This chapter attempts to cover some of the common issues involved when writing 32-bit code, to run under Win32 or Unix, or to be linked with C code generated by a Unix-style C compiler such as DJGPP. It covers how to write assembly code to interface with 32-bit C routines, and how to write position-independent code for shared libraries.
Almost all 32-bit code, and in particular all code running under
Win32, DJGPP or any of the PC Unix variants, runs
in flat memory model. This means that the segment registers and
paging have already been set up to give you the same 32-bit 4Gb address
space no matter what segment you work relative to, and that you should
ignore all segment registers completely. When writing flat-model
application code, you never need to use a segment override or modify any
segment register, and the code-section addresses you pass to
CALL and JMP live in the same address space as
the data-section addresses you access your variables by and the
stack-section addresses you access local variables and procedure parameters
by. Every address is 32 bits long and contains only an offset part.
A lot of the discussion in section 9.4, about interfacing to 16-bit C programs, still applies when working in 32 bits. The absence of memory models or segmentation worries simplifies things a lot.
Most 32-bit C compilers share the convention used by 16-bit compilers,
that the names of all global symbols (functions or data) they define are
formed by prefixing an underscore to the name as it appears in the C
program. However, not all of them do: the ELF specification
states that C symbols do not have a leading underscore on their
assembly-language names.
The older Linux a.out C compiler, all Win32
compilers, DJGPP, and NetBSD and
FreeBSD, all use the leading underscore; for these compilers,
the macros cextern and cglobal, as given in
section 9.4.1, will still work.
For ELF, though, the leading underscore should not be used.
See also section 2.1.28.
The C calling convention in 32-bit programs is as follows. In the following description, the words caller and callee are used to denote the function doing the calling and the function which gets called.
The caller pushes the function's parameters on the stack, one after another, in reverse order (right to left, so that the first argument specified to the function is pushed last).
The caller then executes a near CALL instruction to pass
control to the callee.
The callee receives control, and typically (although this is not
actually necessary, in functions which do not need to access their
parameters) starts by saving the value of ESP in
EBP so as to be able to use EBP as a base pointer
to find its parameters on the stack. However, the caller was probably doing
this too, so part of the calling convention states that EBP
must be preserved by any C function. Hence the callee, if it is going to
set up EBP as a frame pointer, must push the previous value
first.
The callee may then access its parameters relative to EBP.
The doubleword at [EBP] holds the previous value of
EBP as it was pushed; the next doubleword, at
[EBP+4], holds the return address, pushed implicitly by
CALL. The parameters start after that, at
[EBP+8]. The leftmost parameter of the function, since it was
pushed last, is accessible at this offset from EBP; the others
follow, at successively greater offsets. Thus, in a function such as
printf which takes a variable number of parameters, the
pushing of the parameters in reverse order means that the function knows
where to find its first parameter, which tells it the number and type of
the remaining ones.
The callee may also wish to decrease ESP further, so as to
allocate space on the stack for local variables, which will then be
accessible at negative offsets from EBP.
The callee, if it wishes to return a value to the caller, should leave
the value in AL, AX or EAX depending
on the size of the value. Floating-point results are typically returned in
ST0.
Once the callee has finished processing, it restores ESP
from EBP if it had allocated local stack space, then pops the
previous value of EBP, and returns via RET
(equivalently, RETN).
When the caller regains control from the callee, the function parameters
are still on the stack, so it typically adds an immediate constant to
ESP to remove them (instead of executing a number of slow
POP instructions). Thus, if a function is accidentally called
with the wrong number of parameters due to a prototype mismatch, the stack
will still be returned to a sensible state since the caller, which
knows how many parameters it pushed, does the removing.
There is an alternative calling convention used by Win32 programs for
Windows API calls, and also for functions called by the Windows
API such as window procedures: they follow what Microsoft calls the
__stdcall convention. This is slightly closer to the Pascal
convention, in that the callee clears the stack by passing a parameter to
the RET instruction. However, the parameters are still pushed
in right-to-left order.
Thus, you would define a function in C style in the following way:
global _myfunc
_myfunc:
push ebp
mov ebp,esp
sub esp,0x40 ; 64 bytes of local stack space
mov ebx,[ebp+8] ; first parameter to function
; some more code
leave ; mov esp,ebp / pop ebp
ret
At the other end of the process, to call a C function from your assembly code, you would do something like this:
extern _printf
; and then, further down...
push dword [myint] ; one of my integer variables
push dword mystring ; pointer into my data segment
call _printf
add esp,byte 8 ; `byte' saves space
; then those data items...
segment _DATA
myint dd 1234
mystring db 'This number -> %d <- should be 1234',10,0
This piece of code is the assembly equivalent of the C code
int myint = 1234;
printf("This number -> %d <- should be 1234\n", myint);
To get at the contents of C variables, or to declare variables which C
can access, you need only declare the names as GLOBAL or
EXTERN. (Again, the names require leading underscores, as
stated in section 10.1.1.) Thus, a C variable
declared as int i can be accessed from assembler as
extern _i
mov eax,[_i]
And to declare your own integer variable which C programs can access as
extern int j, you do this (making sure you are assembling in
the _DATA segment, if necessary):
global _j
_j dd 0
To access a C array, you need to know the size of the components of the
array. For example, int variables are four bytes long, so if a
C program declares an array as int a[10], you can access
a[3] by coding mov ax,[_a+12]. (The byte offset
12 is obtained by multiplying the desired array index, 3, by the size of
the array element, 4.) The sizes of the C base types in 32-bit compilers
are: 1 for char, 2 for short, 4 for
int, long and float, and 8 for
double. Pointers, being 32-bit addresses, are also 4 bytes
long.
