nasm system calls Linux

Question

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!

Try using google next time: docs.cs.up.ac.za/programming/asm/derick_tut/syscalls.html blog.rchapman.org/post/36801038863/… — Michael
@Michael Thank you, but I already know these sites, and they didn't actually tell me which "int flags" I have to set in ecx for syscall open or how must be the char *const filename ... I've searched on google obviously, but I couldn't find what I am searching. That's why I'm asking here. The docs.cs.up.ac.za site has a link for the source but when I'm clicking on it it does nothing (on linux) — pippo
"they didn't actually tell me which "int flags" I have to set in ecx for syscall open" google "man sys_open". "or how must be the char *const filename" I don't understand the comment. A const char* would just be the address of a NUL-terminated string. — Michael
@Michael So isn't there any .h file or something like that on my own machine for the description of syscalls? Only with google? — pippo
On a Linux system you can run man from the terminal. The system calls are in section 2 of the man pages, so e.g. to view the man page for open you'd run man 2 open in a terminal. — Michael

Peter Cordes Peter Cordes · Accepted Answer · 2016-02-08T03:37:07

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.

nasm system calls Linux

2 Answers