Using Variables in Program Memory

Variables in program memory can be used almost exactly like variables in data memory. The exceptions are presented below:

• The __pmem qualifier cannot be used in a structure declaration because a structure can have all its members either in program memory or in data memory, but not in both memory spaces. The compiler will issue an error message in this case. For example:

  typedef struct // simple structure declaration
  {
     int i;
     char __pmem *p; // error, __pmem not allowed here
     long l;
  } test;

• The compiler will signal an error when an implicit conversion between a pointer to data in data memory and a pointer to data in program memory is attempted. For example, using the previous definitions, the compiler gives an error for this assignment:

  pxp1 = ppx1;

Explicit conversions are allowed, but they should be used with care. An explicit conversion for the previous assignment that is accepted by the compiler is given below:

pxp1 = ( __pmem int * )ppx1;

Another consequence of this restriction is that an important part of the MSL functions that have at least an argument that is a pointer will not work with variables in program memory. For example:

char *c1; // pointer in data memory to char in data memory
char __pmem *c2; // pointer in data memory to char in program
memoryvstrcat( c1, c2 ); // error, the second argument can't be converted to 'const char *'

If variable argument lists are used, this problem is generally hidden. The program is compiled with no errors from the compiler, but it doesn't work as expected. The most common example is the printf function:

char *c1 = "xmem"; // pointer in data memory to char in data memory
char __pmem *c2 = "pmem"; // pointer in data memory to char in program memory
printf( "%s\n", c1 );     // works as expected
printf( "%s\n", c2 );     // doesn't work as expected

Here, the type of the arguments is lost because printf uses a variable argument list. Thus the compiler can not signal a type mismatch and the program will compile without errors, but it won't work as expected, because printf assumes that all the data is stored in data memory.