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char *fgets(char *dich, int n, FILE *f);

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

int main(void) {
    FILE *f = fopen("du-lieu.txt", "r");
    if (!f) { perror("du-lieu.txt"); return 1; }

    char   dong[1024];
    size_t so_dong = 0;

    while (fgets(dong, sizeof dong, f)) {
        dong[strcspn(dong, "\n")] = '\0';

        printf("%zu: %s\n", ++so_dong, dong);
    }

    if (ferror(f)) { perror("du-lieu.txt"); fclose(f); return 1; }

    fclose(f);

    return 0;
}

while (fgets(dong, sizeof dong, f)) {
    xu_ly(dong);
}

/* Toi day fgets da tra NULL. Vi sao? */

if (ferror(f)) {
    fprintf(stderr, "%s: loi khi doc: %s\n", ten, strerror(errno));
    fclose(f);

    return -1;                      /* KHONG bao la thanh cong */
}

/* Khong co loi, vay la het tep binh thuong */
fclose(f);

return 0;

#include <string.h>

#define DONG_TOI_DA 1024

/* Doc mot dong. Tra ve:
     1  doc duoc mot dong day du
     0  het tep
    -1  dong qua dai hoac loi doc */
int doc_mot_dong(FILE *f, char *dong, size_t co) {
    if (!fgets(dong, (int)co, f)) return ferror(f) ? -1 : 0;

    size_t n = strcspn(dong, "\n");

    if (dong[n] == '\n') {                /* co ky tu xuong dong: du roi */
        dong[n] = '\0';

        return 1;
    }

    /* Khong co ky tu xuong dong. Hai kha nang:
         a. dong dai hon bo dem
         b. dong cuoi tep khong ket thuc bang xuong dong */
    int c = fgetc(f);

    if (c == EOF) return ferror(f) ? -1 : 1;   /* truong hop b, van hop le */

    ungetc(c, f);                              /* tra lai byte vua doc */

    /* Truong hop a: nuot phan con lai cua dong roi bao loi */
    while ((c = fgetc(f)) != EOF && c != '\n') { }

    return -1;
}

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

/* Doc mot dong do dai bat ky. Nguoi goi phai free ket qua.
   Tra ve NULL khi het tep hoac loi. */
char *doc_dong_dong(FILE *f) {
    size_t co = 128, n = 0;
    char  *bo_dem = malloc(co);

    if (!bo_dem) return NULL;

    for (;;) {
        if (!fgets(bo_dem + n, (int)(co - n), f)) {
            if (n == 0) { free(bo_dem); return NULL; }   /* chua doc gi */

            break;                                       /* dong cuoi khong co '\n' */
        }

        n += strlen(bo_dem + n);

        if (n > 0 && bo_dem[n - 1] == '\n') { bo_dem[--n] = '\0'; break; }

        if (n + 1 < co) break;                  /* fgets dung vi het tep */

        char *moi = realloc(bo_dem, co * 2);    /* Bai 14.8 */
        if (!moi) { free(bo_dem); return NULL; }

        bo_dem = moi;
        co    *= 2;
    }

    return bo_dem;
}

int fputs(const char *s, FILE *f);      /* KHONG tu them xuong dong */
int puts(const char *s);                /* TU them xuong dong */

int fputc(int c, FILE *f);
int fgetc(FILE *f);                     /* tra int, xem phan tren */

int chep(const char *tu, const char *toi) {
    FILE *vao = fopen(tu, "rb");
    if (!vao) { perror(tu); return -1; }

    FILE *ra = fopen(toi, "wb");
    if (!ra) { perror(toi); fclose(vao); return -1; }

    int c;
    while ((c = fgetc(vao)) != EOF)
        if (fputc(c, ra) == EOF) break;      /* dung ngay khi ghi hong */

    int loi = ferror(vao) || ferror(ra);

    fclose(vao);
    if (fclose(ra) != 0) loi = 1;

    return loi ? -1 : 0;
}