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:
Pipe to hexdump:
Strace:
From printf.nasm , output to STDOUT:
Pipe to hexdump - no output:
Strace:
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:Jump to 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....2 Replies
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.Of course thanks . You know someone told me the issue was that ; the exit
syscall is preventing any data my C library buffered as in via printf to be properly flushed before the process terminates. Rather than me using syscall to exit, use the C library exit function.
That didn't really fix it. thanks man @Enthernet Code