C stdio.h fsetpos() Function

fsetpos() Function

The fsetpos() function in C is used to restore the position indicator of a file stream to a previously saved location. This function is particularly useful when you need to return to a specific point in a file after performing some operations. It also clears the end-of-file indicator and discards any effects from previous calls to ungetc(), ensuring a clean state for subsequent file operations. On streams open for update (read+write), this function helps in switching between reading and writing.

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Syntax of fsetpos()

int fsetpos(FILE *stream, const fpos_t *pos);

Parameters

Parameter	Description
stream	Pointer to a FILE object that identifies the stream.
pos	Pointer to an fpos_t object that contains a file position obtained previously with fgetpos().

The fsetpos() function resets the internal file position indicator to the position stored in the object pointed to by pos. This function is particularly effective when you need to re-read data or switch between different modes of operation in a file stream.

Return Value

If successful, the fsetpos() function returns zero. On failure, it returns a nonzero value and sets errno to a system-specific positive value.

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Examples for fsetpos()

Example 1: Restoring File Position After Reading

This example demonstrates how to save and restore the file position using fgetpos() and fsetpos() after reading data from a file example.txt.

Program

#include <stdio.h>

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

    fpos_t pos;
    char buffer[100];

    // Save the current file position
    if (fgetpos(fp, &pos) != 0) {
        perror("Error getting file position");
        fclose(fp);
        return 1;
    }

    // Read the first line from the file
    fgets(buffer, sizeof(buffer), fp);
    printf("First read: %s", buffer);

    // Restore the file position to the saved state
    if (fsetpos(fp, &pos) != 0) {
        perror("Error setting file position");
        fclose(fp);
        return 1;
    }

    // Read the line again after resetting position
    fgets(buffer, sizeof(buffer), fp);
    printf("\nSecond read: %s", buffer);

    fclose(fp);
    return 0;
}

Explanation:

1. The file "example.txt" is opened in read mode.
2. The current file position is saved using fgetpos() into pos.
3. The first line is read from the file and printed.
4. fsetpos() restores the file position to the saved state.
5. The line is read again from the beginning and printed, demonstrating that the position was reset.

Program Output:

Example 2: Switching Between Reading and Writing in Update Mode

This example illustrates how fsetpos() is useful in a file data.txt opened in update mode (read+write), allowing the program to switch between reading and writing operations.

Program

#include <stdio.h>

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

    fpos_t pos;

    // Save the current position in the file
    if (fgetpos(fp, &pos) != 0) {
        perror("Error getting file position");
        fclose(fp);
        return 1;
    }

    // Read a line from the file
    char buffer[100];
    fgets(buffer, sizeof(buffer), fp);
    printf("Read: %s", buffer);

    // Restore the position to switch to writing mode
    if (fsetpos(fp, &pos) != 0) {
        perror("Error setting file position");
        fclose(fp);
        return 1;
    }

    // Write new data at the restored position
    fputs("Hello Arjun!\n", fp);

    fclose(fp);
    return 0;
}

Explanation:

1. The file "data.txt" is opened in update mode.
2. The current file position is saved using fgetpos().
3. A line is read from the file and printed.
4. fsetpos() restores the file position, allowing the program to switch back to writing at the original position.
5. New data is written into the file, replacing or appending to the original content based on the file mode.

Program Output:

Updated data.txt file:

Example 3: Clearing End-of-File Indicator with fsetpos()

This example demonstrates how fsetpos() clears the end-of-file indicator, allowing further reading from a file after reaching EOF.

Program

#include <stdio.h>

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

    fpos_t pos;

    // Save the position before reading
    if (fgetpos(fp, &pos) != 0) {
        perror("Error getting file position");
        fclose(fp);
        return 1;
    }

    // Read until EOF
    while (!feof(fp)) {
        fgetc(fp);
    }
    printf("Reached EOF.\n");

    // Restore position and clear EOF indicator
    if (fsetpos(fp, &pos) != 0) {
        perror("Error setting file position");
        fclose(fp);
        return 1;
    }

    // Read again after clearing EOF
    char buffer[100];
    fgets(buffer, sizeof(buffer), fp);
    printf("After resetting, first line: %s", buffer);

    fclose(fp);
    return 0;
}

Explanation:

1. The file "log.txt" is opened in read mode.
2. The current file position is saved before reading the entire file.
3. The program reads characters until the end-of-file is reached.
4. fsetpos() restores the file position and clears the EOF indicator.
5. The program then reads from the restored position, demonstrating that further reading is possible.

Program Output: