DevHeads IoT Integration ServerDIIS
DevHeads IoT Integration Server•17mo ago
Marvee Amasi

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 system running Ubuntu 22.04 that requires precise control over output formatting. As part of this tool, I'm implementing a custom printf like function in assembly using NASM assembler and GNU Linker.

I've encountered unexpected behavior when piping the output of my assembly printf function to hexdump . The output is displayed correctly when printed directly to the terminal, but piping it to hexdump results in no output.
From printf.c , output to STDOUT:
 $ ./printf
TEST

Pipe to hexdump:
 $ ./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 +++


From printf.nasm , output to STDOUT:
 $ ./printf
TEST

Pipe to hexdump - no output:
 $ ./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 +++

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
file0.jpg
file1.jpg
Solution
@Marvee Amasi The issue you're encountering is due to the buffering behavior of the standard output stream (stdout). In C, 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 the output.
To make sure that your assembly implementation behaves consistently, you need to explicitly flush the stdout buffer. You can achieve this by using a write system call directly instead of relying on the C printf function.
Jump to solution
Was this page helpful?