Error in reading the file in C [closed] - c++

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I'm trying to write and read from file. writing works fine, but when it tries to read, the program crashed. tell me whats the error?
The objective of this program is to store the record of students in a file.
Calculate the percentage, and find the highest and lowest percentage of student.
#include <stdio.h>
#include <conio.h>
#include <string.h>
struct result
{
char name[15];
int batchNo;
int rgtNo;
int marks;
float perc;
};
int main()
{
char answer, i = 1;
FILE *ptr, *abc;
result s, st;
int j;
ptr = fopen("rec.txt","a");
abc = fopen("rec.txt","r");
if(!ptr)
printf("Error opening file");
else if(!abc)
printf("Error opening read file");
else {
do
{
int sum = 0;
printf("\nEnter name of student %d: ", i);
scanf("%s", s.name);
printf("\nEnter Batch #: ");
scanf("%d", &s.batchNo);
printf("\nEnter Registeration no: ");
fflush(stdin);
scanf("%d", &s.rgtNo);
printf("\nEnter marks of Five subjects: ");
scanf("%d",&s.marks);
/*for(j = 0; j < 5; j++)
{
scanf("%d",&s.marks[j]);
sum += s.marks[j];
}*/
s.perc = sum / 5.0;
printf("Percentage of student %d: %f", i+1, s.perc);
fprintf(ptr, "%s %d %d %d %f", s.name, s.batchNo, s.rgtNo, s.marks, s.perc);
printf("\n\n\n");
printf("Do you want to add another record? (y/n) ");
answer = getche();
i++;
} while(answer == 'y');
printf("outside loop\n");
fclose(ptr);
fflush(stdin);
rewind(abc);
do
{
fscanf(abc, "%s%d%d%d%f", s.name, s.batchNo, s.rgtNo, s.marks, s.perc);
printf("in loop");
} while( !feof(abc));
fclose(abc);
}
}

Answer 1

So many defects, so little time.
C is not C++
They are two different languages. Remove the C++ tag.
But since you tagged it as C++, you should use the fstream and iostream libraries as well as the std::string type.
One pointer per physical file
You have one FILE * for each different mode of the same file. In many cases, this will confuse the operating system and heck some won't allow it.
Instead, use one FILE * and open the file as rw+ for read, write and append.
See fseek function.
Buffer overflow with scanf
You have reserved 15 letters for the name. What happens if the user types in a name longer than 15 letters? Buffer overflow. You could write over other variables that follow the name member.
You could resolve this by using fscanf with the C language or std::cin and std::string with the C++ language.
Flushing the input
The flush function only works with output buffers. Don't use with input streams.
In the C language you have to read characters until the buffer is empty or until your terminator character is found (such as 'n').
In C++ this is accomplished by cin.ignore(1000000, '\n').
Checking input functions for errors
You can't trust the User. You must check the scanf return value for errors. If you ask for a number and the User types in a number, the scanf will fail. The value is undefined.

Answer 2

the program crashes whenever you type something different than 'y' at the last question Do you want to add another record? (y/n).
think what happens after this stage in your program...
hint - take a look at your ptr and abc File pointers - do they point to the same file?

Answer 3

The issue which makes your program segfault is the fscanf call: you're supposed to pass pointers to variables for arguments. I guess you quickly C&P without double checking your code.
As mentioned in another answer: fflush behaviour isn't specified for input streams, though many implementations behave as you expect. Another way to flush is to do a loop on the stream and read char by char:
void flush_stdin(){
char c;
while (c != '\n' && c != EOF)
c = getchar();
}
Note: I'm using getchar, as getche isn't available on my system. I suppose you should have it too, and would recommend using it for portability.
Regarding multiple opening of the same file in different modes: I don't think it should be a problem, though it seems much more straightforward to open it in append mode first, close it when the input session is done, and then reopen it for reading afterwards. In a more complex program, you'd probably split these two functionalities in separate functions, each doing its own I/O, thus following that pattern.
One last thing: you probably want to test the result of your calls to scanf and the like. Perhaps a small macro like the following would help:
#define CHECK_SCAN(x) do { \
int err = (x); \
if (err <= 0) { \
printf("error on scan: %s", strerror(err)); \
exit(1); \
} } while(0)