xref: /openbmc/linux/tools/include/nolibc/nolibc.h (revision 6d1970e1)
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. All input arguments
17  *     are castto a long stored in a register. These expressions always return
18  *     the syscall's return value as a signed long value which is often either
19  *     a pointer or the negated errno value.
20  *
21  *   - The second level is mostly architecture-independent. It is made of
22  *     static functions called sys_<name>() which rely on my_syscallN()
23  *     depending on the syscall definition. These functions are responsible
24  *     for exposing the appropriate types for the syscall arguments (int,
25  *     pointers, etc) and for setting the appropriate return type (often int).
26  *     A few of them are architecture-specific because the syscalls are not all
27  *     mapped exactly the same among architectures. For example, some archs do
28  *     not implement select() and need pselect6() instead, so the sys_select()
29  *     function will have to abstract this.
30  *
31  *   - The third level is the libc call definition. It exposes the lower raw
32  *     sys_<name>() calls in a way that looks like what a libc usually does,
33  *     takes care of specific input values, and of setting errno upon error.
34  *     There can be minor variations compared to standard libc calls. For
35  *     example the open() call always takes 3 args here.
36  *
37  * The errno variable is declared static and unused. This way it can be
38  * optimized away if not used. However this means that a program made of
39  * multiple C files may observe different errno values (one per C file). For
40  * the type of programs this project targets it usually is not a problem. The
41  * resulting program may even be reduced by defining the NOLIBC_IGNORE_ERRNO
42  * macro, in which case the errno value will never be assigned.
43  *
44  * Some stdint-like integer types are defined. These are valid on all currently
45  * supported architectures, because signs are enforced, ints are assumed to be
46  * 32 bits, longs the size of a pointer and long long 64 bits. If more
47  * architectures have to be supported, this may need to be adapted.
48  *
49  * Some macro definitions like the O_* values passed to open(), and some
50  * structures like the sys_stat struct depend on the architecture.
51  *
52  * The definitions start with the architecture-specific parts, which are picked
53  * based on what the compiler knows about the target architecture, and are
54  * completed with the generic code. Since it is the compiler which sets the
55  * target architecture, cross-compiling normally works out of the box without
56  * having to specify anything.
57  *
58  * Finally some very common libc-level functions are provided. It is the case
59  * for a few functions usually found in string.h, ctype.h, or stdlib.h.
60  *
61  * The nolibc.h file is only a convenient entry point which includes all other
62  * files. It also defines the NOLIBC macro, so that it is possible for a
63  * program to check this macro to know if it is being built against and decide
64  * to disable some features or simply not to include some standard libc files.
65  *
66  * A simple static executable may be built this way :
67  *      $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \
68  *            -static -include nolibc.h -o hello hello.c -lgcc
69  *
70  * Simple programs meant to be reasonably portable to various libc and using
71  * only a few common includes, may also be built by simply making the include
72  * path point to the nolibc directory:
73  *      $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \
74  *            -I../nolibc -o hello hello.c -lgcc
75  *
76  * The available standard (but limited) include files are:
77  *   ctype.h, errno.h, signal.h, stdio.h, stdlib.h, string.h, time.h
78  *
79  * In addition, the following ones are expected to be provided by the compiler:
80  *   float.h, stdarg.h, stddef.h
81  *
82  * The following ones which are part to the C standard are not provided:
83  *   assert.h, locale.h, math.h, setjmp.h, limits.h
84  *
85  * A very useful calling convention table may be found here :
86  *      http://man7.org/linux/man-pages/man2/syscall.2.html
87  *
88  * This doc is quite convenient though not necessarily up to date :
89  *      https://w3challs.com/syscalls/
90  *
91  */
92 #ifndef _NOLIBC_H
93 #define _NOLIBC_H
94 
95 #include "std.h"
96 #include "arch.h"
97 #include "types.h"
98 #include "sys.h"
99 #include "ctype.h"
100 #include "signal.h"
101 #include "unistd.h"
102 #include "stdio.h"
103 #include "stdlib.h"
104 #include "string.h"
105 #include "time.h"
106 #include "stackprotector.h"
107 
108 /* Used by programs to avoid std includes */
109 #define NOLIBC
110 
111 #endif /* _NOLIBC_H */
112