What does the GCC error message, "Error: unsupported for `mov'", mean?
Solution 1
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.
Solution 2
you should use "movq" to move 64 bits value.
see like this:
int main(void)
{
long str_len = 15;
const char* str = "hello world!\n\r";
asm volatile(
"movl $4, %%eax\n\t"
"movl $1, %%ebx\n\t"
"movq %0, %%rcx\n\t"
"movq %1, %%rdx\n\t"
"int $0x80\n\t"
:
:"r"(str), "r"(str_len)
:"eax","ebx", "rcx", "rdx"
);
return 0;
}
Dan
Updated on July 09, 2022Comments
-
Dan almost 2 years
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, %%ebx\n\t" 17 "movl %2, %%ecx\n\t" 18 "movl %3, %%edx\n\t" 19 "movl $103, %%eax\n\t" 20 "int $128\n\t" 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("%s\n", buffer); 26 27 } 28 29
The code is supposed to invoke the
syslog()
system call to read the last 128 bytes from the kernelprintk()
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'
-
Peter Cordes over 2 yearsYes, that would get it to compile, but note that it's pointless to use 64-bit registers for
int 0x80
; the 32-bit system call is only going to look at the 32-bit register. (What happens if you use the 32-bit int 0x80 Linux ABI in 64-bit code?). So you could domov %k0, %%ecx
to force it to print the 32-bit register name when expanding%0
, instead of whatever width it picked based on the C var. -
Peter Cordes over 2 yearsOr better, use a
"c"(str)
constraint to ask for the pointer in ECX/RCX in the first place, so you don't need anymov
, justasm("int $0x80" : "=a"(retval) : ... )
as the template with no clobbers needed, not forcing the compiler to find different registers for the inputs. -
Peter Cordes over 2 yearsAlso note that the key thing was using a 64-bit destination register, not just the
movq
mnemonic. You could use plainmov
and let the assembler infer the operand-size. So yes, this does answer the question aboutmov
sizes, but for a system call wrapper this is the worst possible way to go about it, and basically wrong to use int 0x80 instead of syscall for 64-bit code. Hopefully useful to people usingmov
for other reasons, though. -
Peter Cordes over 2 yearsAlso, Linux uses just
\n
for newline, not DOS\r\n
, and certainly not\n\r
(used nowhere).