alphabetical sorting from a file found somewhere on the internet

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

int cmp_func(const void *a, const void *b){
    return strcmp(*(char**)a, *(char**)b);
}

int main(void){
    FILE *f;
    long num, i;
    char **arr, buf[BUFSIZ], *p;

    SetConsoleCP(1251);
    SetConsoleOutputCP(1251);

    printf("file name: ");
    if ( ! fgets(buf, BUFSIZ, stdin) ){
        fprintf(stderr, "read error!\n");
        exit(1);
    }
    if ( p = strrchr(buf, '\n') )
        *p = '\0';
    if ( ! *buf ){
        fprintf(stderr, "empty line!\n");
        exit(1);
    }
    if ( ( f = fopen(buf, "r") ) == NULL ){
        fprintf(stderr, "open file error!\n");
        exit(1);
    }

    arr = NULL;
    num = 0;
    while ( fgets(buf, BUFSIZ, f) ){
        if ( p = strrchr(buf, '\n') )
            *p = '\0';
        if ( ! *buf )
            continue;
        if ( ( arr = realloc(arr, sizeof(char*) * (num + 1)) ) == NULL ){
            fprintf(stderr, "memory error!\n");
            fclose(f);
            exit(1);
        }
        if ( ( arr[num++] = strdup(buf) ) == NULL ){
            fprintf(stderr, "memory error!\n");
            fclose(f);
            exit(1);
        }
    }
    fclose(f);

    printf("\nlines read from file:\n");
    for ( i = 0; i < num; ++i )
        printf("%s\n", arr[i]);

    qsort(arr, num, sizeof(char*), cmp_func);

    printf("\nsorted lines:\n");
    for ( i = 0; i < num; ++i )
        printf("%s\n", arr[i]);

    for ( i = 0; i < num; ++i )
        free(arr[i]);
    free(arr);

    exit(0);
}