1 | @c -*- Texinfo -*- |
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2 | @node Syscalls |
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3 | @chapter System Calls |
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4 | |
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5 | @cindex linking the C library |
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6 | The C subroutine library depends on a handful of subroutine calls for |
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7 | operating system services. If you use the C library on a system that |
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8 | complies with the POSIX.1 standard (also known as IEEE 1003.1), most of |
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9 | these subroutines are supplied with your operating system. |
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10 | |
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11 | If some of these subroutines are not provided with your system---in |
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12 | the extreme case, if you are developing software for a ``bare board'' |
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13 | system, without an OS---you will at least need to provide do-nothing |
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14 | stubs (or subroutines with minimal functionality) to allow your |
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15 | programs to link with the subroutines in @code{libc.a}. |
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16 | |
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17 | @menu |
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18 | * Stubs:: Definitions for OS interface |
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19 | * Reentrant Syscalls:: Reentrant covers for OS subroutines |
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20 | @end menu |
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21 | |
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22 | @node Stubs |
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23 | @section Definitions for OS interface |
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24 | @cindex stubs |
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25 | |
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26 | @cindex subroutines for OS interface |
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27 | @cindex OS interface subroutines |
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28 | This is the complete set of system definitions (primarily subroutines) |
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29 | required; the examples shown implement the minimal functionality |
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30 | required to allow @code{libc} to link, and fail gracefully where OS |
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31 | services are not available. |
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32 | |
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33 | Graceful failure is permitted by returning an error code. A minor |
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34 | complication arises here: the C library must be compatible with |
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35 | development environments that supply fully functional versions of these |
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36 | subroutines. Such environments usually return error codes in a global |
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37 | @code{errno}. However, the Red Hat newlib C library provides a @emph{macro} |
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38 | definition for @code{errno} in the header file @file{errno.h}, as part |
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39 | of its support for reentrant routines (@pxref{Reentrancy,,Reentrancy}). |
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40 | |
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41 | @cindex @code{errno} global vs macro |
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42 | The bridge between these two interpretations of @code{errno} is |
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43 | straightforward: the C library routines with OS interface calls |
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44 | capture the @code{errno} values returned globally, and record them in |
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45 | the appropriate field of the reentrancy structure (so that you can query |
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46 | them using the @code{errno} macro from @file{errno.h}). |
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47 | |
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48 | This mechanism becomes visible when you write stub routines for OS |
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49 | interfaces. You must include @file{errno.h}, then disable the macro, |
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50 | like this: |
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51 | |
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52 | @example |
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53 | #include <errno.h> |
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54 | #undef errno |
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55 | extern int errno; |
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56 | @end example |
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57 | |
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58 | @noindent |
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59 | The examples in this chapter include this treatment of @code{errno}. |
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60 | |
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61 | @ftable @code |
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62 | @item _exit |
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63 | Exit a program without cleaning up files. If your system doesn't |
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64 | provide this, it is best to avoid linking with subroutines that require |
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65 | it (@code{exit}, @code{system}). |
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66 | |
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67 | @item close |
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68 | Close a file. Minimal implementation: |
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69 | |
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70 | @example |
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71 | int close(int file) @{ |
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72 | return -1; |
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73 | @} |
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74 | @end example |
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75 | |
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76 | @item environ |
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77 | A pointer to a list of environment variables and their values. For a |
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78 | minimal environment, this empty list is adequate: |
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79 | |
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80 | @example |
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81 | char *__env[1] = @{ 0 @}; |
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82 | char **environ = __env; |
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83 | @end example |
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84 | |
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85 | @item execve |
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86 | Transfer control to a new process. Minimal implementation (for a system |
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87 | without processes): |
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88 | |
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89 | @example |
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90 | #include <errno.h> |
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91 | #undef errno |
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92 | extern int errno; |
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93 | int execve(char *name, char **argv, char **env) @{ |
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94 | errno = ENOMEM; |
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95 | return -1; |
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96 | @} |
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97 | @end example |
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98 | |
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99 | @item fork |
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100 | Create a new process. Minimal implementation (for a system without processes): |
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101 | |
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102 | @example |
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103 | #include <errno.h> |
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104 | #undef errno |
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105 | extern int errno; |
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106 | int fork(void) @{ |
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107 | errno = EAGAIN; |
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108 | return -1; |
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109 | @} |
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110 | @end example |
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111 | |
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112 | @item fstat |
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113 | Status of an open file. For consistency with other minimal |
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114 | implementations in these examples, all files are regarded as character |
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115 | special devices. The @file{sys/stat.h} header file required is |
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116 | distributed in the @file{include} subdirectory for this C library. |
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117 | |
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118 | @example |
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119 | #include <sys/stat.h> |
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120 | int fstat(int file, struct stat *st) @{ |
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121 | st->st_mode = S_IFCHR; |
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122 | return 0; |
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123 | @} |
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124 | @end example |
