linux - Using printf in assembly leads to an empty ouput -

i try use printf assembler code, minimal example should print hello stdout:

.section  .rodata hello:     .ascii  "hello\n\0" .section .text     .globl _start         _start:     movq $hello, %rdi #first parameter     xorl %eax, %eax #0 - number of used vector registers     call printf         #exit        movq $60, %rax     movq $0, %rdi     syscall 

i build with

gcc -nostdlib try_printf.s -o try_printf -lc 

and when run it, seems work: string hello printed out , exit status 0:

xxx$ ./try_printf hello xxx$ echo $? 0 xxx$ 

but when try capture text, obvious, not working properly:

xxx$ output=$(./try_printf)  xxx$ echo $output  xxx$  

the variable output should have value hello, empty.

what wrong usage of printf?

as michael explained, ok link c-library dynamically. how introduced in "programming bottom up" book (see chapter 8).

however important call exit c-library in order end program , not bypass it, wrongly did calling exit-syscall. hinted michael, exit lot of clean up flushing streams.

that happened: explained here, c-library buffers the standard streams follows:

1. no buffering standard error.
2. if standard out/in terminal, line-buffered.
3. if standard out/in not terminal, fully-buffered , flush needed in end of writing.

which case applies decided when printf called first time stream.

so if printf_try called directly in terminal, output of program can seen because hello has \n @ end (which triggers flush in line-buffered mode) , terminal, 2. case.

calling printf_try via $(./printf_try) means stdout no longer terminal (actually don't know whether is temp file or memory file) , 3. case in effect - there need explicit flush i.e. call c-exit.