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/* SPDX-License-Identifier: LGPL-2.1 OR MIT */
/* nolibc.h
* Copyright (C) 2017-2018 Willy Tarreau <w@1wt.eu>
*/
/*
* 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.
*
* The nolibc.h file is only a convenient entry point which includes all other
* files. It also defines the NOLIBC macro, 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.
*
* A simple static executable may be built this way :
* $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \
* -static -include nolibc.h -o hello hello.c -lgcc
*
* Simple programs meant to be reasonably portable to various libc and using
* only a few common includes, may also be built by simply making the include
* path point to the nolibc directory:
* $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \
* -I../nolibc -o hello hello.c -lgcc
*
* The available standard (but limited) include files are:
* ctype.h, errno.h, signal.h, stdio.h, stdlib.h, string.h, time.h
*
* In addition, the following ones are expected to be provided by the compiler:
* float.h, stdarg.h, stddef.h
*
* The following ones which are part to the C standard are not provided:
* assert.h, locale.h, math.h, setjmp.h, limits.h
*
* 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/
*
*/
#ifndef _NOLIBC_H
#define _NOLIBC_H
#include "std.h"
#include "arch.h"
#include "types.h"
#include "sys.h"
#include "ctype.h"
#include "signal.h"
#include "stdio.h"
#include "stdlib.h"
#include "string.h"
#include "time.h"
#include "unistd.h"
/* Used by programs to avoid std includes */
#define NOLIBC
#endif /* _NOLIBC_H */
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