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125 | |
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126 | @item getpid |
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127 | Process-ID; this is sometimes used to generate strings unlikely to |
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128 | conflict with other processes. Minimal implementation, for a system |
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129 | without processes: |
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130 | |
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131 | @example |
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132 | int getpid(void) @{ |
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133 | return 1; |
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134 | @} |
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135 | @end example |
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136 | |
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137 | @item isatty |
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138 | Query whether output stream is a terminal. For consistency with the |
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139 | other minimal implementations, which only support output to |
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140 | @code{stdout}, this minimal implementation is suggested: |
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141 | |
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142 | @example |
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143 | int isatty(int file) @{ |
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144 | return 1; |
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145 | @} |
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146 | @end example |
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147 | |
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148 | @item kill |
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149 | Send a signal. Minimal implementation: |
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150 | |
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151 | @example |
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152 | #include <errno.h> |
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153 | #undef errno |
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154 | extern int errno; |
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155 | int kill(int pid, int sig) @{ |
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156 | errno = EINVAL; |
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157 | return -1; |
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158 | @} |
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159 | @end example |
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160 | |
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161 | @item link |
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162 | Establish a new name for an existing file. Minimal implementation: |
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163 | |
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164 | @example |
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165 | #include <errno.h> |
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166 | #undef errno |
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167 | extern int errno; |
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168 | int link(char *old, char *new) @{ |
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169 | errno = EMLINK; |
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170 | return -1; |
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171 | @} |
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172 | @end example |
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173 | |
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174 | @item lseek |
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175 | Set position in a file. Minimal implementation: |
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176 | |
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177 | @example |
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178 | int lseek(int file, int ptr, int dir) @{ |
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179 | return 0; |
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180 | @} |
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181 | @end example |
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182 | |
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183 | @item open |
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184 | Open a file. Minimal implementation: |
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185 | |
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186 | @example |
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187 | int open(const char *name, int flags, int mode) @{ |
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188 | return -1; |
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189 | @} |
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190 | @end example |
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191 | |
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192 | @item read |
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193 | Read from a file. Minimal implementation: |
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194 | |
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195 | @example |
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196 | int read(int file, char *ptr, int len) @{ |
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197 | return 0; |
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198 | @} |
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199 | @end example |
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200 | |
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201 | @item sbrk |
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202 | Increase program data space. As @code{malloc} and related functions |
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203 | depend on this, it is useful to have a working implementation. The |
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204 | following suffices for a standalone system; it exploits the symbol |
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205 | @code{_end} automatically defined by the GNU linker. |
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206 | |
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207 | @example |
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208 | @group |
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209 | caddr_t sbrk(int incr) @{ |
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210 | extern char _end; /* @r{Defined by the linker} */ |
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211 | static char *heap_end; |
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212 | char *prev_heap_end; |
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213 | |
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214 | if (heap_end == 0) @{ |
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215 | heap_end = &_end; |
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216 | @} |
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217 | prev_heap_end = heap_end; |
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218 | if (heap_end + incr > stack_ptr) @{ |
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219 | write (1, "Heap and stack collision\n", 25); |
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220 | abort (); |
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221 | @} |
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222 | |
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223 | heap_end += incr; |
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224 | return (caddr_t) prev_heap_end; |
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225 | @} |
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226 | @end group |
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227 | @end example |
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228 | |
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229 | @item stat |
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230 | Status of a file (by name). Minimal implementation: |
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231 | |
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232 | @example |
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233 | int stat(char *file, struct stat *st) @{ |
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234 | st->st_mode = S_IFCHR; |
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235 | return 0; |
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236 | @} |
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237 | @end example |
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238 | |
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239 | @item times |
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240 | Timing information for current process. Minimal implementation: |
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241 | |
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242 | @example |
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243 | int times(struct tms *buf) @{ |
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244 | return -1; |
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245 | @} |
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246 | @end example |
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247 | |
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248 | @item unlink |
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249 | Remove a file's directory entry. Minimal implementation: |
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250 | |
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251 | @example |
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252 | #include <errno.h> |
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253 | #undef errno |
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254 | extern int errno; |
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255 | int unlink(char *name) @{ |
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256 | errno = ENOENT; |
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257 | return -1; |
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258 | @} |
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259 | @end example |
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260 | |
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261 | @item wait |
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262 | Wait for a child process. Minimal implementation: |
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263 | @example |
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264 | #include <errno.h> |
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265 | #undef errno |
