On Sun, Jun 23, 2019 at 10:17 AM Dmitry V. Levin <ldv@xxxxxxxxxxxx> wrote: > > Hi, > > On Sun, Jun 23, 2019 at 09:03:31AM -0700, Naveen Iyer wrote: > > Hi, > > > > As suggested by > > https://www.kernel.org/doc/man-pages/reporting_bugs.html , I looked up > > the online man page as well: > > http://man7.org/linux/man-pages/man2/syscall.2.html > > > > In the table in "Architecture calling conventions" section: > > arch/ABI instruction syscall # retval error Notes > > ──────────────────────────────────── > > x86-64 syscall rax rax - [5] > > > > Retval must be placed in rdi > > Why? > > > as per the following link: > > https://w3challs.com/syscalls/?arch=x86_64 > > This link doesn't claim that. > The link states that rdi contains the int error_code. I am able to return whatever int I want to return by placing the int value of my choice in rdi. Why do you say that this link doesn't claim that when it clearly does? Am I misreading it? > > } > > > > Step 2) Compile and execute: > > $ gcc -nostdlib -c startup.c > > $ ld startup.o -o startup > > $ ./startup > > $ echo $? > > 97 > > So your test invokes "exit" syscall with 97 as its first argument. > The return value of this syscall has no meaning because it does not return. > Most of syscalls do return and their return value is in %rax on x86_64. I am checking for the program's return status and not the return of syscall itself.
