我不明白这个简单的循环如何改变文件指针位置

Question

This simple program opens file with letters A to Z that i already created, changes one letter to a star *, than prints list of letters again. Everything works but i dont understand how this loop works:

for (i = 0; i < 26; i++)
    {
        letter = fgetc(fptr);
        printf("The next letter is %c.\n", letter);
    }

Particularly where does it say that fptr - filepointer is connected or equals to i?? I understand that loop will increase i from 0 to 26 but i have no idea why fptr should increase too?? The whole program is below:

FILE * fptr;

main()
{
    char letter;
    int i;

    fptr = fopen("//Users//nik//Desktop//letters.txt", "r+");

    if (fptr == 0)
    {
        printf("There was an error while opening the file.\n");
        exit(1);
    }

    printf("Which # letter would you like to change (1-26)? ");
    scanf(" %d", &i);

    // Seeks that position from the beginning of the file

    fseek(fptr, (i-1), SEEK_SET); // Subtract 1 from the position
    // because array starts at 0

    // Write an * over the letter in that position
    fputc('*', fptr);

    // Now jump back to the beginning of the array and print it out

    fseek(fptr, 0, SEEK_SET);

    printf("Here is the file now:\n");

    for (i = 0; i < 26; i++)
    {
        letter = fgetc(fptr);
        printf("The next letter is %c.\n", letter);
    }

    fclose(fptr); 

    return(0);
}

Answer 1

Based on the next source... http://www.cplusplus.com/reference/cstdio/fgetc/

This function will return the current character which is pointed by the position indicator and then it will move to the next char. So, it is rational that if you loop 26 times you will call the function 26 times and the function will return you the corresponding letters.

Answer 2

FILE is struct ure with the following field:

unsigned char *_p;  /* current position in (some) buffer */

When you open a file, this variable points to a buffer which holds the contents of the file. Consider the following program ( main.c ):

#include <stdio.h>

int main(void)
{
    int ch;
    FILE *pf = fopen("main.c", "r");

    rewind(pf);
    printf("%p\n", pf->_p);

    ch = fgetc(pf);
    printf("%p: %c\n", pf->_p, ch);

    ch = fgetc(pf);
    printf("%p: %c\n", pf->_p, ch);

    fclose(pf);

    return 0;

}

Executing the program produces the following output (on my machine):

0x7f7fe1803200
0x7f7fe1803201: #
0x7f7fe1803202: i

As you can see, the address that pf->_p points to is incremented by each call to fgetc . How fgetc is called (for example, in a for loop) is irrelevant.