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266 | extern int errno; |
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267 | int wait(int *status) @{ |
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268 | errno = ECHILD; |
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269 | return -1; |
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270 | @} |
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271 | @end example |
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272 | |
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273 | @item write |
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274 | Write to a file. @file{libc} subroutines will use this |
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275 | system routine for output to all files, @emph{including} |
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276 | @code{stdout}---so if you need to generate any output, for example to a |
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277 | serial port for debugging, you should make your minimal @code{write} |
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278 | capable of doing this. The following minimal implementation is an |
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279 | incomplete example; it relies on a @code{outbyte} subroutine (not |
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280 | shown; typically, you must write this in assembler from examples |
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281 | provided by your hardware manufacturer) to actually perform the output. |
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282 | |
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283 | @example |
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284 | @group |
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285 | int write(int file, char *ptr, int len) @{ |
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286 | int todo; |
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287 | |
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288 | for (todo = 0; todo < len; todo++) @{ |
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289 | outbyte (*ptr++); |
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290 | @} |
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291 | return len; |
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292 | @} |
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293 | @end group |
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294 | @end example |
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295 | |
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296 | @end ftable |
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297 | |
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298 | @page |
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299 | @node Reentrant Syscalls |
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300 | @section Reentrant covers for OS subroutines |
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301 | |
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302 | Since the system subroutines are used by other library routines that |
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303 | require reentrancy, @file{libc.a} provides cover routines (for example, |
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304 | the reentrant version of @code{fork} is @code{_fork_r}). These cover |
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305 | routines are consistent with the other reentrant subroutines in this |
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306 | library, and achieve reentrancy by using a reserved global data block |
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307 | (@pxref{Reentrancy,,Reentrancy}). |
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308 | |
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309 | @menu |
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310 | * _close_r:: Reentrant version of close |
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311 | * _execve_r:: Reentrant version of execve |
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312 | * _fork_r:: Reentrant version of fork |
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313 | @ifset STDIO64 |
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314 | * _fstat64_r:: Reentrant version of fstat64 |
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315 | @end ifset |
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316 | * _fstat_r:: Reentrant version of fstat |
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317 | * _getpid_r:: Reentrant version of getpid |
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318 | * _kill_r:: Reentrant version of kill |
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319 | * _link_r:: Reentrant version of link |
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320 | @ifset STDIO64 |
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321 | * _lseek64_r:: Reentrant version of lseek64 |
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322 | @end ifset |
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323 | * _lseek_r:: Reentrant version of lseek |
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324 | @ifset STDIO64 |
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325 | * _open64_r:: Reentrant version of open64 |
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326 | @end ifset |
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327 | * _open_r:: Reentrant version of open |
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328 | * _read_r:: Reentrant version of read |
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329 | * _sbrk_r:: Reentrant version of sbrk |
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330 | @ifset STDIO64 |
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331 | * _stat64_r:: Reentrant version of stat64 |
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332 | @end ifset |
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333 | * _stat_r:: Reentrant version of stat |
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334 | * _times_r:: Reentrant version of times |
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335 | * _unlink_r:: Reentrant version of unlink |
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336 | * _wait_r:: Reentrant version of wait |
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337 | * _write_r:: Reentrant version of write |
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338 | @end menu |
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339 | |
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340 | @lowersections |
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341 | @page |
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342 | @include reent/closer.def |
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343 | |
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344 | @page |
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345 | @include reent/execr.def |
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346 | |
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347 | @ifset STDIO64 |
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348 | @page |
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349 | @include reent/fstat64r.def |
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350 | @end ifset |
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351 | |
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352 | @page |
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353 | @include reent/fstatr.def |
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354 | |
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355 | @page |
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356 | @include reent/linkr.def |
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357 | |
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358 | @ifset STDIO64 |
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359 | @page |
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360 | @include reent/lseek64r.def |
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361 | @end ifset |
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362 | |
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363 | @page |
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364 | @include reent/lseekr.def |
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365 | |
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366 | @ifset STDIO64 |
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367 | @page |
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368 | @include reent/open64r.def |
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369 | @end ifset |
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370 | |
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371 | @page |
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372 | @include reent/openr.def |
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373 | |
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374 | @page |
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375 | @include reent/readr.def |
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376 | |
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377 | @page |
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378 | @include reent/sbrkr.def |
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379 | |
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380 | @page |
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381 | @include reent/signalr.def |
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382 | |
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383 | @ifset STDIO64 |
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384 | @page |
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385 | @include reent/stat64r.def |
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386 | @end ifset |
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387 | |
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388 | @page |
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389 | @include reent/statr.def |
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390 | |
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391 | @page |
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392 | @include reent/timesr.def |
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393 | |
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394 | @page |
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395 | @include reent/unlinkr.def |
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396 | |
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397 | @page |
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398 | @include reent/writer.def |
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399 | @raisesections |
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