RealView Compilation Tools Compiler Reference Guide - ARM ...

Compiler-specific Features
4.1.20 __writeonly
The __writeonly type qualifier indicates that a data object cannot be read from.
In the C and C++ type system it behaves as a cv-qualifier like const or volatile. Its
specific effect is that an lvalue with __writeonly type cannot be converted to an rvalue.
Assignment to a __writeonly bitfield is not allowed if the assignment is implemented as
read-modify-write. This is implementation-dependent.
Example
void foo(__writeonly int *ptr)
{
*ptr = 0;
// allowed
printf("ptr value = %d\n", *ptr); // error
}
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