C: fread does not read the full block - c

Question

Here's my code so far:
#include <stdio.h>
#include <stdint.h>
#define N_DET 432
#define N_PROJ 500
int readSinogram(uint16_t mess[],char filename[], int sample){
//init
FILE *fp;
int i;
//open file
fp=fopen(filename,"r");
//read sinogram
fseek(fp,sizeof(uint16_t)*N_DET*N_PROJ*sample*2,SEEK_SET); //move pointer
fread(mess,sizeof(uint16_t),N_DET*N_PROJ,fp); //read sinogram
return 0;
};
int main()
{
uint16_t mess[N_DET*N_PROJ];
char filename[]="C:\\Path\\MyFile.fxc";
double curr;
int i,DET,PROJ;
readSinogram(mess,filename,0); //read the data
printf("\nDET?"); //ask for input
scanf("%u", &DET);
printf("\nPROJ?");
scanf("%u", &PROJ);
curr=mess[DET+N_DET*PROJ]; //get the data
printf("Data: %f",curr); //print the data
return 0;
}
This reads in the .fxc file, which is just a binary file containing uint16_t formatted numbers. "readSinogram" reads one data set which contains N_DET*N_PROJ numbers. (The pointer is moved by twice the block size, because there are alternating blocks of two measurements in the file.)
After reading it, one can put in a DET and PROJ and have a look at the data at this point.
So far, everything works fine. The data is correct for a certain data range but when asking for too big DET and/or PROJ the data is incorrect.
I do have the same file read in in Matlab and can confirm that the data in it is fine.
To be more exact: every index above [DET+N_DET*PROJ] > 248835 will return 52428 instead of the correct value (ranging from 0 to 4088). The values up to those work fine. So there's got to be something wrong with the "mess" array above that index, I guess.
What am I doing wrong? Thanks in advance!

Answer 1

you need allocate a larger array to get to index value greater than 248835
currently you have the following defines
#define N_DET 432
#define N_PROJ 500
which leads to the array size of 432*500 = 216000
now if your indexing the array at value 248835 248835 > 216000 you would cause to access the memory out of the allocated memory for the array which would lead to undefined behavior. What you need is a bigger size array which can accommodate more than 248835 entries. a simple #define N_DET 500 would do that, but you have to make sure that is the requirement. A basic index check would be helpful to avoid having a out of range problem something on the lines of
#define MAX_RANGE 500*500
if((DET+N_DET*PROJ) < MAX_RANGE)
curr=mess[DET+N_DET*PROJ]; //get the data
else
//error handling
you also need error handling in fopen fseek and check the return for fread plus optionally you can close the opened file discriptor by close(fd)