Invalid conversion between named address space pointers - pointer assignment

Defined requirement

Under the Embedded C address-space model, an implicit conversion between a pointer to a named address space and a generic pointer, in either direction, is a constraint violation. A conforming implementation must emit a diagnostic for assignments such as:

__address_space(__mem) int *src;
int *dst = src; /* invalid: implicit named-address-space conversion */

The permitted fix is an explicit cast:

int *dst = (int *)src;

The cast removes the diagnostic, but the pointer value itself remains implementation-defined unless the toolchain documents how named and generic address spaces relate.

Confirmed facts versus open behavior

Confirmed: the constraint-violation rule applies to pointer assignment, initialization, parameter passing, and return values when the types differ only by named address-space qualification.

Open: the Embedded C technical report does not define how memcpy, memmove, strcpy, or related functions treat named-address-space pointers. The standard therefore does not require an implementation to diagnose or reject those calls. Each toolchain chooses one of these approaches:

• Library prototypes require generic pointers, so a named-address-space argument is a constraint violation unless cast.
• The toolchain provides overloads or intrinsics that accept named-address-space pointers and document address-space-aware semantics.
• The implementation accepts the pointers but treats them as generic, which may produce incorrect copies for physically separate memories.

Steps for this case

1. Replace the implicit assignment with an explicit cast to a generic pointer and confirm the diagnostic disappears.
2. Before calling any standard library routine, cast named-address-space pointers to generic pointers if the toolchain's prototypes require generic arguments.
3. If the copy must preserve address-space mapping, use the toolchain's documented address-space-aware copy routine or implement a byte loop using the original pointer types.
4. Consult the target compiler manual under "named address spaces" and "library functions" to determine which of the three behaviors above applies.

Portability assessment

Portable code should not depend on undocumented library acceptance of named-address-space pointers. Use explicit casts to generic pointers for standard library calls, or isolate address-space-specific copies behind vendor-specific wrappers. Relying on implicit library support makes the program non-portable across compilers that implement the Embedded C extensions minimally.

Missing diagnostic detail

Which embedded compiler and version are you targeting? This determines whether a cast is sufficient for library calls or whether a vendor-specific copy routine is required.