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struct return

How a large struct comes back from a function - the caller passes a hidden address and the function writes the struct there.

Struct-return (sret) passes the address of caller-allocated result storage as a hidden argument (rdi on System V, rcx on Microsoft x64, a0 on RISC-V; x8 on AAPCS64, not counting as an argument) and returns that address in rax on x86-64; small structs instead come back in registers.

Examples

  • A 32-byte struct returned on x86-64 Linux is written to memory the caller provides, whose address arrives in rdi.

See also

AAPCS64 struct
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