Question

I am trying to print a string in assembly by calling printf.

My assembly code:

mov dword[ebx + 0], '"'
mov dword[ebx + 4], 'h'
mov dword[ebx + 8], 'e'
mov dword[ebx + 12], 'l'
mov dword[ebx + 16], 'l'
mov dword[ebx + 20], 'o'
mov dword[ebx + 24], '"'
mov dword[ebx + 28], 0
push ebx
push formatString
call printf
add esp, 8
...
formatString    db    '%s', 10, 0

However when I run this it only prints the first character – ‘”‘, not the whole word (“hello”).

Many thanks

Answer 1

My assembly is rusty, but those should probably be byte-wise moves. You need a byte array in memory since that is what printf is expecting for a %s. The string in memory is probably "\0\0\0h\0\0\0e\0\0\0l\0\0\0l\0\0\0o\0\0\0"\0\0\0\0\0\0\0.

mov dword[ebx + 0], '"'        ; moves the 32-bit value 0x22000000 to EBX
mov dword[ebx + 4], 'h'        ; moves the 32-bit value 0x68000000 to EBX + 4
...

So, if ebx contains the address 0x123456, then you would have something like the following in memory:

0123456 | 22 00 00 00  68 00 00 00   65 00 00 00  6c 00 00 00 | "...h...e...l... |
0123466 | 6c 00 00 00  6f 00 00 00   22 00 00 00  00 00 00 00 | l...o..."....... |

Even though you are passing 0x123456 into printf as the address, it is only seeing a single character before it hits the first NUL byte. The following should work:

mov dword[ebx + 0], 577267052   ; 0x2268656c = "hel
mov dword[ebx + 4], 1819222528  ; 0x6c6f2200 = lo"\0
push ebx
push formatString
call printf
add esp, 8

There is probably a much better way to load bytes into an indirect address based on ebx but I haven’t looked at assembly in more years than I can count.