How to Use fseek in C: Syntax, Example, and Tips

In C, fseek moves the file pointer to a specific location in a file. You use it by passing a file pointer, an offset, and a position indicator like SEEK_SET, SEEK_CUR, or SEEK_END. This lets you read or write at any point in the file.

Syntax

The fseek function has this syntax:

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

• stream: The file pointer returned by fopen.
• offset: Number of bytes to move the pointer.
• whence: Starting point for offset. It can be SEEK_SET (start of file), SEEK_CUR (current position), or SEEK_END (end of file).

The function returns 0 on success and non-zero on failure.

c

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

Example

This example opens a file, moves the pointer 10 bytes from the start, reads a character, and prints it.

c

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "r");
    if (!file) {
        perror("Failed to open file");
        return 1;
    }

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

    int ch = fgetc(file);
    if (ch == EOF) {
        printf("No character at this position or read error.\n");
    } else {
        printf("Character at byte 10: %c\n", ch);
    }

    fclose(file);
    return 0;
}

Output

Character at byte 10: X

Common Pitfalls

• Not checking if fseek returns 0 can cause unnoticed errors.
• Using an invalid whence value leads to undefined behavior.
• Seeking beyond the file size may cause read/write errors.
• Remember to open the file in a mode that supports the intended operation (read/write).

c

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "r");
    if (!file) return 1;

    // Wrong: using invalid whence
    // fseek(file, 5, 100); // Invalid whence, do not do this

    // Right:
    if (fseek(file, 5, SEEK_CUR) != 0) {
        perror("fseek failed");
    }

    fclose(file);
    return 0;
}

Quick Reference

Parameter	Description	Common Values
stream	File pointer from fopen	FILE *
offset	Bytes to move pointer	Positive or negative long
whence	Reference point for offset	SEEK_SET, SEEK_CUR, SEEK_END

Key Takeaways

Use fseek to move the file pointer to a specific byte position in a file.

Always check if fseek returns 0 to confirm success.

Use SEEK_SET, SEEK_CUR, or SEEK_END to specify the starting point for the offset.

Opening the file in the correct mode is essential for fseek to work properly.

Avoid invalid whence values and seeking beyond file boundaries.