ead ead - 4 months ago 18
Linux Question

Using printf in assembly leads to an empty ouput

I try to use

printf
from my assembler code, this is a minimal example which should just print
hello
to 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 it with

gcc -nostdlib try_printf.s -o try_printf -lc


and when I run it, it seems to work: the string
hello
is printed out and the exit status is
0
:

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


But when I try to capture the text, it is obvious, that something is not working properly:

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

XXX$


The variable
output
should have the value
hello
, but is empty.

What is wrong with my usage of
printf
?

ead ead
Answer

As Michael explained, it is OK to link the C-library dynamically. This is also how it is introduced in the "Programming bottom up" book (see chapter 8).

However it is important to call exit from the C-library in order to end the program and not to bypass it, which was what I wrongly did by calling exit-syscall. As hinted by Michael, exit does a lot of clean up like flushing streams.

That is what happened: As explained here, the C-library buffers the the standard streams as follows:

  1. No buffering for standard error.
  2. If standard out/in is a terminal, it is line-buffered.
  3. If standard out/in is a not a terminal, it is fully-buffered and thus flush is needed in the end of writing.

Which case applies is decided when printf is called for the first time for a stream.

So if printf_try is called directly in the terminal, the output of the program can be seen because hello has \n at the end (which triggers the flush in the line-buffered mode) and it is a terminal, also the 2. case.

Calling printf_try via $(./printf_try) means that the stdout is no longer a terminal (actually I don't know whether is is a temp file or a memory file) and thus the 3. case is in effect - there is need for an explicit flush i.e. call to C-exit.

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