Files in C, accessing pointers, reading and writing in files - c

Question

I'm trying to make a program in C which tracks the books borrowed by a student. I am having a hard time in accessing pointers with files. When I use files, I don't usually you fscanf( ), instead I use the usual scanf. I have this data struct:
typedef struct{
char fName[24], mInitial, lName[16];
}nameType;
typedef struct{
unsigned long idNo;
nameType studName;
char course[8];
int yrLevel;
books borrowedBooks;
int bksCtr;
}student;
typedef struct{
student *studs;
int studCtr;
}studList;
I have created two functions as of now, which is addStudToFile(void), which adds students to the file, and displayStudsFromFile(void), which basically prints out the students that were added in the file. These are my newbie function code:
void addStudToFile(void)
{
FILE *fp;
studList myStud;
fp = fopen("students.db", "w");
if(fp!=NULL){
/* ask for student details and adds these to the file */
printf("Enter ID number: ");
fflush(stdin);
scanf(,"%lu", &myStud.studs->idNo);
printf("Enter First Name: ");
fflush(stdin);
gets(myStud.studs->studName.fName);
printf("Enter Last Name: ");
fflush(stdin);
gets(myStud.studs->studName.lName);
printf("Enter Middle Initial: ");
fflush(stdin);
scanf("%c", &(myStud.studs->studName.mInitial));
printf("Enter Course: ");
fflush(stdin);
gets(myStud.studs->course);
printf("Enter Year: ");
fflush(stdin);
scanf("%d", &(myStud.studs->yrLevel));
fwrite(&myStud, sizeof(studList),1,fp);
fclose(fp);
}
}
and
void displayStudsFromFile(void)
{
FILE *fp;
studList myStud;
fp = fopen("students.db", "r");
if(fp!=NULL){
while (fread(&myStud, sizeof(studList), 1, fp)){
printf("%lu\t %s, %s %s\t %s-%d", myStud.studs->idNo, myStud.studs->studName.lName,
myStud.studs->studName.fName, myStud.studs->studName.mInitial,
myStud.studs->course, myStud.studs->yrLevel);
printf("borrowed %d books", myStud.studs->bksCtr);
}
fclose(fp);
}
}
Now, my problem here is in accessing my List which is myStud. In my addStudToFile( ) function, everytime I input my ID number, my program stops working. Why does it stops working? Do I have to malloc something? Or is my accessing in scanf() wrong? Another situation where I encounter my program to stop working again, is when I call my display function. It displays something, but alien/garbage values.
Here is a screenshot on where I encountered my problem in my scanning function:
And here is on my display function:
I hope someone can help me with this. Thanks!

Answer 1

Your hunch is right, you do need to malloc something :)
typedef struct{
student *studs;
int studCtr;
}studList;
Here's your problem. You're defining studs as a pointer to a student struct, but you are not actually allocating any memory for it, so you can later reference it with the -> operator.
You can either allow for a preset number of entries, so you could then define studs like,
student studs[10];
to allow for 10 entries, or in addStudToFile() you could ask the user to input how many entries he would like to give. In this case you would leave the definition as it is and as soon as you have the user input do:
myStud.studs = (student *) malloc( sizeof(student) * how_many );
There may be more bugs along the code you have posted, but for the time being the above is what keeps you back.
edit: if you follow the malloc() route, before returning from addStudToFile() for whatever reason you should make sure you call
free(myStud.studs);
or you get a memory leak...
Update
All right, going further down, when you fwrite() everything, remember, you malloc()ed the memory for studs. sizeof(studlist) is computed at compile time and can't possibly know of the additional memory used at run time. In addition, the two memory regions are not guaranteed to be continuous, so still, one fwrite wouldn't cut it.
With your code structured as it is, you would be better off fwrite()ing the studCtr first, then the memory you malloced for studs.
For the displayStudsFromFile() since there is just a loop there and nothing is really stored for later, I'd just use
student myStud;
ie, use just one instance of student structure instead of a studlist. In this scenario you do one fread() to read in the studCtr from the disk file and then use that to loop around fread() for one student object at a time into myStud. Within that loop you print the fields of interest like so:
printf("borrowed %d books", myStud.bksCtr);
Hope this will get you going... First steps in C are a bit tough :D

Answer 2

myStud.studs is a pointer to a student, but I don't see where you're actually allocating that student. You need to malloc a student before you can do things like &myStud.studs->idNo.

