error: expected string-literal before ‘command’ / reusing asm volatile - c++

Question

Im trying to run the following code, which writes to a port.
void write(uint8_t size, uint8_t data, uint16_t port_number){
char command[] = "out_ %0, %1";
command[3] = size; //outb, outw or outl
__asm__ volatile(command: : "a"(data), "Nd"(port_number) );
}
i get tet the followng error message:
test.cpp:32:22: error: expected string-literal before ‘command’
__asm__ volatile(command : : "a"(data), "Nd"(port_number) );
if i try running this line:
__asm__ volatile("outb %0, %1": : "a"(data), "Nd"(port_number) );
it compiles without problem.
Now, i know i can solve this by creating 3 versions of the write function, one for each form of 'out', but is there a better way of solving this problem without copy-pasting code?

Answer 1

The first argument of __asm__ volatile(), AsmTemplate, must be a string literal.
For each output sizes, you just need to call the function with the corresponding literal. You can use a branching here using if-else or switch-case:
void write(uint8_t size, uint8_t data, uint16_t port_number){
char command[] = "out_ %0, %1";
command[3] = size; //outb, outw or out
switch(size) {
case 'b': __asm__ volatile("outb %0, %1": :"a"(data), "Nd"(port_number) ); break;
case 'w' : __asm__ volatile("outw %0, %1": :"a"(data), "Nd"(port_number) ); break;
case 'l' : __asm__ volatile("outl %0, %1": :"a"(data), "Nd"(port_number) ); break;
}
}
Add the cases to cover every possible 'size' values.
You can shorten each line by using a #define with size as its argument