程序未写入文件，加载时崩溃

Question

So i need to write and read to and from a binary file, but cant seem to do it, and when using readFromFile, the program crashes. I need help write to binary file, and then reading from it and resuming my work later after turning off the program.I have no idea what i am doing wrong, and i have googled for a long time now, but with no results. Here is the code of my program:

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

typedef struct{
    char *subjName;
    char *lectName;
    char *lectSurname;
    int credits;
    int num_students;
}Subject;

typedef struct{
        Subject **subjs;
        int num_subjs;
}Subjects;

int numOfSubjs=0;

void listInput();
void listEdit();
void listDelete();
void listPrint();
int userChoice(int select);
int enterNumber(char *name);
void saveToFile(Subjects *subjects);
void readFromFile(Subjects *subjects);

int main() {

    Subjects *subjects = malloc(sizeof(Subjects));
    subjects->num_subjs = 0;
    subjects->subjs = NULL;
    readFromFile(subjects);
    int r=1;
    while(r!=0){
        int select=userChoice(select);
        switch(select){
            case 1:
            listPrint(subjects);
            break;

            case 2:
            listInput(&subjects);
            break;

            case 3:
            listDelete(subjects);
            break;

            case 4:
            listEdit(subjects);
            break;

            case 0:
            r=0;
            break;
        }
    }
    saveToFile(subjects);
    return 0;
}

int userChoice(int select){                                                 // menu options
    int choice,input=0;
    printf("(1). View all the data\n");
    printf("(2). Enter new data\n");
    printf("(3). Delete data\n");
    printf("(4). Edit data\n");
    printf("(0). Exit\n");
    printf("-----------------------------\n");

    while(input!=1){
        choice = enterNumber("menu");
        if(choice>4 || choice<0){
            printf("Invalid input \n");
        }
        else
            input = 1;
    }
    return choice;
}

void listPrint(Subjects *subjects){                         // print data
    int i;
    for(i=0; i< numOfSubjs; i++){
        printf("%d, %s, %s, %s, %d, %d\n",i+1, subjects->subjs[i]->subjName, subjects->subjs[i]->lectName, subjects->subjs[i]->lectSurname, subjects->subjs[i]->credits, subjects->subjs[i]->num_students);
    }
    printf("Number of entries: %d \n", numOfSubjs);
}

char *getln()                                               //dynamically allocate input string
{
    char *line = NULL, *tmp = NULL;
    size_t size = 0, index = 0;
    int ch = 1;

    while (ch) {
        ch = getc(stdin);
        if (ch == '\n')
            ch = 0;

        if (size <= index) {
            size += 1;
            tmp = realloc(line, size);
            if (!tmp) {
                free(line);
                line = NULL;
                break;
            }
            line = tmp;
        }

        line[index++] = ch;
    }

    return line;
}



void saveToFile(Subjects *subjects){
    FILE *data;
    data = fopen("data.bin","wb");
    printf("%s", subjects->subjs[0]);
    for(int i=0; i<numOfSubjs; i++){
        fwrite(&subjects->subjs[i],sizeof(Subject*),1,data);
    }
    fclose(data);
}

void readFromFile(Subjects *subjects){
    FILE *data;
    int i=0;
    data = fopen("data.bin","rb");
    while(!feof(data))
    {
         fread(&subjects->subjs[i],sizeof(Subject*),1,data);
         i++;
    }
    fclose(data);
}

int isText(char *str,char *name){                                   // check if is text
    for(int i = 0; i < strlen(str);i++){
        if((str[i]<'A' || str[i]>'z') && str[i]!=' '){
            printf("Error, %s must be a text \n",name);
            return 0;
        }
    }
    return 1;
}

int enterNumber(char *name){                                // enter number and check if is number
    int input=0, crash=0, num=0;
        while(input!=1)
        {
             crash=0;
             printf("Enter the number of %s\n", name);
             scanf("%d", &num);
             while(getchar()!='\n')
             {
                crash++;
             }
             if(crash>0 || num<0)
                printf("Error, enter a not negative number of %s\n", name);
             else if(crash==0)
                input=1;
        }
    return num;
}

void listInput(Subjects **p_subjects){                              // input new data

    Subject *new_subj = malloc(sizeof(Subject));
    new_subj->subjName = NULL;
    new_subj->lectName = NULL;
    new_subj->lectSurname = NULL;
    new_subj->credits = 0;
    new_subj->num_students = 0;

    do{
        printf("Enter the name of the subject \n");
        new_subj->subjName = getln();
    }while(!isText(new_subj->subjName,"Subject name"));

    do{
        printf("Enter the name of the lecturer \n");
        new_subj->lectName = getln();
        new_subj->lectName[0] &= '_';
    }while(!isText(new_subj->lectName,"Lecturer's name"));

    do{

        printf("Enter the surname of the lecturer\n");
        new_subj->lectSurname = getln();
        new_subj->lectSurname[0] &= '_';                                            //Convert to uppercase if lowercase
    }while(!isText(new_subj->lectSurname,"Lecturer's name"));

    new_subj->credits = enterNumber("credits");

    new_subj->num_students = enterNumber("students");