Answer 3

In short, don't write pointers to files, they will be meaningless later on.
The typical approach is to write out the count of the items first, then loop over each item in the list and write them out individually.
On the reader end:
read the number of items.
Allocate enough memory to hold all the items.
Read each item in.

Answer 4

along with the problems mentioned already,
this function has its' own set of troubles.
I have inserted '<--' and a comment at each problem
fflush(stdin) though works on some implementations, it's still undefined behaviour.
According to the standard, fflush only works with output/update streams
( for your code, since the printf format strings do not end in '\n'
( which would have forced the actual output to occur
( change these lines to 'fflush(stdout)'
A ' ' in a scanf() format string will consume any white space found at that
point in the input. Therefore, for almost all cases, the first char in
the format string should be: ' '. Then newlines, spaces, etc
will be consumed, as if they were never there. It is even correct to
use the leading ' ' when there is no white space to consume.
gets() is depreciated and will corrupt/overrun a input buffer, so NEVER
use gets, rather, use fgets(), where the amount of input can be limited
and similar good things.
void addStudToFile(void)
{
FILE *fp;
studList myStud;
fp = fopen("students.db", "w");
if(fp!=NULL)
{
/* ask for student details and adds these to the file */
printf("Enter ID number: ");
fflush(stdin); <-- change to stdout
scanf(,"%lu", &myStud.studs->idNo);
<-- change format string to: " %lu"
<-- add check of returned value to assure operation successful
printf("Enter First Name: ");
fflush(stdin); <-- change to stdout
gets(myStud.studs->studName.fName);
<-- replace gets with fgets() +appropriate parms)
<-- add check of returned value to assure operation successful
printf("Enter Last Name: ");
fflush(stdin); <-- change to stdout
gets(myStud.studs->studName.lName);
<-- replace gets with fgets() +appropriate parms)
<-- add check of returned value to assure operation successful
printf("Enter Middle Initial: ");
fflush(stdin); <-- change to stdout
scanf("%c", &(myStud.studs->studName.mInitial));
<-- replace format string with " %c"
<-- add check of returned value to assure operation successful
printf("Enter Course: ");
fflush(stdin); <-- change to stdout
gets(myStud.studs->course);
<-- replace gets with fgets() +appropriate parms
<-- add check of returned value to assure operation successful
printf("Enter Year: ");
fflush(stdin); <-- change to stdout
scanf("%d", &(myStud.studs->yrLevel));
<-- change format string to: " %d"
<-- add check of returned value to assure operation successful
fwrite(&myStud, sizeof(studList),1,fp);
<-- add check of returned value to assure operation successful
fclose(fp);
<-- add else clause so use knows what happened. I.E.
} else { perror( "fopen failed for write"); exit(EXIT_FAILURE);
} // end if
} // end function: addStudToFile

Answer 5

Here are my comments, prefixed by '<--'
void displayStudsFromFile(void)
{
FILE *fp;
studList myStud;
fp = fopen("students.db", "r");
if(fp!=NULL)
{
while (fread(&myStud, sizeof(studList), 1, fp))
<-- add check of returned value to assure operation successful
{
printf("%lu\t %s, %s %s\t %s-%d",
myStud.studs->idNo,
myStud.studs->studName.lName,
myStud.studs->studName.fName,
myStud.studs->studName.mInitial,
myStud.studs->course,
myStud.studs->yrLevel);
printf("borrowed %d books", myStud.studs->bksCtr);
}
fclose(fp);
<-- to let user know about error
<-- insert: }else{ perror( "fopen failed for read"); exit(EXIT_FAILURE);
} // end if
} // end function: displayStudsFromFile