I'm reading an article that explains how to call bare syscalls without passing through glibc. To call chmod and exit, use:
#include <linux/unistd.h>
_syscall2(int,chmod,char*,f,int,m)
_syscall1(int,exit,int,r)
My gcc complains about them. What are their use, how do they work?
$ gcc --version
gcc (Ubuntu 7.4.0-1ubuntu1~18.04) 7.4.0
$ gcc e.c
e.c:2:15: error: unknown type name ‘setresuid’; did you mean ‘__NR_setresuid’?
_syscall3(int,setresuid,int,r,int,e,int,s)
^~~~~~~~~
__NR_setresuid
e.c:2:29: error: unknown type name ‘r’
_syscall3(int,setresuid,int,r,int,e,int,s)
^
e.c:2:35: error: unknown type name ‘e’
_syscall3(int,setresuid,int,r,int,e,int,s)
^
e.c:2:41: error: unknown type name ‘s’
_syscall3(int,setresuid,int,r,int,e,int,s)
^
Your article is probably obsolete.
If you code in C, there is no reason to avoid using the syscalls(2) (notice the plural) as documented. Be also aware of the vdso(7). You could use some other C standard library than the glibc (e.g. musl-libc, dietlibc, etc...) and you might (but that is not recommended) statically link it.
You might use syscall(2) (notice the singular) instead. I see no reason to do that, e.g. use read(2) or mmap(2) without syscall.
The Assembly HowTo might be an interesting read (beware, it might be too 32 bits centric, most Linux PCs today are 64 bits x86-64).
See also osdev.org
BTW, some old Unixes (e.g. Solaris) had a libsys providing just the syscalls, and their libc linked to it. I would like a libsys too! But on current Linux systems, it does not really matter, since almost every process (running some dynamically linked ELF executable) is mmap(2)-ing, after ld-linux.so(8), several segments and sections of your libc.so.6; for details, read Drepper's How to write a shared library (since it also explains in details how shared libraries actually work). Use also pmap(1) on some running process (e.g. pmap $$ in a shell).
Some rare syscalls (e.g. userfaultfd(2) today 2Q2019) are not known by the glibc. They are an exception, because most system calls are wrapped by your libc (the wrapping usually just deals with errno(3) setting on failure). Be aware of strace(1).
And you also should read Operating Systems: Three Easy Pieces (it is a freely downloadable book, explaining the role of, and reason for, system calls)
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