How does fread() in C work inside a for loop? - c

Question

I am new to C programming, but I need it to read some binary file which I describe below.
The India Meteorological Department (IMD) has provided historical weather data in .GRD files in their website. They have also provided sample C code to read those files. From their sample C code, I have written the following code that extracts the daily minimum temperatures on 15 April 1980 recorded on a 31x31 grid over India.
/* This program reads binary data for 365/366 days and writes in ascii file. */
#include <stdio.h>
int main() {
float t[31][31];
int i,j ,k;
FILE *fin,*fout;
fin = fopen("C:\\New folder\\Mintemp_MinT_1980.GRD","rb"); // Input file
fout = fopen("C:\\New folder\\MINT15APR1980.TXT","w"); // Output file
fprintf(fout,"Daily Minimum Tempereture for 15 April 1980\n");
if(fin == NULL) {
printf("Can't open file");
return 0;
}
if(fout == NULL) {
printf("Can't open file");
return 0;
}
for(k=0 ; k<366 ; k++) {
fread(&t,sizeof(t),1,fin);
if(k == 105) {
for(i=0 ; i < 31 ; i++) {
fprintf(fout,"\n") ;
for(j=0 ; j < 31 ; j++)
fprintf(fout,"%6.2f",t[i][j]);
}
}
}
fclose(fin);
fclose(fout);
return 0;
}
/* end of main */
The file Mintemp_MinT_1980.GRD can be downloaded from the IMD website by selecting the year as 1980 against Minimum Temperature.
What I don't understand is that how the fread() function actually works in the line fread(&t,sizeof(t),1,fin) within the loop for(k=0 ; k<366 ; k++). In plain sight, the arguments of fread() here do not depend on the looping variable k, and so it should read the same data to the matrix t[31][31] for every k. However, I have checked that, surprisingly, the data extracted by this program are different for different values of k in the line if(k == 105), i.e., the data extracted for k == 105 and k == 32 are different, for example.
I would very much appreciate if one can please explain the above.

Answer 1

Files contain sequential data. All the file operators are based on the premise that whatever you do to a file, you'll generally be doing it in a sequential way.
So when you read data, and then read more data, you will be getting sequential chunks of the file. The both the FILE datatype and the operating system itself do a number of things for you, including keeping track of your current position in the file and doing block buffering in memory to improve performance.
If you wanted to reread the same data over, or skip around in the file, you would need to use fseek() to change positions in the file before doing your next read.