Lære Using File Pointers for Navigation | Error Handling and File Pointers

A file pointer in C is a special variable of type FILE * that keeps track of your current position within an open file. When you read from or write to a file, the file pointer moves through the file's contents, ensuring that your operations happen at the correct location. Navigating within a file—jumping forward, backward, or returning to the beginning—relies on manipulating this file pointer. Mastering file pointer navigation allows you to efficiently access and modify data anywhere in a file, not just sequentially from start to end.

main.c

sample.txt


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#include <stdio.h>

int main() {
    FILE *fp = fopen("sample.txt", "r");
    if (fp == NULL) {
        printf("Failed to open file.\n");
        return 1;
    }

    // Move file pointer to 5 bytes from the beginning
    fseek(fp, 5, SEEK_SET);

    // Tell current position
    long pos = ftell(fp);
    printf("Current file position: %ld\n", pos);

    // Read and print the next character
    int c = fgetc(fp);
    printf("Character at position %ld: %c\n", pos, c);

    fclose(fp);
    return 0;
}

fseek

Moves the file pointer to a specific location within the file. You specify the number of bytes to move and a reference point (SEEK_SET for the beginning, SEEK_CUR for the current position, or SEEK_END for the end of the file);

ftell

Returns the current position of the file pointer as a long integer, indicating how many bytes from the beginning of the file the pointer is;

rewind

Sets the file pointer back to the very beginning of the file, making it easy to reread or rewrite the file from the start.

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