Suggestions

Parameter and local variable declarations should be indented and lined up:

        public  void  function( parameter1,
                                parameter2 )
            char          parameter1[];
            roi_struct    *parameter2;
        {
            int       i;
            float     current_value;
            float     min, max, average;

            for( i = 0;  i < MAX_ROIS;  ++i )
            {
                ...
            }
        }

Try to avoid using the C ability to perform operations within evaluations:
UNDESIRABLE:

        if( x == c++ )
        {
            function();
        }

        if( (x = y) < threshold )
        {
            function();
        }

        a[i++] = 0;

        for( i = 0, j = n;  i < n;
             ++i, --j )

        j = n;
        for( i = 0;  i < n;  ++i )
        {
            --j;
        }

Use your own judgement, and keep in mind that, in general, the priority is readability.

Use a 'for' loop only for loops that actually function as 'for' loops:
UNDESIRABLE:

        for( i = 0;  str[i] == 'k';  ++i )
        {
            ...
        }

        i = 0;
        while( str[i] \!= 'k' )
        {
            ...

            ++i;
        }

For pointer parameters, use the appropriate choice of '*' or '[ ]' to indicate what the parameter represents:

        public  void  validate( list,
                                num_errors )
            int    list[];
            int    *num_errors;
        {
        }

In this case the first parameter represents an array of integers and the second parameter represents a pointer to a single integer, i.e., passed by reference.

Where efficiency is not an issue, treat arrays as arrays, i.e. array addressing is preferred over pointer addressing:
UNDESIRABLE:

          s = string;

          for( i = 0;  i < strlen( s ); ++i )
          {
              if( *s == 'a' )
              {
                  break;
              }
              ++s;
          }

          s = string;

          for( i = 0;  i < strlen( s ); ++i )
          {
              if( s[i] == 'a' )
              {
                  break;
              }
          }