C stdio.h fseek() Function

fseek() Function

The fseek() function in C repositions the stream position indicator for a given file stream. It is used to set the pointer to a specific location in a file, which is particularly useful when you need to navigate through files in both binary and text modes. This function clears the end-of-file indicator and discards any effects from previous ungetc calls, ensuring that subsequent I/O operations work correctly.

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Syntax of fseek()

int fseek(FILE *stream, long int offset, int origin);

Parameters

Parameter	Description
stream	A pointer to a FILE object that identifies the stream.
offset	For binary files: The number of bytes to offset from the reference position. For text files: Either zero or a value returned by ftell().
origin	The reference position used for the offset. It can be one of the following constants: SEEK_SET – Beginning of file SEEK_CUR – Current position of the file pointer SEEK_END – End of file (note: support for this may vary)

It is worth noting that for streams opened in text mode, the offset must either be zero or a value returned by ftell(), and the origin must be SEEK_SET. For binary streams, the offset is simply added to the reference position specified by the origin.

Return Value

If fseek() is successful, it returns zero. Otherwise, it returns a non-zero value, and if an error occurs during reading or writing, the error indicator is set for the stream.

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Examples for fseek()

Example 1: Repositioning in a Binary File

This example demonstrates how to use fseek() to move the file pointer in a binary file to a specified offset from the beginning of the file.

Program

#include <stdio.h>

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

    // Move the file pointer 10 bytes from the beginning (SEEK_SET)
    if (fseek(fp, 10L, SEEK_SET) != 0) {
        perror("fseek failed");
        fclose(fp);
        return 1;
    }

    // Further file operations can be performed here
    printf("File pointer repositioned successfully.\n");

    fclose(fp);
    return 0;
}

Explanation:

1. The program opens a binary file named “example.bin” in read mode.
2. fseek() is used to move the file pointer 10 bytes from the beginning of the file (SEEK_SET).
3. Error checking is performed to ensure the pointer was repositioned successfully.
4. A success message is printed if the operation is successful.

Program Output:

File pointer repositioned successfully.

Example 2: Using fseek in a Text File to Reposition for Reading

This example shows how to use fseek() with a text file example.txt, where the offset is zero or a value obtained from ftell(), to reposition the file pointer before reading data.

Program

#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "r");
    if (fp == NULL) {
        perror("Error opening file");
        return 1;
    }

    // Read and display first 20 characters
    char buffer[21] = {0};
    fread(buffer, sizeof(char), 20, fp);
    printf("Initial read: %s\n", buffer);

    // Reposition to the beginning using ftell value 0 and SEEK_SET
    if (fseek(fp, 0L, SEEK_SET) != 0) {
        perror("fseek failed");
        fclose(fp);
        return 1;
    }

    // Read again from the beginning
    fread(buffer, sizeof(char), 20, fp);
    printf("Re-read after fseek: %s\n", buffer);

    fclose(fp);
    return 0;
}

Explanation:

1. The program opens a text file “example.txt” in read mode.
2. It reads the first 20 characters and prints them.
3. fseek() is called with an offset of 0 and SEEK_SET to reposition the file pointer to the beginning of the file.
4. The file is read again from the beginning and the output is printed.

Program Output:

Example 3: Moving to the End of a File and Back to the Beginning

This example demonstrates repositioning the file pointer to the end of the file using SEEK_END and then moving it back to the beginning with SEEK_SET, which is useful for determining file size or re-reading a file.

Program

#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "r");
    if (fp == NULL) {
        perror("Error opening file");
        return 1;
    }

    // Move to the end of the file
    if (fseek(fp, 0L, SEEK_END) != 0) {
        perror("fseek failed");
        fclose(fp);
        return 1;
    }

    // Optionally, you can use ftell() here to get the file size

    // Reposition back to the beginning
    if (fseek(fp, 0L, SEEK_SET) != 0) {
        perror("fseek failed");
        fclose(fp);
        return 1;
    }

    printf("File pointer repositioned back to the beginning.\n");

    fclose(fp);
    return 0;
}

Explanation:

1. The program opens the text file “example.txt” in read mode.
2. fseek() is used with SEEK_END to move the file pointer to the end of the file.
3. The pointer is then repositioned to the beginning using fseek() with an offset of 0 and SEEK_SET.
4. A success message is printed indicating the pointer has been repositioned.

Program Output:

File pointer repositioned back to the beginning.