Why doesn't gcc compile this code sequentially?

Question

I am working on a lab assignment for the C programming class, which I take. I wrote the code in my local Cygwin directory, compiled it with gcc, and the executable that is created works exactly the way I want, without errors.

When I copy my code to my school UNIX server and compile it with gcc, I get no errors, but when I try to run it, nothing happens.

I tried to do gcc 2darray.c -Wall -pedantic, and this is what was returned:

2darray.c: In function 'main':
2darray.c:5:3: warning: missing braces around initializer [-Wmissing-braces]
2darray.c:5:3: warning: (near initialization for 'M[0]') [-Wmissing-braces]
2darray.c:5:24: warning: C++ style comments are not allowed in ISO C90 [enabled by default]
2darray.c:5:24: warning: (this will be reported only once per input file) [enabled by default]

Errors mention something about array initialization M, but I don't see any problems with the way I initialized it. Here is the code I'm trying to compile:

#include <stdio.h>

int main(void)
{
  int M[10][10] = {0}; // creating a 10x10 array and initializing it to 0
  int i, j; // loop variables
  int sum[10] = {0}; // creating an array to hold the sums of each column of 2d array M

  for (i = 1; i < 10; i++) // assigning values to array M as specified in directions
    {
      for (j = i - 1; j < i; j++)
        {
          M[i][j] = -i;
          M[i][j+1] = i;
          M[i][j+2] = -i;
        }
    }

  for (i = 0; i < 10; i++) // printing array M
    {
      for(j = 0; j < 10; j++)
        {
          printf("%3d", M[i][j]);
        }
      printf("\n");
    }

  printf("\n");
  for (i = 0; i < 10; i++) // calculating sum of each column
    {
      for (j = 0; j < 10; j++)
        {
          sum[i] = M[j][i] + sum[i];
        }
      printf("%3d", sum[i]);  // printing array sum
    }

  return 0;
}

printf for , , , - ?

, Cygwin UNIX:

  0  0  0  0  0  0  0  0  0  0
 -1  1 -1  0  0  0  0  0  0  0
  0 -2  2 -2  0  0  0  0  0  0
  0  0 -3  3 -3  0  0  0  0  0
  0  0  0 -4  4 -4  0  0  0  0
  0  0  0  0 -5  5 -5  0  0  0
  0  0  0  0  0 -6  6 -6  0  0
  0  0  0  0  0  0 -7  7 -7  0
  0  0  0  0  0  0  0 -8  8 -8
  0  0  0  0  0  0  0  0 -9  9

 -1 -1 -2 -3 -4 -5 -6 -7 -8  1

Answer 1

M , undefined.

for (i = 1; i < 10; i++) 
// i ranges from 1 to 9
  {
    for (j = i - 1; j < i; j++)
    // j ranges from i-1 (0 when i==1) to i-1 (8 when i==9)
    // Consider what happens when j==8
      {
        M[i][j] = -i;       // j == 8
        M[i][j+1] = i;      // j == 9
        M[i][j+2] = -i;     // j == 10, out of bounds
      }
  }

, j+2 . :

      M[i][j] = -i;
      M[i][j+1] = i;
      if (j+2 >= 10) puts("OUT OF RANGE");
      M[i][j+2] = -i;

, OUT OF RANGE.

, gcc -Wall -pedantic, . // , -std=c99. " " . , {0} (array, struct, union) . gcc ( , 5.2.0) .

Answer 2

clang gcc , . , .

$ gcc s.c -Wall -Wextra -fsanitize=undefined
$ ./a.out
s.c:15:15: runtime error: index 10 out of bounds for type 'int [10]'
s.c:15:21: runtime error: store to address 0x7fff6c53f2c0 with insufficient space for an object of type 'int'
0x7fff6c53f2c0: note: pointer points here
 09 00 00 00  e5 ef 74 8a 11 7f 00 00  08 00 00 00 09 00 00 00  b0 10 40 00 00 00 00 00  00 00 00 00
              ^

-, cygwin libubsan, -fsanitize-undefined-trap-on-error, , gdb. , - , , , .