To access a C data structure, you need to know the offset from the base
of the structure to the field you are interested in. You can either do this
by converting the C structure definition into a NASM structure definition
(using STRUC), or by calculating the one offset and using just
that.
To do either of these, you should read your C compiler's manual to find
out how it organizes data structures. NASM gives no special alignment to
structure members in its own STRUC macro, so you have to
specify alignment yourself if the C compiler generates it. Typically, you
might find that a structure like
struct {
char c;
int i;
} foo;
might be eight bytes long rather than five, since the int
field would be aligned to a four-byte boundary. However, this sort of
feature is sometimes a configurable option in the C compiler, either using
command-line options or #pragma lines, so you have to find out
how your own compiler does it.
c32.mac: Helper Macros for the 32-bit C InterfaceIncluded in the NASM archives, in the misc directory, is a
file c32.mac of macros. It defines three macros:
proc, arg and endproc. These are
intended to be used for C-style procedure definitions, and they automate a
lot of the work involved in keeping track of the calling convention.
An example of an assembly function using the macro set is given here:
proc _proc32
%$i arg
%$j arg
mov eax,[ebp + %$i]
mov ebx,[ebp + %$j]
add eax,[ebx]
endproc
This defines _proc32 to be a procedure taking two
arguments, the first (i) an integer and the second
(j) a pointer to an integer. It returns i + *j.
Note that the arg macro has an EQU as the
first line of its expansion, and since the label before the macro call gets
prepended to the first line of the expanded macro, the EQU
works, defining %$i to be an offset from BP. A
context-local variable is used, local to the context pushed by the
proc macro and popped by the endproc macro, so
that the same argument name can be used in later procedures. Of course, you
don't have to do that.
arg can take an optional parameter, giving the size of the
argument. If no size is given, 4 is assumed, since it is likely that many
function parameters will be of type int or pointers.
ELF replaced the older a.out object file
format under Linux because it contains support for position-independent
code (PIC), which makes writing shared libraries much easier. NASM supports
the ELF position-independent code features, so you can write
Linux ELF shared libraries in NASM.
NetBSD, and its close cousins FreeBSD and OpenBSD, take a different
approach by hacking PIC support into the a.out format. NASM
supports this as the aoutb output format, so you can write BSD
shared libraries in NASM too.
The operating system loads a PIC shared library by memory-mapping the library file at an arbitrarily chosen point in the address space of the running process. The contents of the library's code section must therefore not depend on where it is loaded in memory.
Therefore, you cannot get at your variables by writing code like this:
mov eax,[myvar] ; WRONG
Instead, the linker provides an area of memory called the global
offset table, or GOT; the GOT is situated at a constant distance from
your library's code, so if you can find out where your library is loaded
(which is typically done using a CALL and POP
combination), you can obtain the address of the GOT, and you can then load
the addresses of your variables out of linker-generated entries in the GOT.
The data section of a PIC shared library does not have these restrictions: since the data section is writable, it has to be copied into memory anyway rather than just paged in from the library file, so as long as it's being copied it can be relocated too. So you can put ordinary types of relocation in the data section without too much worry (but see section 10.2.4 for a caveat).
Each code module in your shared library should define the GOT as an external symbol:
extern _GLOBAL_OFFSET_TABLE_ ; in ELF extern __GLOBAL_OFFSET_TABLE_ ; in BSD a.out
At the beginning of any function in your shared library which plans to access your data or BSS sections, you must first calculate the address of the GOT. This is typically done by writing the function in this form:
func: push ebp
mov ebp,esp
push ebx
call .get_GOT
.get_GOT:
pop ebx
add ebx,_GLOBAL_OFFSET_TABLE_+$$-.get_GOT wrt ..gotpc
; the function body comes here
mov ebx,[ebp-4]
mov esp,ebp
pop ebp
ret
(For BSD, again, the symbol _GLOBAL_OFFSET_TABLE requires a
second leading underscore.)
The first two lines of this function are simply the standard C prologue
to set up a stack frame, and the last three lines are standard C function
epilogue. The third line, and the fourth to last line, save and restore the
EBX register, because PIC shared libraries use this register
to store the address of the GOT.
