I have got a question about linux x86 system calls in assembly.
When I am creating a new assembly program with nasm on linux, I'd like to know which system calls I have to use for doing a specific task (for example reading a file, writing output, or simple exiting...). I know some syscall because I've read them on some examples taken around internet (such as eax=0, ebx=1 int 0x80 exit with return value of 1), but nothing more... How could I know if there are other arguments for exit syscall? Or for another syscall? I'm looking for a docs that explain which syscalls have which arguments to pass in which registers.
I've read the man page about exit function etc. but it didn't explain to me what I'm asking.
Hope I was clear enough,
Thank you!
The x86 wiki (which I just updated again :) has links to the system call ABI (what the numbers are for every call, where to put the params, what instruction to run, and which registers will clobbered on return). This is not documented in the man page because it's architecture-specific. Same for binary constants: they don't have to be the same on every architecture.
grep -r O_APPEND /usr/include for your target architecture to recursively search the .h files.
Even better is to set things up so you can use the symbolic constants in your asm source, for readability and to avoid the risk of errors.
The gcc actually does use the C Preprocessor when processing .S files, but including most C header files will also get you some C prototypes.
Or convert the #defines to NASM macros with sed or something. Maybe feed some #include<> lines to the C preprocessor and have it print out just the macro definitions.
printf '#include <%s>\n' unistd.h sys/stat.h |
gcc -dD -E - |
sed -ne 's/^#define \([A-Za-z_0-9]*\) \(.\)/\1\tequ \2/p'
That turns every non-empty #define into a NASM symbol equ value. The resulting file has many lines of error: expression syntax error when I tried to run NASM on it, but manually selecting some valid lines from that may work.
Some constants are defined in multiple steps, e.g. #define S_IRGRP (S_IRUSR >> 3). This might or might not work when converted to NASM equ symbol definitions.
Also note that in C 0666, is an octal constant. In NASM, you need either 0o666 or 666o; a leading 0 is not special. Otherwise, NASM syntax for hex and decimal constants is compatible with C.
Perhaps you are looking for something like linux/syscalls.h[1], which you have on your system if you've installed the Linux source code via apt-get or whatever your distro uses.
[1] http://lxr.free-electrons.com/source/include/linux/syscalls.h#L326