Segmentation error without defining an unused array

Question

I am trying to write a simple program to output in hexadecimal the first 16 kilobytes of a binary file (Boy Boy ROM) in 16-bit chunks. However, during the for loop, my program will invariably execute segfault, however it always segfaults at another point in the array. Here is the code:

 #include <stdio.h> #include <stdint.h> int main () { uint16_t buffer[8000]; FILE* ROM = fopen("rom.gb", "rb"); if (ROM == NULL) { printf("Error"); fclose(ROM); return 1; } fread(buffer, sizeof(buffer), 1, ROM); int i; for(i = 0; i < sizeof(buffer); ++i) { if (buffer[i] < 16) { printf("000%x ", buffer[i]); } else if (buffer[i] < 256) { printf("00%x ", buffer[i]); } else if (buffer[i] < 4096) { printf("0%x ", buffer[i]); } else { printf("%x ", buffer[i]); } } fclose(ROM); return 0; } 

Before I switched to using uint16_t instead of char (since Game Boy has a 16-bit address space), this did not happen, and in fact, if I include an ad

 unsigned char buffer2[16000]; 

next to the declaration of the first buffer, I get the expected result. So my questions are: why does adding an unused variable stop the program from segfault? And how can I avoid this and declare a huge array that is not completely used in the program?

Answer 1

In this line:

 for(i = 0; i < sizeof(buffer); ++i) 

sizeof(buffer) - the size of the array in bytes, if you want the number of elements to use

i < (sizeof(buffer) / sizeof(buffer[0]))

Answer 2

I'm not sure if you are using fread correctly if you look at the documentation , the second argument is the size of each element, while you pass the size of the entire buffer.

You will probably want to exchange the second and third arguments, see here for usage examples.

Also note, as Alter Mann pointed out, that you need to divide the buffer size by the size of its type.