C stdio.h fread() Function

fread() Function

The fread() function in C reads a block of data from a given stream into a specified memory block. It is commonly used for reading binary data from files, allowing you to specify both the size of each element and the number of elements to read. The function advances the stream’s position indicator by the total number of bytes read.

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Syntax of fread()

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

Parameters

Parameter	Description
ptr	Pointer to a memory block with a size of at least (size * count) bytes where the read data will be stored.
size	The size, in bytes, of each element to be read.
count	The number of elements, each of size size, to be read.
stream	Pointer to a FILE object that specifies the input stream to read from.

It is important to note that if either size or count is zero, fread() returns zero without modifying the stream or the content of the memory pointed to by ptr.

Return Value

The function returns the total number of elements successfully read. If this number differs from the count parameter, it indicates that either a reading error occurred or the end-of-file was reached. The indicators for these situations can be checked using ferror() and feof(), respectively.

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Examples for fread()

Example 1: Reading a Binary File into a Buffer

This example demonstrates how to open a binary file and use fread() to read its contents into a buffer.

Program

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

int main() {
    FILE *fp = fopen("data.bin", "rb");
    if (fp == NULL) {
        perror("Error opening file");
        return EXIT_FAILURE;
    }

    // Allocate buffer for 100 integers
    int buffer[100];
    size_t elementsRead = fread(buffer, sizeof(int), 100, fp);

    printf("Number of elements read: %zu\n", elementsRead);

    fclose(fp);
    return 0;
}

Explanation:

1. The file data.bin is opened in binary read mode.
2. A buffer for 100 integers is allocated.
3. fread() reads up to 100 integers from the file into the buffer.
4. The program prints the number of elements successfully read.
5. The file is then closed.

Program Output:

Number of elements read: 100

Example 2: Reading a Text File into a Character Array

This example shows how to read contents from a text file into a character array using fread().

Program

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

int main() {
    FILE *fp = fopen("text.txt", "r");
    if (fp == NULL) {
        perror("Error opening file");
        return EXIT_FAILURE;
    }

    char buffer[256];
    size_t elementsRead = fread(buffer, sizeof(char), 255, fp);
    buffer[elementsRead] = '\0';  // Null terminate the string

    printf("File content:\n%s\n", buffer);

    fclose(fp);
    return 0;
}

Explanation:

1. The file text.txt is opened in read mode.
2. A character array buffer is allocated to hold up to 255 characters.
3. fread() reads characters from the file into the buffer.
4. A null terminator is added to the end of the read content to form a proper C string.
5. The file content is printed to the console.
6. The file is closed after reading.

Program Output:

File content:
[Content of text.txt]

Example 3: Handling End-of-File and Read Errors

This example illustrates how to handle situations where fewer elements are read than expected, indicating either an error or the end-of-file.

Program

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

int main() {
    FILE *fp = fopen("data.bin", "rb");
    if (fp == NULL) {
        perror("Error opening file");
        return EXIT_FAILURE;
    }

    int buffer[50];
    size_t elementsRead = fread(buffer, sizeof(int), 50, fp);

    if (elementsRead != 50) {
        if (feof(fp)) {
            printf("End-of-file reached after reading %zu elements.\n", elementsRead);
        } else if (ferror(fp)) {
            perror("Error reading file");
        }
    } else {
        printf("Successfully read 50 elements.\n");
    }

    fclose(fp);
    return 0;
}

Explanation:

1. The file data.bin is opened in binary mode.
2. A buffer for 50 integers is allocated.
3. fread() attempts to read 50 elements from the file.
4. If fewer than 50 elements are read, the program checks if the end-of-file was reached using feof() or if a read error occurred using ferror().
5. An appropriate message is printed based on the outcome.
6. The file is closed after the operation.

Program Output:

End-of-file reached after reading [number] elements.