And yet one more way to do it

myway.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define ARR_SZ 1024
#define _TOSTR(a) #a
#define TOSTR(a) _TOSTR(a)

int main()
{
    FILE *f = fopen("ctext.txt", "rb");
    if (f == NULL) {
        fprintf(stderr, "error opening the file\n");
        return 1;
    }

    fseek(f, 0, SEEK_END);
    size_t sz = ftell(f);
    fseek(f, 0, SEEK_SET);

    char *data = malloc(sz + 1);
    fread(data, 1, sz, f);
    data[sz] = '\0';

    char *longest = NULL;
    char *current = NULL;
    size_t longest_sz = 0;
    for (size_t i = 0; i < sz; i++) {
        if (current == NULL) {
            current = data + i;
        }

        if (data[i] == ' ' || data[i] == '\t' ||
            data[i] == '\n' || data[i] == '\r') {
            data[i] = '\0';
            size_t current_sz = (size_t)(&data[i] - current);
            if (current_sz > longest_sz) {
                longest = current;
                longest_sz = current_sz;
                printf("found a new longest word: %s\n", longest);
            }
            current = NULL;
        }
    }

    printf("longest word: %s\n", longest);
    printf("longest word size: %i\n", (int)longest_sz); // shh...
    free(data);
    return 0;
}