1/* SPDX-License-Identifier: LGPL-2.1 OR MIT */
2/* nolibc.h
3 * Copyright (C) 2017-2018 Willy Tarreau <w@1wt.eu>
4 */
5
6/*
7 * This file is designed to be used as a libc alternative for minimal programs
8 * with very limited requirements. It consists of a small number of syscall and
9 * type definitions, and the minimal startup code needed to call main().
10 * All syscalls are declared as static functions so that they can be optimized
11 * away by the compiler when not used.
12 *
13 * Syscalls are split into 3 levels:
14 *   - The lower level is the arch-specific syscall() definition, consisting in
15 *     assembly code in compound expressions. These are called my_syscall0() to
16 *     my_syscall6() depending on the number of arguments. The MIPS
17 *     implementation is limited to 5 arguments. All input arguments are cast
18 *     to a long stored in a register. These expressions always return the
19 *     syscall's return value as a signed long value which is often either a
20 *     pointer or the negated errno value.
21 *
22 *   - The second level is mostly architecture-independent. It is made of
23 *     static functions called sys_<name>() which rely on my_syscallN()
24 *     depending on the syscall definition. These functions are responsible
25 *     for exposing the appropriate types for the syscall arguments (int,
26 *     pointers, etc) and for setting the appropriate return type (often int).
27 *     A few of them are architecture-specific because the syscalls are not all
28 *     mapped exactly the same among architectures. For example, some archs do
29 *     not implement select() and need pselect6() instead, so the sys_select()
30 *     function will have to abstract this.
31 *
32 *   - The third level is the libc call definition. It exposes the lower raw
33 *     sys_<name>() calls in a way that looks like what a libc usually does,
34 *     takes care of specific input values, and of setting errno upon error.
35 *     There can be minor variations compared to standard libc calls. For
36 *     example the open() call always takes 3 args here.
37 *
38 * The errno variable is declared static and unused. This way it can be
39 * optimized away if not used. However this means that a program made of
40 * multiple C files may observe different errno values (one per C file). For
41 * the type of programs this project targets it usually is not a problem. The
42 * resulting program may even be reduced by defining the NOLIBC_IGNORE_ERRNO
43 * macro, in which case the errno value will never be assigned.
44 *
45 * Some stdint-like integer types are defined. These are valid on all currently
46 * supported architectures, because signs are enforced, ints are assumed to be
47 * 32 bits, longs the size of a pointer and long long 64 bits. If more
48 * architectures have to be supported, this may need to be adapted.
49 *
50 * Some macro definitions like the O_* values passed to open(), and some
51 * structures like the sys_stat struct depend on the architecture.
52 *
53 * The definitions start with the architecture-specific parts, which are picked
54 * based on what the compiler knows about the target architecture, and are
55 * completed with the generic code. Since it is the compiler which sets the
56 * target architecture, cross-compiling normally works out of the box without
57 * having to specify anything.
58 *
59 * Finally some very common libc-level functions are provided. It is the case
60 * for a few functions usually found in string.h, ctype.h, or stdlib.h.
61 *
62 * The nolibc.h file is only a convenient entry point which includes all other
63 * files. It also defines the NOLIBC macro, so that it is possible for a
64 * program to check this macro to know if it is being built against and decide
65 * to disable some features or simply not to include some standard libc files.
66 *
67 * A simple static executable may be built this way :
68 *      $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \
69 *            -static -include nolibc.h -o hello hello.c -lgcc
70 *
71 * Simple programs meant to be reasonably portable to various libc and using
72 * only a few common includes, may also be built by simply making the include
73 * path point to the nolibc directory:
74 *      $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \
75 *            -I../nolibc -o hello hello.c -lgcc
76 *
77 * The available standard (but limited) include files are:
78 *   ctype.h, errno.h, signal.h, stdio.h, stdlib.h, string.h, time.h
79 *
80 * In addition, the following ones are expected to be provided by the compiler:
81 *   float.h, stdarg.h, stddef.h
82 *
83 * The following ones which are part to the C standard are not provided:
84 *   assert.h, locale.h, math.h, setjmp.h, limits.h
85 *
86 * A very useful calling convention table may be found here :
87 *      http://man7.org/linux/man-pages/man2/syscall.2.html
88 *
89 * This doc is quite convenient though not necessarily up to date :
90 *      https://w3challs.com/syscalls/
91 *
92 */
93#ifndef _NOLIBC_H
94#define _NOLIBC_H
95
96#include "std.h"
97#include "arch.h"
98#include "types.h"
99#include "sys.h"
100#include "ctype.h"
101#include "signal.h"
102#include "stdio.h"
103#include "stdlib.h"
104#include "string.h"
105#include "time.h"
106#include "unistd.h"
107
108/* Used by programs to avoid std includes */
109#define NOLIBC
110
111#endif /* _NOLIBC_H */
112