C rewind() Function – stdio.h

rewind() Function

The rewind() function in C stdio.h resets the position indicator for a given file stream, effectively setting it back to the beginning of the file. This is useful for re-reading a file or clearing any previous error or end-of-file indicators, and it facilitates switching between reading and writing on update streams.

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Syntax of rewind()

void rewind(FILE *stream);

Parameters

Parameter	Description
stream	A pointer to a FILE object that identifies the stream.

After a successful call, the rewind() function clears the error and end-of-file indicators for the stream and discards any effects from previous calls to ungetc(). It also enables switching between reading and writing in streams opened for update.

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Return Value

The rewind() function does not return any value.

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Examples for rewind()

Example 1: Basic Usage of rewind() to Reset File Stream

This example demonstrates how to use rewind() to reset the file pointer, allowing a file to be read from the beginning after initial reading.

Program

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

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

    char buffer[50];

    // Read first line from the file
    if (fgets(buffer, sizeof(buffer), fp) != NULL) {
        printf("First read: %s", buffer);
    }

    // Reset file pointer to the beginning
    rewind(fp);

    // Read the first line again after rewinding
    if (fgets(buffer, sizeof(buffer), fp) != NULL) {
        printf("After rewind: %s", buffer);
    }

    fclose(fp);
    return 0;
}

Explanation:

1. The program opens a file named "example.txt" for reading.
2. It reads the first line and prints it.
3. The rewind() function is called to reset the file pointer to the beginning.
4. The first line is read again and printed, demonstrating that the stream position was successfully reset.

Program Output:

First read: This is the first line of the file.
After rewind: This is the first line of the file.

Example 2: Switching Between Reading and Writing Modes

This example shows how rewind() can be used to switch from reading to writing on a file opened for update.

Program

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

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

    // Write data to the file
    fputs("Initial Data\n", fp);

    // Rewind to read what was written
    rewind(fp);

    char buffer[50];
    if (fgets(buffer, sizeof(buffer), fp) != NULL) {
        printf("Data after rewind: %s", buffer);
    }

    fclose(fp);
    return 0;
}

Explanation:

1. A file "update.txt" is opened in update mode ("w+").
2. The program writes some initial data to the file.
3. The rewind() function resets the stream to the beginning.
4. The program reads the written data from the start and prints it.

Program Output:

Data after rewind: Initial Data

Example 3: Clearing Error and EOF Indicators

This example illustrates how rewind() clears the end-of-file and error indicators, allowing further operations on the stream.

Program

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

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

    // Move to the end to simulate EOF
    fseek(fp, 0, SEEK_END);
    if (feof(fp)) {
        printf("Reached end-of-file.\n");
    }

    // Now rewind the file to clear EOF indicator
    rewind(fp);
    if (!feof(fp)) {
        printf("EOF indicator cleared after rewind.\n");
    }

    fclose(fp);
    return 0;
}

Explanation:

1. The file "data.txt" is opened for reading.
2. The file pointer is moved to the end of the file using fseek(), setting the EOF indicator.
3. The program checks and confirms that the end-of-file has been reached.
4. The rewind() function is called, which clears the EOF (and error) indicator.
5. A subsequent check confirms that the EOF indicator has been cleared.

Program Output:

Reached end-of-file.
EOF indicator cleared after rewind.