fread() not working correctly? - c

Question

I have this code that is meant to read from a text file, store the information in a bin file and then read the information from the bin file and project it onto the screen.
I have the writing to the bin file all correct except when I go to print the tempStudents to the screen it always says the LAST option that is in the text file. So it's as if the only student it's saving is the last student.
#include<stdlib.h>
#include<stdio.h>
struct student {
char name[200];
float marks;
};
int main() {
FILE * txtFile;
struct student tempStudent;
// File pointer to binary file
FILE * binFile;
int searchNum;
if ((txtFile = fopen("/Users/Ash/Desktop/Lab 8B/input.txt", "r")) == NULL) {
printf("Can not open file input.txt\n");
}
else {
FILE * binFile;
binFile = fopen("/Users/Ash/Desktop/Lab 8B/binFile.bin","w+b");
while (fscanf(txtFile,"%s %f", tempStudent.name, &(tempStudent.marks)) == 2) {
fwrite(tempStudent.name,sizeof(char),sizeof(tempStudent.name),binFile);
fwrite(&tempStudent.marks,sizeof(int),1,binFile);
}
printf("Please enter the student you want to search for\n");
printf("For example if you want the first student type 1\n");
scanf("%d", &searchNum);
int i = 0;
for (i = 0; i <= searchNum; i++)
{
fread(tempStudent.name, 60, sizeof(char),binFile);
fread(&tempStudent.marks,60, sizeof(int),binFile);
}
// write code that reads in the student structure that the user asked for
// from the binary file and store it in the variable tempStudent
printf("The student name retreived is: %s\n", tempStudent.name);
printf("The student mark retreived is: %.2f\n", tempStudent.marks);
fclose(binFile);
fclose(txtFile);
}
return 0;
}

Answer 1

A file has something like a current position. After writing the binary data to the file this position is at the end. When you (try to) read the bin data in this state fread will read nothing.
Check the return values!

Answer 2

You are always searching one too far:
for (i = 0; i <= searchNum; i++)
if you want the first student (searchNum = 1), then you will in fact do two reads. Usually this gets you "one more than the value I intended to read".
More critically, if you are reading and writing from a file, you need to make sure you start at the right place. For this you have the fseek() function. Instead of doing lots of reads / writes, just make sure you are in the right place before reading or writing.
More importantly still, you seem to have variable length records for your name and marks - this makes the whole thing a giant mess. Some recommendations:
Make the record length constant - in this way you will be able to fseek to a particular record without having to read all the previous records first.
Be VERY CAREFUL about reading and writing to a file at the same time; consider first writing all the inputs to file, closing the file, then opening for reading
Make sure you read the right number of bytes... don't just hard wire "60".
Learn about fseek(). Google it.

Answer 3

You are writing 60*sizeof(int) bytes of data into a single float element:
struct student {
char name[200];
float marks;
};
struct student tempStudent;
fread(&tempStudent.marks,60,sizeof(int),binFile);
Surely you can't expect this code to work!!!