[SOLVED] How can I call printf function correctly from assembly?

How can I call printf function correctly from assembly?

I am trying to call an x86 C function, printf, in assembly, but there is a linking error: error LNK2019: unresolved external symbol _printf referenced in function _main.

.model flat, C
.code
extern printf: proc

main proc
    push offset msg
    push offset fmt
    call printf
    ret
main endp

.data
    msg db "Hello world!", 0
    fmt db "%s", 0
end

How can I solve that problem?

Solution

here can be several ways. if want use existing libs - _printf was in legacy_stdio_definitions.lib but if use it need also link to legacy_stdio_wide_specifiers.lib and ucrt.lib, call __initterm ( or set entry point to _wmainCRTStartup ) complete code in this case can look like

.686

.model flat

extern _printf: PROC
extern __imp__exit: DWORD
extern __imp___initterm: DWORD

.code

ep proc

    push offset __xc_z
    push offset __xc_a
    call __imp___initterm
    add esp,8
    
    push offset msg
    push offset fmt
    call _printf
    add esp,8
    
    push eax
    call __imp__exit
    
    ret
ep endp

CRT$XIA SEGMENT ALIAS(".CRT$XIA")
__xc_a DD 0
CRT$XIA ENDS

CRT$XIZ SEGMENT ALIAS(".CRT$XIZ")
__xc_z DD 0
CRT$XIZ ENDS

.const
    msg db "Hello world!", 0
    fmt db "%s", 0
end

(note that better use .const section instead .data here for constand data )

and link file with


link.exe /MAP /MERGE:.CRT=.rdata /SUBSYSTEM:CONSOLE /ENTRY:ep /LARGEADDRESSAWARE /DYNAMICBASE /MACHINE:X86 /SAFESEH:NO /OPT:REF /OPT:ICF /NODEFAULTLIB /EMITPOGOPHASEINFO /EMITVOLATILEMETADATA:NO /LIBPATH:"C:\lib\um\x86" /LIBPATH:"C:\msvc\lib\x86" /LIBPATH:"C:\lib\ucrt\x86" test.obj legacy_stdio_definitions.lib legacy_stdio_wide_specifiers.lib ucrt.lib

another way - build my self msvcrt.lib ( standard msvcrt.lib have no printf despite it was exported by msvcrt.dll and then use it ) minimal way


.686
.model flat
API MACRO name
name proc
name endp
ENDM
.code
API _printf
end

and msvcrt.def :

EXPORTS
printf

build with

link.exe /DLL /NOENTRY:ep /LARGEADDRESSAWARE /DYNAMICBASE /MACHINE:X86 /SAFESEH:NO /OPT:REF /OPT:ICF /NODEFAULTLIB /EMITPOGOPHASEINFO /EMITVOLATILEMETADATA:NO /DEF:msvcrt.def msvcrt.obj

and than can use this

.686

.model flat

extern __imp__printf: DWORD
extern __imp__ExitProcess@4: DWORD

.code

ep proc

    push offset msg
    push offset fmt
    call __imp__printf
    add esp,8
    
    push eax
    call __imp__ExitProcess@4
    
    ret
ep endp

.const
    msg db "Hello world!", 0
    fmt db "%s", 0
end

link with:

link.exe /MAP /SUBSYSTEM:CONSOLE /ENTRY:ep /LARGEADDRESSAWARE /DYNAMICBASE /MACHINE:X86 /SAFESEH:NO /OPT:REF /OPT:ICF /NODEFAULTLIB /EMITPOGOPHASEINFO /EMITVOLATILEMETADATA:NO /LIBPATH:"C:\lib\um\x86" /LIBPATH:"C:\msvc\lib\x86" /LIBPATH:"C:\lib\ucrt\x86" test2.obj msvcrt.lib kernel32.lib

(more general way auto build lib for dll - here)