Calling C functions

EXTERN and linking

EXTERN declares a symbol defined in another object, like a libc function. Link against libc with gcc, which adds the startup code and the dynamic linker flags:

extern printf section .text global main main: ; ... call printf wrt ..plt

The System V ABI

Integer arguments go in RDI, RSI, RDX, RCX, R8, R9; floating-point arguments in XMM0-XMM7; the return value in RAX (or XMM0). AL must hold the number of vector registers used when calling a variadic function like printf:

mov rdi, fmt mov rsi, rcx xor eax, eax ; zero vector regs for printf call printf wrt ..plt

A printf example

The wrt ..plt suffix routes the call through the procedure linkage table, which the dynamic linker fills in. For a program that only uses syscalls, plain ld works; with libc, link with gcc instead.

Next: Debugging with GDB

Article author: Arthur Isaev