Error reading file");
   } else {
       for (int i = 0; i < 5; i++) {
           printf("Number %d: %d\n", i + 1, numbers[i]);
       }
   }
   
   fclose(file);
   return 0;
}

Output

The above code produces following result−

Number 1: 1
Number 2: 2
Number 3: 3
Number 4: 4
Number 5: 5

Example 2: Reading a Structure from a Binary File

The program reads a structure Employee from a binary file and prints its members.

#include <stdio.h>

typedef struct {
   int id;
   char name[20];
   float salary;
} Employee;

int main() {
   FILE *file;
   Employee emp;
   
   file = fopen("employee.bin", "rb");
   if (file == NULL) {
       perror("Error opening file");
       return 1;
   }

   size_t result = fread(&emp, sizeof(Employee), 1, file);
   if (result != 1) {
       perror("Error reading file");
   } else {
       printf("Employee ID: %d\n", emp.id);
       printf("Employee Name: %s\n", emp.name);
       printf("Employee Salary: %.2f\n", emp.salary);
   }
   
   fclose(file);
   return 0;
}

Output

After execution of above code, we get the following result

Employee ID: 101
Employee Name: John Doe
Employee Salary: 55000.00