rcutorture/nolibc: Add a bit of documentation to explain how to use nolibc

 * Copyright (C) 2017-2018 Willy Tarreau <w@1wt.eu>

 */

/* some archs (at least aarch64) don't expose the regular syscalls anymore by

/*

 * This file is designed to be used as a libc alternative for minimal programs

 * with very limited requirements. It consists of a small number of syscall and

 * type definitions, and the minimal startup code needed to call main().

 * All syscalls are declared as static functions so that they can be optimized

 * away by the compiler when not used.

 *

 * Syscalls are split into 3 levels:

 *   - The lower level is the arch-specific syscall() definition, consisting in

 *     assembly code in compound expressions. These are called my_syscall0() to

 *     my_syscall6() depending on the number of arguments. The MIPS

 *     implementation is limited to 5 arguments. All input arguments are cast

 *     to a long stored in a register. These expressions always return the

 *     syscall's return value as a signed long value which is often either a

 *     pointer or the negated errno value.

 *

 *   - The second level is mostly architecture-independent. It is made of

 *     static functions called sys_<name>() which rely on my_syscallN()

 *     depending on the syscall definition. These functions are responsible

 *     for exposing the appropriate types for the syscall arguments (int,

 *     pointers, etc) and for setting the appropriate return type (often int).

 *     A few of them are architecture-specific because the syscalls are not all

 *     mapped exactly the same among architectures. For example, some archs do

 *     not implement select() and need pselect6() instead, so the sys_select()

 *     function will have to abstract this.

 *

 *   - The third level is the libc call definition. It exposes the lower raw

 *     sys_<name>() calls in a way that looks like what a libc usually does,

 *     takes care of specific input values, and of setting errno upon error.

 *     There can be minor variations compared to standard libc calls. For

 *     example the open() call always takes 3 args here.

 *

 * The errno variable is declared static and unused. This way it can be

 * optimized away if not used. However this means that a program made of

 * multiple C files may observe different errno values (one per C file). For

 * the type of programs this project targets it usually is not a problem. The

 * resulting program may even be reduced by defining the NOLIBC_IGNORE_ERRNO

 * macro, in which case the errno value will never be assigned.

 *

 * Some stdint-like integer types are defined. These are valid on all currently

 * supported architectures, because signs are enforced, ints are assumed to be

 * 32 bits, longs the size of a pointer and long long 64 bits. If more

 * architectures have to be supported, this may need to be adapted.

 *

 * Some macro definitions like the O_* values passed to open(), and some

 * structures like the sys_stat struct depend on the architecture.

 *

 * The definitions start with the architecture-specific parts, which are picked

 * based on what the compiler knows about the target architecture, and are

 * completed with the generic code. Since it is the compiler which sets the

 * target architecture, cross-compiling normally works out of the box without

 * having to specify anything.

 *

 * Finally some very common libc-level functions are provided. It is the case

 * for a few functions usually found in string.h, ctype.h, or stdlib.h. Nothing

 * is currently provided regarding stdio emulation.

 *

 * The macro NOLIBC is always defined, so that it is possible for a program to

 * check this macro to know if it is being built against and decide to disable

 * some features or simply not to include some standard libc files.

 *

 * Ideally this file should be split in multiple files for easier long term

 * maintenance, but provided as a single file as it is now, it's quite

 * convenient to use. Maybe some variations involving a set of includes at the

 * top could work.

 *

 * A simple static executable may be built this way :

 *      $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \

 *            -static -include nolibc.h -lgcc -o hello hello.c

 *

 * A very useful calling convention table may be found here :

 *      http://man7.org/linux/man-pages/man2/syscall.2.html

 *

 * This doc is quite convenient though not necessarily up to date :

 *      https://w3challs.com/syscalls/

 *

 */

/* Some archs (at least aarch64) don't expose the regular syscalls anymore by

 * default, either because they have an "_at" replacement, or because there are

 * more modern alternatives. For now we'd rather still use them.

 */

#define NOLIBC

/* Build a static executable this way :

 *      $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \

 *            -static -include nolibc.h -lgcc -o hello hello.c

 *

 * Useful calling convention table found here :

 *      http://man7.org/linux/man-pages/man2/syscall.2.html

 *

 * This doc is even better :

 *      https://w3challs.com/syscalls/

 */

/* this way it will be removed if unused */

static int errno;