不用libc, 使用汇编实现的memcpy

main.c

void memcpy(void* dst, void* src, unsigned len);
int _start()
{
  int a[]={1,2,3,4};
  int b[4];
  memcpy(b,a,sizeof(a));
asm("movl $1,%eax;"
        "xorl %ebx,%ebx;"
        "int  $0x80"
    );
}

Makefile

AS = gcc
CC = gcc
LD = ld
ASFLAGS += -g -m32
CFLAGS += -g -O0
CFLAGS  += -march=i386 -m32 -fno-stack-protector
CFLAGS  += -fcf-protection=none # remove endbr32 in Ubuntu 20.04 with a CPU newer than Comet Lake
LDFLAGS += -melf_i386
main: main.o memcpy.o
	$(LD) $(LDFLAGS)  -o main $^
memcpy.o: memcpy.S
	$(AS) $(ASFLAGS) -c -o $@ $<
main.o: main.c
	$(CC) $(CFLAGS) -c -o $@ $<

run: main
	./main

clean:
	rm -rf *.o ./main

memcpy.S

#define PARMS	12 // 因为有2个callee-saved寄存器
#define RTN	PARMS
#define DEST	RTN
#define SRC	DEST+4
#define LEN	SRC+4

.section        .text
.global memcpy
memcpy:
  pushl %edi
  pushl %esi
	movl	DEST(%esp), %edi
	movl	SRC(%esp), %esi
	movl	LEN(%esp), %ecx

	/* We need this in any case.  */
	cld
  rep; movsb

  popl %esi
  popl %edi

	ret