3 replies

Why Does Piping Output to Hexdump Fail for My Assembly printf Function?

Here is a low level debugging tool for an

Intel Core i7 12700K

Intel Core i7 12700K

system running

Ubuntu 22.04

Ubuntu 22.04

that requires precise control over output formatting. As part of this tool, I'm implementing a custom

printf

printf

like function in assembly using

NASM

NASM

assembler and

GNU

GNU

Linker.

I've encountered unexpected behavior when piping the output of my assembly

printf

printf

function to

hexdump

hexdump

. The output is displayed correctly when printed directly to the terminal, but piping it to

hexdump

hexdump

results in no output.
From printf.c , output to STDOUT:

 $ ./printf
TEST

 $ ./printf
TEST

Pipe to hexdump:

 $ ./printf | hexdump -v -C
00000000  54 45 53 54 0a                                    |TEST.|
00000005

 $ ./printf | hexdump -v -C
00000000  54 45 53 54 0a                                    |TEST.|
00000005

Strace:

 $ strace -ttT -f -e trace="write" ./printf
17:44:33.841789 write(1, "TEST\n", 5TEST
)   = 5 <0.000050>
17:44:33.842320 +++ exited with 0 +++

 $ strace -ttT -f -e trace="write" ./printf
17:44:33.841789 write(1, "TEST\n", 5TEST
)   = 5 <0.000050>
17:44:33.842320 +++ exited with 0 +++

From printf.nasm , output to STDOUT:

 $ ./printf
TEST

 $ ./printf
TEST

Pipe to hexdump - no output:

 $ ./printf | hexdump -v -C

 $ ./printf | hexdump -v -C

Strace:

 $ strace -ttT -f -e trace="write" ./printf
17:46:00.828887 write(1, "TEST\n", 5TEST
)   = 5 <0.000050>
17:46:00.829282 +++ exited with 0 +++

 $ strace -ttT -f -e trace="write" ./printf
17:46:00.828887 write(1, "TEST\n", 5TEST
)   = 5 <0.000050>
17:46:00.829282 +++ exited with 0 +++

Ryt now I suspect the system call interactions between C and assembly.

I want to understanding the underlying reasons for this and potential solutions to ensure consistent output behavior in both C and assembly implementations

Solution

@Marvee Amasi The issue you're encountering is due to the buffering behavior of the standard

output

output

stream (

stdout

stdout

). In

, the standard library handles the buffering of stdout, which ensures that the buffer is flushed when the program exits or when the buffer is full. However, when you implement printf in assembly, you're bypassing these standard library mechanisms, which can lead to different behavior, especially when

piping

piping

the

output

output

.
To make sure that your assembly implementation behaves consistently, you need to explicitly flush the

stdout

stdout

buffer. You can achieve this by using a write system call directly instead of relying on the