问题：谁调用了 main函数？
在C语言的层面来看，main函数是一个程序的起始入口点，而实际上，ELF可执行文件的入口点并不是main而是_start。
mdb也可以反汇编_start：
_start::dis ;从_start 的地址开始反汇编
_start: pushl $0
_start+2: pushl $0
_start+4: movl %esp,%ebp
_start+6: pushl %edx
_start+7: movl $0x80504b0,%eax
_start+0xc: testl %eax,%eax
_start+0xe: je +0xf _start+0x1d
_start+0x10: pushl $0x80504b0
_start+0x15: call -0x75
_start+0x1a: addl $4,%esp
_start+0x1d: movl $0x8060710,%eax
_start+0x22: testl %eax,%eax
_start+0x24: je +7 _start+0x2b
_start+0x26: call -0x86
_start+0x2b: pushl $0x80506cd
_start+0x30: call -0x90
_start+0x35: movl +8(%ebp),%eax
_start+0x38: leal +0x10(%ebp,%eax,4),%edx
_start+0x3c: movl %edx,0x8060804
_start+0x42: andl $0xf0,%esp
_start+0x45: subl $4,%esp
_start+0x48: pushl %edx
_start+0x49: leal +0xc(%ebp),%edx
_start+0x4c: pushl %edx
_start+0x4d: pushl %eax
_start+0x4e: call +0x152 _init
_start+0x53: call -0xa3 __fpstart
_start+0x58: call +0xfb ;在这里调用了main函数
_start+0x5d: addl $0xc,%esp
_start+0x60: pushl %eax
_start+0x61: call -0xa1
_start+0x66: pushl $0
_start+0x68: movl $1,%eax
_start+0x6d: lcall $7,$0
_start+0x74: hlt
