Why do I get a false "array subscript is above array bounds" warning message when compiling my MEX file with GCC on MATLAB 7.10 (R2010a)?

When I compile the following code:

typedef struct {

    int chunks[3];

} S;

void f(int y[])

{

      int nc;

      int i = 0;

      for (nc = 0; nc < 3; nc = nc + 1) {

        i = i + 1;

      }

      if (i < 3) {

        y[i] = 0;

      }

}

int main(void)

{

     S tmp;

     f(tmp.chunks);

     return 0;

}

with this command:

gcc -Wall -O3 s3.c

I get the following warning message:

 s3.c: In function 'main':

 s3.c:15: warning: array subscript is above array bounds

Answer 1

[MathWorks Support Team]

This is a bug in GCC 4.3.4. For more information on this, please see the following webpage:

 <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43949>

GCC 4.4.3 has this bug fixed.

This is a false warning, and can be ignored. The -O3 switch inlines the function f() within the function main(). Now, when analyzing the inlined code, the compiler knows that y has 3 elements. However, it failes to consider the fact that on line 15, i is guaranteed to be less than 3 by the condition on line 14, so out of bounds subscript is not possible.

If the -Werror switch is specified, the warning is treated as an error, and the compilation will fail.

To work around this issue, you can either:

1. Compile the code with the –Wno-array-bounds switch

2. If in a header file, add the following code fragment to the header

// suppress bogus warning when compiling with gcc 4.3

#if (__GNUC__ == 4 && __GNUC_MINOR__ == 3)

#pragma GCC diagnostic ignored "-Warray-bounds"

#endif