how c compiler treats a struct return value from a function, in ASM

Question

When speaking about C function\'s return value, the return value is stored in the EAX register. Suppose we are speaking about 32 bit register, integers are welcomed

Answer 1

In common x86 calling conventions, objects that fit in two registers are returned in RDX:RAX. This is the same register pair that is an implicit input/output for div and mul instructions, and for cdq / cqo (sign extend e/rax into e/rdx).

The i386 Linux (SysV) calling convention only returns 64bit integers that way. Structs (even a struct consisting of a single int32_t) use the hidden-parameter method instead of being packed eax or edx:eax. 64bit Linux, and Microsoft's current standard __vectorcall, both pack structs into e/rax, or e/rdx:e/rax.

Many calling conventions handle larger objects by adding a hidden extra parameter: a pointer to space for storing the return value. Consult the ABI documentation for the specific ABI you are using. (links at in the x86 wiki).

Compared to other calling conventions discussed in comments (e.g. implicitly using space on the stack to store large objects being returned), passing a pointer can save a copy, because the pointer can point to the final destination instead of scratch space on the stack.