fseek，fread并返回二进制文件功能中的指针（C）不能正常工作

Question

I want to return the pointer to the structure that I read from the file (it goes to another functions that basicaly just prints the data.) Function gets the (serial number -1) through int sem, and then the idea was, to set the file pointer to the beginning of the structure that I want to read with the fseek, and simply read the whole structure and return the pointer. (Example: If the number of the structures written in the file is 5 and I want to read the structure no. 4, the sem value that function gets is 3, then with fseek pointer skips the first 3 structures and function reads and return the strucutre no.4). It works fine if the sem value is 0, but not so fine when I seek for any other structure. I have no idea what goes wrong! Help? :)

type_seminar *file_seminars_search(int sem) {
     type_seminar *temp_s = (type_seminar*) malloc(sizeof(type_seminar));
     FILE *f_sem;
     if ((f_sem = fopen("seminars.bin", "r")) != NULL ) {
          fseek(f_sem, sem * sizeof(type_seminar), SEEK_SET);
          fread(temp_s, sizeof(type_seminar), 1, f_sem);
          fclose(f_sem);
          return (temp_s);
     } else
          printf("Cannot access file. \n ");
     return 0;
}

The definition of strucutre:

typedef struct {
     char s_title[A];
     char s_street[B];
     char s_town[C];
     int max_no_teachers;
     int no_applied_teachers;
} type_seminar;

• Contents of the seminars.txt file
• What it prints on the screen if I choose the second structure in the file

And the function that prints data if the problem is in here:

void seminar_details(type_seminar *temp_s) {
      printf("   SEMINAR TITLE : %s", temp_s->s_title);
      printf("   Street: %s ", temp_s->s_street);
      printf("   Town: %s", temp_s->s_town);
      printf("   Max no of applied teachers: %d \n", temp_s->max_no_teachers);
      printf("   No of applied teachers: %d \n", temp_s->no_applied_teachers);
}

Answer 1

I feel that the way seminars.bin was created will determine the way the file is read. It may also be helpful to open your file in binary mode as compared to ascii mode.

I used the following function to create the seminars.bin

void createData()
{
FILE    *fcre;
type_seminar element;
int counter;

fcre = fopen("seminars.bin", "wb");
for(counter = 0; counter < 5; counter++)
{
    printf("Enter type_seminar.s_title:");
    scanf("%s", element.s_title);
    printf("Enter type_seminar.s_street:");
    scanf("%s", element.s_street);
    printf("Enter type_seminar.s_town:");
    scanf("%s", element.s_town);
    printf("Enter type_seminar.max_no_teachers:");
    scanf("%d", &element.max_no_teachers);
    printf("Enter type_seminar.no_applied_teachers:");
    scanf("%d", &element.no_applied_teachers);

    fwrite(&element, sizeof(type_seminar), 1, fcre);
}
fclose(fcre);
}

I modified a couple of lines in your original file as

if ((f_sem = fopen("seminars.bin", "rb")) != NULL ) 

and

 fread(temp_s, sizeof(type_seminar), 1, f_sem);

With these changes, I could get your code to work as per your expected design!!

Answer 2

Problem is at

return 0;

line. because you're not returning int.