I'm trying to call the function fgets to read from standard input. How do I get the FILE* stdin in assembly?

I'm using nasm on cygwin.
Thanks in advance.

8 Years
Discussion Span
Last Post by Salem

So, I have been doing some digging, and figured out how to use the system calls _write and _read, that use file descriptors instead of a FILE*.

I found some references to a function called fileptr that returns a FILE* corresponding to a file descriptor, but that doesn't seem to be part of the C standard library.

Anyway, here is my test code. It builds and runs correctly under Cygwin.

; test_io.asm
; Test _read and _write system calls
; nasm -o test_io.o -fwin32 test_io.asm
; gcc -o test_io test_io.o
section .data
prompt      db 'Enter a string: ',0
prompt_len  equ $ - prompt

BUFSZ   equ 1024
STDIN   equ 0
STDOUT  equ 1

section .bss
buffer  resb BUFSZ

section .text
    global _main
    extern _read, _write
    push    prompt_len
    push    prompt
    push    STDOUT
    call    _write
    add     esp, 12
    push    BUFSZ - 1   ; leave space for terminating 0
    push    buffer
    push    STDIN
    call    _read
    add     esp, 12
    push    eax         ; _read returns string length in eax
    push    buffer
    push    STDOUT
    call    _write
    add     esp, 12
    mov     eax, 0

That would seem to be the way to go.

FILE*'s are constructed by the C runtime library, which may necessitate you dragging in a whole lot of stuff you didn't want. If you did want it, there would seem to be little point in writing this bit in ASM.

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