What does the GCC error message, “Error: unsupported for `mov’”, mean?

I am just trying to compile some simple example code I typed in from a book, and GCC gives me the above error. Here’s my code:

$ cat -n test.cpp

 1  #define READ_COMMAND    3
 2  
 3  #define MSG_LENGTH  128
 4  
 5  #include <stdlib.h>
 6  #include <stdio.h>
 7  
 8  int main(int argc, char *arg[])
 9  {
10      int syslog_command = READ_COMMAND;
11      int bytes_to_read = MSG_LENGTH;
12      int retval;
13      char buffer[MSG_LENGTH];
14  
15      asm volatile(
16          "movl %1, %%ebxnt"
17          "movl %2, %%ecxnt"
18          "movl %3, %%edxnt"
19          "movl $103, %%eaxnt"
20          "int $128nt"
21          "movl %%eax, %0"
22          :"=r" (retval)
23          :"m"(syslog_command),"r"(buffer),"m"(bytes_to_read)
24          :"%eax","%ebx","%ecx","%edx");
25      if (retval > 0) printf("%sn", buffer);
26  
27  }
28  
29

The code is supposed to invoke the syslog() system call to read the last 128 bytes from the kernel printk() ring buffer. Here is some information about my OS and system configuration:

uname -a:

Linux 3.2.0-26-generic #41-Ubuntu SMP Thu Jun 14 17:49:24 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

gcc -v:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6/lto-wrapper
Target: x86_64-linux-gnu

Configured with: ../src/configure -v –with-pkgversion=’Ubuntu/Linaro 4.6.3-1ubuntu5′ –with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs –enable-languages=c,c++,fortran,objc,obj-c++ –prefix=/usr –program-suffix=-4.6 –enable-shared –enable-linker-build-id –with-system-zlib –libexecdir=/usr/lib –without-included-gettext –enable-threads=posix –with-gxx-include-dir=/usr/include/c++/4.6 –libdir=/usr/lib –enable-nls –with-sysroot=/ –enable-clocale=gnu –enable-libstdcxx-debug –enable-libstdcxx-time=yes –enable-gnu-unique-object –enable-plugin –enable-objc-gc –disable-werror –with-arch-32=i686 –with-tune=generic –enable-checking=release –build=x86_64-linux-gnu –host=x86_64-linux-gnu –target=x86_64-linux-gnu

Thread model: posix

gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5)

Heres the full error:

$ gcc test.cpp

test.cpp: Assembler messages:

test.cpp:25: Error: unsupported for `mov'

Answer

You are attempting to compile 32-bit assembly code on a 64-bit machine. The inline assembly you list compiles to:

movl -24(%rbp), %ebx
movl %rsi, %ecx       <--- error here
movl -28(%rbp), %edx
movl $103, %eax
int $128
movl %eax, %r12d

As you can see, you’re attempting to store a 64-bit register in a 32-bit register, which is illegal. More importantly, this isn’t the 64-bit ABI system call protocol either.

Try compiling with -m32 to force 32-bit ABI.

What does the GCC error message, “Error: unsupported for `mov'”, mean?

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Answer