于 2011/8/27 下午 10:40, Maciej Bliziński 写道:
2011/8/27 uulinux<uulinux@xxxxxxxxx>:
localhost array # gcc -o main main.c
localhost array # ./main
1 2 3 -1218524971 -944870872
===============================================
I want to know, what is wrong with it?
I'm not an expert, but it seems to me that in the expression "int
a[5]={1, 2, 3, a[2], a[0]+a[1]};" when you say e.g. a[2], you are
referring an uninitialized array. Your code, as far as initialization
goes, would be equivalent to:
int b[5];
int a[5]={1, 2, 3, b[2], b[0]+b[1]};
In this example, it's clear what's wrong. Your code has the same
problem, but slightly obfuscated.
If you try to do the same in Python, you get:
a = [1, 2, 3, a[2], a[0]+a[1]]
Traceback (most recent call last):
File "<stdin>", line 1, in<module>
NameError: name 'a' is not defined
Which is exactly the same problem you get in C, but exposed better.
Maciej
Dear Maciej:
Thanks very mach. I am so glad to get a replay. I seem to know what you
mean. The auto variable allocated from stack, whose value is initialized
by code in runtime, not by compiler.
But look the assembler code compiled by gcc-4.4.5.
=========================================================
.file "main.c"
.section .rodata
.LC0:
.string "\t%d"
.text
.globl main
.type main, @function
main:
pushl %ebp
movl %esp, %ebp
andl $-16, %esp
subl $48, %esp
movl 32(%esp), %eax
movl 24(%esp), %ecx
movl 28(%esp), %edx
leal (%ecx,%edx), %edx
movl $1, 24(%esp)
movl $2, 28(%esp)
movl $3, 32(%esp)
movl %eax, 36(%esp)
movl %edx, 40(%esp)
movl $0, 44(%esp)
movl $0, 44(%esp)
jmp .L2
.L3:
movl 44(%esp), %eax
movl 24(%esp,%eax,4), %edx
movl $.LC0, %eax
movl %edx, 4(%esp)
movl %eax, (%esp)
call printf
addl $1, 44(%esp)
.L2:
cmpl $4, 44(%esp)
jle .L3
movl $10, (%esp)
call putchar
movl $0, %eax
leave
ret
.size main, .-main
.ident "GCC: (Gentoo 4.4.5 p1.2, pie-0.4.5) 4.4.5"
.section .note.GNU-stack,"",@progbits
=========================================================
Please notice this code compiled from "int a[5] = [1, 2, 3, a[2],
a[0]+a[1]]":
movl $1, 24(%esp)
movl $2, 28(%esp)
movl $3, 32(%esp)
movl %eax, 36(%esp)
movl %edx, 40(%esp)
I just doubt whether there is something wrong or not very good. I
expect code as following:
movl $1, 24(%esp)
movl $2, 28(%esp)
movl $3, 32(%esp)
movl 32(%esp), %eax
movl %eax, 36(%esp)
movl 24(%esp), %edx
movl 28(%esp), %eax
leal (%edx,%eax), %eax
movl %eax, 40(%esp)
And I expect "int a[5] = [1, 2, 3, a[2], a[0]+a[1]]" is equivalent to:
int a[5] = [1, 2, 3];
a[3] = a[2];
a[4] = a[0]+a[1];
If the array is global variable and which will be put in .data
segment by compiler, gcc-4.4.5 will report error as following. But I
think this kind method of initialization is useful.
=========================================================
localhost array # gcc -o main main.c
main.c:28: error: initializer element is not constant
main.c:28: error: (near initialization for 'a[3]')
main.c:28: error: initializer element is not constant
main.c:28: error: (near initialization for 'a[4]')
=========================================================
