我正在尝试编写一个简单的程序来打印出一个 C 字符串，而不使用其中一个 linux 系统调用或标准 C 库函数。这仅用于学习目的，我永远不会在生产中这样做（除非我真的很擅长它=））。
首先我的系统信息：
[mehoggan@fedora sandbox-print_chars]$ uname -a
Linux fedora.laptop 2.6.35.14-106.fc14.i686.PAE #1 SMP Wed Nov 23 13:39:51 UTC 2011 i686 i686 i386 GNU/Linux
[mehoggan@fedora sandbox-print_chars]$ gcc --version
gcc (GCC) 4.5.1 20100924 (Red Hat 4.5.1-4)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
接下来是代码：
#include <unistd.h>
#include <sys/syscall.h>
void main()
{
char *str = "Hello World";
while(*(str) != '\0') {
//printf("%c", *(str++));
//syscall(__NR_write, 1, *(str++), 1);
__asm__( "movl %0, %%ecx" :"=c" (str));
__asm__( "movl $0X4, %eax" );
__asm__( "movl $0X1, %ebx" );
__asm__( "movl $0X1, %edx" );
__asm__( "int $0X80" );
str++;
}
return;
}
使用以下 makefile 编译：
all: sandbox_c
sandbox_c: sandbox.c
gcc -Wall -o sandbox_c ./sandbox.c
gcc -S -Wall -o sandbox_c.asm ./sandbox.c
事情编译得很好，我只是无法正确使用语法来让事情正常工作。非常感谢您的更正，但如果您也可以指出您是如何获得解决方案的，那就太好了。我正在尝试更好地使用手册页等。
添加通过gdb 运行可执行文件我可以看到 ecx 没有指向正确的地址：
[mehoggan@fedora sandbox-print_chars]$ gdb ./sandbox_c
GNU gdb (GDB) Fedora (7.2-52.fc14)
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-redhat-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/mehoggan/Code/Assembly/sandbox/sandbox-print_chars/sandbox_c...done.
(gdb) break sandbox.c:8
Breakpoint 1 at 0x80483a2: file ./sandbox.c, line 8.
(gdb) run
Starting program: /home/mehoggan/Code/Assembly/sandbox/sandbox-print_chars/sandbox_c
Breakpoint 1, main () at ./sandbox.c:8
8 while(*(str) != '\0') {
Missing separate debuginfos, use: debuginfo-install glibc-2.13-2.i686
(gdb) step
11 __asm__( "movl %0, %%ecx" :"=c" (str));
(gdb) info registers
eax 0x48 72
ecx 0x34092fad 873017261
edx 0x1 1
ebx 0x567ff4 5668852
esp 0xbffff2a4 0xbffff2a4
ebp 0xbffff2b8 0xbffff2b8
esi 0x0 0
edi 0x0 0
eip 0x80483a4 0x80483a4 <main+16>
eflags 0x200206 [ PF IF ID ]
cs 0x73 115
ss 0x7b 123
ds 0x7b 123
es 0x7b 123
fs 0x0 0
gs 0x33 51
(gdb) step
12 __asm__( "movl $0X4, %eax" );
(gdb) info registers
eax 0x48 72
ecx 0x34092fad 873017261
edx 0x1 1
ebx 0x34092fad 873017261
esp 0xbffff2a4 0xbffff2a4
ebp 0xbffff2b8 0xbffff2b8
esi 0x0 0
edi 0x0 0
eip 0x80483ab 0x80483ab <main+23>
eflags 0x200206 [ PF IF ID ]
cs 0x73 115
ss 0x7b 123
ds 0x7b 123
es 0x7b 123
fs 0x0 0
gs 0x33 51
(gdb) step
13 __asm__( "movl $0X1, %ebx" );
(gdb) info registers
eax 0x4 4
ecx 0x34092fad 873017261
edx 0x1 1
ebx 0x34092fad 873017261
esp 0xbffff2a4 0xbffff2a4
ebp 0xbffff2b8 0xbffff2b8
esi 0x0 0
edi 0x0 0
eip 0x80483b0 0x80483b0 <main+28>
eflags 0x200206 [ PF IF ID ]
cs 0x73 115
ss 0x7b 123
ds 0x7b 123
es 0x7b 123
fs 0x0 0
gs 0x33 51
(gdb) step
14 __asm__( "movl $0X1, %edx" );
(gdb) info registers
eax 0x4 4
ecx 0x34092fad 873017261
edx 0x1 1
ebx 0x1 1
esp 0xbffff2a4 0xbffff2a4
ebp 0xbffff2b8 0xbffff2b8
esi 0x0 0
edi 0x0 0
eip 0x80483b5 0x80483b5 <main+33>
eflags 0x200206 [ PF IF ID ]
cs 0x73 115
ss 0x7b 123
ds 0x7b 123
es 0x7b 123
fs 0x0 0
gs 0x33 51
(gdb) step
15 __asm__( "int $0X80" );
(gdb) info registers
eax 0x4 4
ecx 0x34092fad 873017261
edx 0x1 1
ebx 0x1 1
esp 0xbffff2a4 0xbffff2a4
ebp 0xbffff2b8 0xbffff2b8
esi 0x0 0
edi 0x0 0
eip 0x80483ba 0x80483ba <main+38>
eflags 0x200206 [ PF IF ID ]
cs 0x73 115
ss 0x7b 123
ds 0x7b 123
es 0x7b 123
fs 0x0 0
gs 0x33 51
(gdb) step
16 str++;
(gdb) info registers
eax 0xfffffff2 -14
ecx 0x34092fad 873017261
edx 0x1 1
ebx 0x1 1
esp 0xbffff2a4 0xbffff2a4
ebp 0xbffff2b8 0xbffff2b8
esi 0x0 0
edi 0x0 0
eip 0x80483bc 0x80483bc <main+40>
eflags 0x200206 [ PF IF ID ]
cs 0x73 115
ss 0x7b 123
ds 0x7b 123
es 0x7b 123
fs 0x0 0
gs 0x33 51