x86 - How to print the length of a string in assembly -

i learning assembly using following hello world program

section .text     global _start     ;must declared linker (ld)  _start:             ;tells linker entry point    mov  edx,len     ;message length    mov  ecx,msg     ;message write    mov  ebx,1       ;file descriptor (stdout)    mov  eax,4       ;system call number (sys_write)    int  0x80        ;call kernel     mov  eax,1       ;system call number (sys_exit)    int  0x80        ;call kernel  section .data     msg db 'hello, world!', 0xa  ;our string     len equ $ - msg              ;length of our string 

the original question had meant length of string. mean number of chars or length in memory (number of bytes)? check wanted print variable len. how can that? naively tried define new variable

    len2 equ $ - len 

and instead run

   mov  edx,len2     ;message length    mov  ecx,len     ;message write    mov  ebx,1       ;file descriptor (stdout)    mov  eax,4       ;system call number (sys_write)    int  0x80        ;call kernel 

to try print len, printed nothing. how can print number represented len?

   ...    mov  edx,len     ;message length 

this loads edx kind of numeric value, 14 in case. len "equ" constant symbol, #define in c.

   mov  ecx,msg     ;message write 

this loads ecx address of first character (msg label, pointing memory).

   mov  ebx,1       ;file descriptor (stdout)    mov  eax,4       ;system call number (sys_write)    int  0x80        ;call kernel    ...      msg db 'hello, world!', 0xa  ;our string 

this defines 14 bytes of memory, values 72 ('h'), 101 ('e'), ... . first byte pointed @ msg label (memory address of it).

    len equ $ - msg              ;length of our string 

this defines constant len visible during compile time. doesn't define memory content, can't find in executable or during runtime (unless used, mov edx,len, compiled particular instruction of course).

the definition $ - msg, $ in context works "current address", next defined machine code byte compiled, @ place equal msg + 14 (i hope did count number of characters correctly :) ). , ((msg+14) - msg) = 14 = number of bytes defined in memory between definition of len , label msg.

notice how avoid words variable or chars, asm more low level, labels memory , bytes more accurate wording , hope recognize subtle differences.

your len2 equ $ - len after len did define value len2 (msg+14) (still there in memory, no new byte added len definition) minus len 14, defined len2 equal msg.

then:

   mov  edx,len2     ;message length    mov  ecx,len     ;message write    ... 

does call sys_write pointer string equal 14 (invalid memory reference, area of memory off limits ordinary user code), , length equal address msg, on 32b linux value 0x80004000, i.e. on 2g of characters output.

the sys_write naturally doesn't that, fails, , returns error code in eax.

to output console sys_write have first write memory ascii (i think utf8 supported default in ubuntu shell, lazy verify) encoded string, , give sys_write address of memory, , length in bytes (with utf8 string difference between bytes , chars important, sys_write not aware of characters, works binary files , bytes, length amount of bytes).

i'm not going write code output numbers, that's several lines long (simplified printf implementation) , has several q+a on this, hope explanation understand happened , how works.

if learning asm, consider either linking against clib have printf available, or better, use debugger, , verify values straight in registers in debugger, don't bother string output yet, that's bit more advanced topic initial arithmetic, , basic flow control , operating stack. after more comfortable how basic instruction works, , how debug code, more easier try output numbers then.