Cảnh báo của trình biên dịch

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

/* 1. Chuyen doi thu hep im lang */
static void thu_hep(void) {
    long lon = 3000000000L;
    int  nho = lon;                 /* mat du lieu */
    double d = 3.99;
    int    i = d;                   /* cat phan thap phan */
    printf("thu_hep: %d %d\n", nho, i);
}

/* 2. So sanh co dau va khong dau */
static void dau(void) {
    int  a = -1;
    unsigned b = 1;
    printf("dau: -1 < 1 la %s\n", (a < b) ? "dung" : "SAI");
}

/* 3. Khai bao cu kieu K&R, khong co nguyen mau */
static int cong();                  /* () khong phai (void) */

/* 4. Bo cuc struct lang phi */
struct Xau { char a; double b; char c; };

/* 5. Ham khong lam gi */
static void ham_khong_doi(void) { }

/* 6. Bien an bien ngoai */
static int dem = 0;
static void an(void) {
    int dem = 5;                    /* che bien toan cuc */
    printf("an: %d\n", dem);
}

int main(void) {
    thu_hep();
    dau();
    an();
    printf("sizeof(struct Xau) = %d\n", (int)sizeof(struct Xau));
    printf("cong = %d\n", cong(1, 2));
    ham_khong_doi();
    return 0;
}
static int cong(a, b) int a; int b; { return a + b; }

CANH_BAO := -Wall -Wextra -Wpedantic \
            -Wconversion -Wsign-conversion \
            -Wshadow \
            -Wstrict-prototypes -Wmissing-prototypes -Wold-style-definition \
            -Wcast-qual -Wcast-align \
            -Wwrite-strings \
            -Wredundant-decls -Wnested-externs \
            -Wundef \
            -Wdouble-promotion \
            -Wformat=2 \
            -Wnull-dereference \
            -Wswitch-enum -Wswitch-default \
            -Wfloat-equal \
            -Wunreachable-code

CFLAGS := -std=c11 $(CANH_BAO) -g

#include <stddef.h>
#include <string.h>

int      lay_do_dai(const char *s) { return strlen(s); }  /* size_t -> int */
char     lay_byte(int x)           { return x; }          /* int -> char */
unsigned lay_khong_dau(int x)      { return x; }          /* co dau -> khong dau */
float    lay_float(double d)       { return d; }          /* double -> float */
int      lay_tu_ll(long long v)    { return v; }          /* 64 -> 32 bit */

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

/* Ham nay CO Y chuyen size_t sang int, va ta biet chuoi luon ngan. */
static int do_dai_ngan(const char *s) {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wconversion"
    return strlen(s);       /* an toan: goi ham nay chi voi chuoi < 100 */
#pragma GCC diagnostic pop
}

/* Ham nay KHONG duoc tat, canh bao phai con */
static int do_dai_bat_ky(const char *s) {
    return strlen(s);
}

int main(void) {
    printf("%d %d\n", do_dai_ngan("abc"), do_dai_bat_ky("abcd"));
    return 0;
}