Cannot increment the value of a dereferenced pointer

Question

I seem to have a problem with a simple program that is supposed to count various character types in a file. It always prints zeroes even though the file is not empty at all. I think it has something to do with pointers, could be wrong. I would also like to know if initializing variables is necessary in this case?

// fun.h

void count_char(FILE *f, unsigned *newl, unsigned *let, unsigned *num, unsigned *spec_char);

// main.c

#include <stdio.h>
#include "fun.h"

int main()
{
    unsigned newline = 0, number = 0, letter = 0, special_character = 0;
    char path[256];
    FILE *f_read;
    printf("Insert a file path: ");
    gets(path);
    f_read = fopen(path, "r");
    if(f_read == NULL)
    {
        perror("The following error occurred");
        return 1;
    }
    count_char(f_read, &newline, &number, &letter, &special_character);
    printf("File content:\n\tnewline - %u\n\tletters - %u\n\tnumbers - %u\n\tspecial characters - %u\n", newline, number, letter, special_character);
    return 0;
}

// fun.c

#include <stdio.h>
#include <ctype.h>

void count_char(FILE *f, unsigned *newl, unsigned *let, unsigned *num, unsigned *spec_char)
{
    char c;
    while((c = fgetc(f)) != EOF)
    {
        if(c == '\n')
            *newl++;
        else if(isalpha(c))
            *let++;
        else if(isdigit(c))
            *num++;
        else
            *spec_char++;
    }
    return;
}

That worked! Please post your comment as an answer so I can accept it. — Venom, Jan 16 '13 at 20:54
Possible duplicate of [Post-increment on a dereferenced pointer?](https://stackoverflow.com/questions/859770/post-increment-on-a-dereferenced-pointer) — Bernhard Barker, May 25 '17 at 12:07

score 1 · Accepted Answer · answered Jan 16 '13 at 20:58

1

When you do something like this: *newl++;; what happens is that first, the pointer is incremented (i.e. make it point to the next memory location) and THEN it is dereferenced based on operator precedence.

If you want to dereference it and then increment, you have to use parentheses, like this: (*newl)++;

answered Jan 16 '13 at 20:58

imreal

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This is because of operator precedence. `*newl++` is equivalent to `*(newl++)`. – Keith Thompson Sep 18 '16 at 00:07

Cannot increment the value of a dereferenced pointer

1 Answers1

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