Printf without newline in assembly

fflush() flushes buffered output in line or full-buffered stdio streams:

extern fflush
...
xor  edi, edi          ; RDI = 0
call fflush            ; fflush(NULL) flushes all streams
...

Alternatively, mov rdi, [stdout] / call fflush also works to flush only that stream. (Use default rel for efficient RIP-relative addressing, and you'll need extern stdout as well.)

Call fflush(stdout); to display what's currently sitting in the buffers.

For Windows 32-bit mode (FASM):

push [_iob]
call [fflush]   ; call into DLL.  Callee-pops calling convention

GNU/Linux 32-bit mode (NASM)

extern fflush
extern stdout
...
push dword [stdout]
call fflush            ; linker takes care of PLT stub for dynamic linking (in a non-PIE executable)
add  esp, 4            ; caller-pops calling convention
etc...