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

/* Doc toan bo mot tep vao bo nho.
   Tra ve con tro toi vung da cap phat, hoac NULL neu that bai.
   NGUOI GOI phai free ket qua.
   Neu ra_co khac NULL thi so byte doc duoc duoc ghi vao do. */
char *doc_ca_file(const char *ten, size_t *ra_co) {
    FILE *f = fopen(ten, "rb");              /* PHAI la "rb" */
    if (!f) return NULL;

    if (fseek(f, 0, SEEK_END) != 0) { fclose(f); return NULL; }

    long co = ftell(f);
    if (co < 0)                     { fclose(f); return NULL; }

    rewind(f);

    char *bo_dem = malloc((size_t)co + 1);   /* cong 1 cho ky tu ket thuc */
    if (!bo_dem)                    { fclose(f); return NULL; }

    size_t doc = fread(bo_dem, 1, (size_t)co, f);

    if (ferror(f)) { free(bo_dem); fclose(f); return NULL; }

    fclose(f);

    bo_dem[doc] = '\0';                      /* dung doc, KHONG dung co */

    if (ra_co) *ra_co = doc;

    return bo_dem;
}

size_t co;
char  *noi_dung = doc_ca_file("cau-hinh.txt", &co);

if (!noi_dung) { perror("cau-hinh.txt"); return 1; }

printf("Tep dai %zu byte\n", co);

if (strstr(noi_dung, "debug=1")) bat_go_loi();

free(noi_dung);                    /* DUNG QUEN */

fseek(f, 0, SEEK_END);      /* voi ong dan: TRA VE KHAC 0, that bai */
long co = ftell(f);         /* hoac tra ve -1 */

/* Neu ban khong kiem tra thi:
     co la -1
     malloc(-1 + 1) tuc malloc(0), tra ve con tro hop le nhung 0 byte
     fread ghi vao do -> TRAN BO DEM

   Neu ban ep sang size_t:
     (size_t)(-1) la 18446744073709551615
     malloc that bai, tra NULL
     may man hon, nhung van la loi. */

long co = ftell(f);
char *bo_dem = malloc((size_t)co + 1);

/* Nguoi dung tro chuong trinh vao mot tep 40 GB.
   malloc that bai, ban tra NULL, coi nhu on.

   Nhung neu chuong trinh cua ban la mot dich vu mang
   va co la 8 GB thi may chu cua ban vua bi chiem het RAM
   boi mot yeu cau duy nhat. */

#define TOI_DA (64L * 1024 * 1024)      /* 64 MB */

if (co > TOI_DA) {
    fprintf(stderr, "%s: tep qua lon (%ld byte, toi da %ld)\n",
            ten, co, TOI_DA);
    fclose(f);

    return NULL;
}

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

/* Doc ca luong, khong dung fseek nen chay duoc voi ONG DAN.
   Nguoi goi phai free ket qua. */
char *doc_ca_luong(FILE *f, size_t *ra_co) {
    size_t co = 0, suc_chua = 4096;
    char  *bo_dem = malloc(suc_chua);

    if (!bo_dem) return NULL;

    for (;;) {
        if (co + 1 >= suc_chua) {                    /* chua cho ky tu ket thuc */
            size_t moi_sc = suc_chua * 2;
            char  *moi    = realloc(bo_dem, moi_sc);

            if (!moi) { free(bo_dem); return NULL; }

            bo_dem   = moi;
            suc_chua = moi_sc;
        }

        size_t n = fread(bo_dem + co, 1, suc_chua - co - 1, f);

        co += n;

        if (n == 0) break;                           /* het luong hoac loi */
    }

    if (ferror(f)) { free(bo_dem); return NULL; }

    bo_dem[co] = '\0';

    if (ra_co) *ra_co = co;

    return bo_dem;
}