    (*p_subjects)->subjs = realloc((*p_subjects)->subjs,sizeof(Subject*)*(++(*p_subjects)->num_subjs));
    (*p_subjects)->subjs[(*p_subjects)->num_subjs-1] = new_subj;
    numOfSubjs++;
    printf("Added a new entry.\n\n");
}

void listDelete(Subjects *subjects){                                                // delete entries
    int del;
    if(numOfSubjs==0)
        printf("Number of entries is 0, can't delete anything\n");
    else{
        printf("Enter 0 to exit. Number of subjects : %d \n", numOfSubjs);
        while(1){
            del = enterNumber("entry which you would like to delete");
            if(del<=numOfSubjs && del>0){
                    for(int i = del-1; i<numOfSubjs-1; i++){
                        subjects->subjs[i]=subjects->subjs[i+1];
                        subjects->subjs = realloc(subjects->subjs,sizeof(Subject*)*(--subjects->num_subjs));
                    }
                numOfSubjs--;
                break;  
            }
            if(del>numOfSubjs)
                printf("Error, input a number between 1 and %d (or enter 0 to exit)\n", numOfSubjs);
            else
                break;
        }
    }
}
void listEdit(Subjects *subjects){                                                                  // edit entries
    int choice=0, editEntry=0, editSubj=0;
    if(numOfSubjs == 0)
        printf("Number of entries is 0, can't edit anthing\n");
    else{
        while(1){
            printf("Number of entry must be between 1 and %d \n", numOfSubjs);
            choice = enterNumber("entry you would like to edit.");
            if(choice>0 && choice<=numOfSubjs){
                while(1){
                    editEntry = enterNumber("what would you like to edit\n 1 - Subject name\n 2 - Lecturer's name\n 3 - Lecturer's surname\n 4 - Number of credits\n 5 - Number of students");
                        if(editEntry>0 && editEntry <=5){
                            switch(editEntry){
                                case 1:
                                    do{
                                        printf("Enter the name of the subject \n");
                                        subjects->subjs[choice-1]->subjName = getln();
                                    }while(!isText(subjects->subjs[choice-1]->subjName,"Subject name"));
                                    break;
                                case 2:
                                    do{
                                        printf("Enter Lecturer's name \n");
                                        subjects->subjs[choice-1]->lectName = getln();
                                    }while(!isText(subjects->subjs[choice-1]->lectName,"Lecturer's name"));
                                    break;
                                case 3:
                                    do{
                                        printf("Enter Lecturer's surname \n");
                                        subjects->subjs[choice-1]->lectSurname = getln();
                                    }while(!isText(subjects->subjs[choice-1]->lectSurname,"Lecturer's surname"));
                                    break;
                                case 4:
                                    subjects->subjs[choice-1]->credits = enterNumber("credits");
                                    break;
                                case 5:
                                    subjects->subjs[choice-1]->num_students = enterNumber("students");
                                    break;

                            }
                        }
                    break;
                }
            }
            break;
        }
    }
}

Answer 1

&subjects->subjs[i], This is Undefined behavior. Accessing garbage value. You need to properly initialize it proper memory address. You ddin't do it anywhere.

(*subjects).subjs or subjects->subjs -> This is not pointing anywhere. It is NULL .

Also here you don't need the double pointer. A single pointer would do the thing you want to do.

typedef struct{
        Subject *subjs;
        int num_subjs;
}Subjects;

For single pointer this would be like

Subjects *subjects = malloc(sizeof(Subjects));
subjects->num_subjs = 10;
subjects->subjs = malloc(subjects->num_subjs * sizeof Subject);
subjects->subjs[0].subjName = malloc(40);

Each of the malloc should be checked with it's return Value. If it's NULL then it would be error to proceed further or access it.

And free() it accordingly when you are done working with it.

---

Some basic things:-

  typedef struct{
          Subject *subjs;
          int num_subjs;
  }Subjects;

Now let's look a bit in the code.

Op asked why OP should initialize and isn;t subjects->num_subjs = 0; subjects->subjs = NULL; subjects->num_subjs = 0; subjects->subjs = NULL; not enough?

A pointer is a variable that is supposed to hold address. here Subject* will hold the address of the variables of type Subject .

Now here initially you initialized it.

You have allocated a memory and assigned it's address to the Subject* variable subjects .

Now let's see what else you do.

subjects->num_subjs = 0;
subjects->subjs = NULL;

You initialized it. And then you try to access it( subjects->subjs[i] ). Can you tell me where it points to? ( subject->subjs )?

Answer is nope. It is pointing to nowhere. It contains NULL value now. Don't you think you should tell it how many subject you want to hold and allocate accordingly? Yes you should and that's what I did precisely in the example shown.

Whenever you have a pointer variable ask yourself what it contains - and if the value is something you know about, not some random garbage value.