github.com/opencontainers/runc@v1.2.0-rc.1.0.20240520010911-492dc558cdd6/libcontainer/dmz/nolibc/nolibc.h (about)

     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 */