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Say, I want to call a function with the following signature in inline assembly of gcc. How can I do that?
int some_function( void * arg );
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There are, in general, two types of inline assembly supported by C/C++ compilers: asm (or __asm__) in GCC. GCC uses a direct extension of the ISO rules: assembly code template is written in strings, with inputs, outputs, and clobbered registers specified after the strings in colons.
In assembly language, the call instruction handles passing the return address for you, and ret handles using that address to return back to where you called the function from. The return value is the main method of transferring data back to the main program.
The __asm keyword invokes the inline assembler and can appear wherever a C or C++ statement is legal. It cannot appear by itself. It must be followed by an assembly instruction, a group of instructions enclosed in braces, or, at the very least, an empty pair of braces.
Generally you'll want to do something like
void *x;
asm(".. code that writes to register %0" : "=r"(x) : ...
int r = some_function(x);
asm(".. code that uses the result..." : ... : "r"(r), ...
That is, you don't want to do the function call in the inline asm at all. That way you don't have to worry about details of the calling conventions, or stack frame management.
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