Jumping to Labels in Inline Assembly
The latest version of this topic can be found at Jumping to Labels in Inline Assembly.
Microsoft Specific
Like an ordinary C or C++ label, a label in an __asm block has scope throughout the function in which it is defined (not only in the block). Both assembly instructions and goto statements can jump to labels inside or outside the __asm block.
Labels defined in __asm blocks are not case sensitive; both goto statements and assembly instructions can refer to those labels without regard to case. C and C++ labels are case sensitive only when used by goto statements. Assembly instructions can jump to a C or C++ label without regard to case.
The following code shows all the permutations:
void func( void )
{
goto C_Dest; /* Legal: correct case */
goto c_dest; /* Error: incorrect case */
goto A_Dest; /* Legal: correct case */
goto a_dest; /* Legal: incorrect case */
__asm
{
jmp C_Dest ; Legal: correct case
jmp c_dest ; Legal: incorrect case
jmp A_Dest ; Legal: correct case
jmp a_dest ; Legal: incorrect case
a_dest: ; __asm label
}
C_Dest: /* C label */
return;
}
int main()
{
}
Don't use C library function names as labels in __asm blocks. For instance, you might be tempted to use exit as a label, as follows:
; BAD TECHNIQUE: using library function name as label
jne exit
.
.
.
exit:
; More __asm code follows
Because exit is the name of a C library function, this code might cause a jump to the exit function instead of to the desired location.
As in MASM programs, the dollar symbol ($) serves as the current location counter. It is a label for the instruction currently being assembled. In __asm blocks, its main use is to make long conditional jumps:
jne $+5 ; next instruction is 5 bytes long
jmp farlabel
; $+5
.
.
.
farlabel:
END Microsoft Specific