How to print integers using syscall

Asked Oct 20 '18 at 12:13

Active Oct 20 '18 at 12:13

Viewed 54 times

I am trying to print single characters to STDOUT using syscalls, however it is giving me a segfault. The following code is part of a larger program which also uses syscall and rdi to print out regular strings. It prints out "I am" and then I want to print 17 for example. Forgive me, I am very new to assembly.

movq    $1, %rax            #sysout
movq    $1, %rdi            #stdout
movq    $'1', (%rsi)        #move char 1 to rsi
movq    $1, %rdx            #move counter(length of string) to rdx (next argument of syscall)
syscall                     #syscall (print)

movq    $1, %rax            #sysout
movq    $1, %rdi            #stdout
movq    $'9', (%rsi)        #move char 9 to rsi
movq    $1, %rdx            #move counter(length of string) to rdx (next argument of syscall)
syscall                     #syscall (print)

movq $60, %rax              # system call 60 is sys exit 
movq $0, %rdx               # normal exit: 0
syscall

asked Oct 20 '18 at 12:13

Vetle Hjelmtvedt

You have not set `rsi` to anything. Not surprising that it faults. – Jester Oct 20 '18 at 12:17
So I move 1 to %rsi instead of (%rsi)? – Vetle Hjelmtvedt Oct 20 '18 at 12:44
No, `(%rsi)` is correct but you need to make sure it points to some writable memory where you can store your `'1'`. – Jester Oct 20 '18 at 12:49
@VetleHjelmtvedt `rsi` needs to contain the address of a buffer to print. So store `'1'` somewhere in RAM (e.g. on the stack) and let `rsi` point to it. – fuz Oct 20 '18 at 12:50

How to print integers using syscall

0 Answers0