The interesting bit is the CALL instruction and the
following two lines. The CALL and POP combination
obtains the address of the label .get_GOT, without having to
know in advance where the program was loaded (since the CALL
instruction is encoded relative to the current position). The
ADD instruction makes use of one of the special PIC relocation
types: GOTPC relocation. With the WRT ..gotpc qualifier
specified, the symbol referenced (here _GLOBAL_OFFSET_TABLE_,
the special symbol assigned to the GOT) is given as an offset from the
beginning of the section. (Actually, ELF encodes it as the
offset from the operand field of the ADD instruction, but NASM
simplifies this deliberately, so you do things the same way for both
ELF and BSD.) So the instruction then
adds the beginning of the section, to get the real address of the
GOT, and subtracts the value of .get_GOT which it knows is in
EBX. Therefore, by the time that instruction has finished,
EBX contains the address of the GOT.
If you didn't follow that, don't worry: it's never necessary to obtain the address of the GOT by any other means, so you can put those three instructions into a macro and safely ignore them:
%macro get_GOT 0
call %%getgot
%%getgot:
pop ebx
add ebx,_GLOBAL_OFFSET_TABLE_+$$-%%getgot wrt ..gotpc
%endmacro
Having got the GOT, you can then use it to obtain the addresses of your
data items. Most variables will reside in the sections you have declared;
they can be accessed using the ..gotoff special
WRT type. The way this works is like this:
lea eax,[ebx+myvar wrt ..gotoff]
The expression myvar wrt ..gotoff is calculated, when the
shared library is linked, to be the offset to the local variable
myvar from the beginning of the GOT. Therefore, adding it to
EBX as above will place the real address of myvar
in EAX.
If you declare variables as GLOBAL without specifying a
size for them, they are shared between code modules in the library, but do
not get exported from the library to the program that loaded it. They will
still be in your ordinary data and BSS sections, so you can access them in
the same way as local variables, using the above ..gotoff
mechanism.
Note that due to a peculiarity of the way BSD a.out format
handles this relocation type, there must be at least one non-local symbol
in the same section as the address you're trying to access.
If your library needs to get at an external variable (external to the
library, not just to one of the modules within it), you must use
the ..got type to get at it. The ..got type,
instead of giving you the offset from the GOT base to the variable, gives
you the offset from the GOT base to a GOT entry containing the
address of the variable. The linker will set up this GOT entry when it
builds the library, and the dynamic linker will place the correct address
in it at load time. So to obtain the address of an external variable
extvar in EAX, you would code
mov eax,[ebx+extvar wrt ..got]
This loads the address of extvar out of an entry in the
GOT. The linker, when it builds the shared library, collects together every
relocation of type ..got, and builds the GOT so as to ensure
it has every necessary entry present.
Common variables must also be accessed in this way.
If you want to export symbols to the user of the library, you have to declare whether they are functions or data, and if they are data, you have to give the size of the data item. This is because the dynamic linker has to build procedure linkage table entries for any exported functions, and also moves exported data items away from the library's data section in which they were declared.
So to export a function to users of the library, you must use
global func:function ; declare it as a function
func: push ebp
; etc.
And to export a data item such as an array, you would have to code
global array:data array.end-array ; give the size too array: resd 128 .end:
Be careful: If you export a variable to the library user, by declaring
it as GLOBAL and supplying a size, the variable will end up
living in the data section of the main program, rather than in your
library's data section, where you declared it. So you will have to access
your own global variable with the ..got mechanism rather than
..gotoff, as if it were external (which, effectively, it has
become).
Equally, if you need to store the address of an exported global in one of your data sections, you can't do it by means of the standard sort of code:
dataptr: dd global_data_item ; WRONG
NASM will interpret this code as an ordinary relocation, in which
global_data_item is merely an offset from the beginning of the
.data section (or whatever); so this reference will end up
pointing at your data section instead of at the exported global which
resides elsewhere.
Instead of the above code, then, you must write
dataptr: dd global_data_item wrt ..sym
which makes use of the special WRT type ..sym
to instruct NASM to search the symbol table for a particular symbol at that
address, rather than just relocating by section base.
Either method will work for functions: referring to one of your functions by means of
funcptr: dd my_function
will give the user the address of the code you wrote, whereas
funcptr: dd my_function wrt ..sym
will give the address of the procedure linkage table for the function, which is where the calling program will believe the function lives. Either address is a valid way to call the function.
Calling procedures outside your shared library has to be done by means of a procedure linkage table, or PLT. The PLT is placed at a known offset from where the library is loaded, so the library code can make calls to the PLT in a position-independent way. Within the PLT there is code to jump to offsets contained in the GOT, so function calls to other shared libraries or to routines in the main program can be transparently passed off to their real destinations.
To call an external routine, you must use another special PIC relocation
type, WRT ..plt. This is much easier than the GOT-based ones:
you simply replace calls such as CALL printf with the
PLT-relative version CALL printf WRT ..plt.
Having written some code modules and assembled them to .o
files, you then generate your shared library with a command such as
ld -shared -o library.so module1.o module2.o # for ELF ld -Bshareable -o library.so module1.o module2.o # for BSD
For ELF, if your shared library is going to reside in system directories
such as /usr/lib or /lib, it is usually worth
using the -soname flag to the linker, to store the final
library file name, with a version number, into the library:
ld -shared -soname library.so.1 -o library.so.1.2 *.o
You would then copy library.so.1.2 into the library
directory, and create library.so.1 as a symbolic link to it.