StackOverflow2013

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POHow can i get the true length of the returned string of syscall getcwd in nasm?
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copied!<p>My program calls function 183 (getcwd) of interrupt 80h which copies an absolute pathname of the current working directory to the memory location pointed to by buff, which is of length 4096. Returned absolute pathname length is usually less than 4096 bytes so i want to get its true length. How can i do that?</p> <pre><code>%define LF 0Ah ; Line feed ASCII code. %define STDOUT_FILENO 1 ; Standard output stream. %define SYS_exit 1 %define SYS_write 4 %define SYS_getcwd 183 SECTION .bss buff resb 4096 SECTION .text global _start _start: mov eax, SYS_getcwd ; getcwd mov ebx, buff mov ecx, 4096 int 80h mov eax, SYS_write ; print result to stdout mov ebx, STDOUT_FILENO mov ecx, buff mov edx, 4096 int 80h mov eax, SYS_exit ; exit mov ebx, 0 int 80h </code></pre> <p>I add code to find length of null terminated string to my program as following and it works:</p> <pre><code>%define LF 0Ah ; Line feed ASCII code. %define STDOUT_FILENO 1 ; Standard output stream. %define SYS_exit 1 %define SYS_write 4 %define SYS_getcwd 183 SECTION .data mesg1 db "Can't not find string length.",LF mesg1_l db $-mesg1 SECTION .bss buff resb 4096 SECTION .text global _start _start: mov eax, SYS_getcwd ; getcwd mov ebx, buff mov ecx, 4096 int 80h mov al, 0 ; find string length with scasb mov edi, buff cld repne scasb jne error1 sub ecx, 4096 neg ecx mov edx,ecx print: mov byte [buff + ecx],LF mov byte [buff + ecx + 1], 0 inc edx mov eax, SYS_write ; print result to stdout mov ebx, STDOUT_FILENO mov ecx, buff int 80h jmp exit error1: mov eax, SYS_write mov ebx, STDOUT_FILENO mov ecx, error1 mov edx, mesg1_l int 80h exit: mov eax, SYS_exit ; exit mov ebx, 0 int 80h </code></pre>

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