1 | RTEMS C++ Library |
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2 | ================= |
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3 | |
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4 | The RTEMS C++ Library or librtems++ is a wrapper for the RTEMS API. |
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5 | The classes provide as close a match to the RTEMS C API, for |
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6 | performance, to share the existing C documentation as much as |
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7 | possible, and to allow easy tracking of any changes to the RTEMS C |
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8 | API. |
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9 | |
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10 | The C++ interface only uses RTEMS API calls. No external references |
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11 | or internal interfaces are used. This allows the classes to be used |
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12 | in separately compiled modules or applications which link to the RTEMS |
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13 | trap interface. |
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14 | |
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15 | (This is the goal, which has not quite been reached. The TOD macro for |
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16 | micro-seconds to ticks is used, and this uses an internal global RTEMS |
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17 | variable) |
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18 | |
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19 | The C++ interface does not deal with RTEMS initialisation or the |
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20 | device driver interface. The current view is these parts of a system |
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21 | are best handled in the current manner. This means BSP for |
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22 | initialisation and the C API for drivers. |
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23 | |
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24 | RTEMS C++ Classes |
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25 | ================= |
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26 | |
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27 | The classes map to the managers of RTEMS. |
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28 | |
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29 | The methods have default values selected which try to fit most cases |
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30 | or follow the documented RTEMS default values. Moving from left to |
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31 | right the parameters become less used, allowing the defaults to be |
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32 | selected. An example is the scope parameter for most classes. This |
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33 | can be local or global. I assume that most RTEMS objects are local, |
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34 | therefore it has been made the last parameter. |
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35 | |
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36 | Inline methods have been used for methods which are commonly used in |
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37 | applications. This tries to add the minimum of overhead. For |
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38 | example, the methods to send or receive events are inline, while all |
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39 | methods for control of a task are not. |
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40 | |
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41 | The RTEMS types, enumerations, and defines are used. If a new type, |
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42 | enumeration or define is made it will map directly to the RTEMS |
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43 | equivalent. For example the enumeration Scope is defined for various |
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44 | classes which can be local or global. The elements of the enumeration |
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45 | are forced to the same value as the RTEMS values. An enumeration is |
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46 | used in this case to allow the compiler to type check a little |
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47 | better. It saves having to check only RTEMS_LOCAL or RTEMS_GLOBAL is |
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48 | passed as a parameter (I am not convinced this is really needed as the |
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49 | goal was to not define anything and to only use what RTEMS provided). |
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50 | |
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51 | Where possible the various parts of an option bit set, or mode can be |
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52 | controlled separately or controlled as a group. An example is the |
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53 | task mode. The RTEMS C API allows a set of modes to be modified at |
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54 | once. The TaskMode class allows this to occur, while also providing |
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55 | methods to control a single mode item. |
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56 | |
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57 | The name of an object is always passed as a string. The classes turn |
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58 | the string into a rtems_name variable. The string does not have to be |
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59 | nul character terminated. |
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60 | |
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61 | The RTEMS C API uses 'delete' to remove or kill an RTEMS object. This |
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62 | is a reserved word in C++, so the word 'destroy' is used instead. |
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63 | |
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64 | Calling the classes from interrupts follows the rules of RTEMS. An |
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65 | exception introduced by the class library is the last status code. |
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66 | There is only one last status code for each instance of the library's |
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67 | classes and it is not protected. This needs to be watched for. Maybe |
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68 | a better solution needs to be found, such as interrupt calls do not set |
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69 | the last status code. |
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70 | |
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71 | RTEMS objects created by the C++ library can be operated on by C code |
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72 | just as any other RTEMS object. If limitations exist they should be |
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73 | documented in under the class. |
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74 | |
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75 | RTEMS Object Ownership |
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76 | ====================== |
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77 | |
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78 | The concept of ownership of an object is not defined as part of the |
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79 | RTEMS C API. A piece of code executing as part a task can create a |
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80 | message queue. Another piece of code running as part of a different |
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81 | task can destroy the message queue. Correct behavior between the code |
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82 | that creates the message queue and the code which destroy's the |
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83 | message queue must be provided by the programmer. |
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84 | |
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85 | The librtems++ supports the concept of ownership of an RTEMS object. |
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86 | Only the C++ object that creates the RTEMS object can destroy it. A |
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87 | C++ object can connect to an existing RTEMS object and control it, |
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88 | how-ever it can not destroy it. |
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89 | |
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90 | Copy constructors and assignment operators are provided to in-force |
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91 | this rule. |
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92 | |
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93 | Ownership only applies to classes that create RTEMS objects. These |
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94 | classes contain a flag which signals ownership of the id. |
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95 | |
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96 | Timeouts |
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97 | ======== |
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98 | |
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99 | The timeout value is specified in micro-seconds. The classes turn the |
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100 | micro-second timeout value into ticks required by the RTEMS C API. |
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101 | |
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102 | This causes a problem for timeout values which are less than one tick. |
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103 | This case is tested for and the timeout value is set to one tick. All |
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104 | other cases round down to the nearest tick. |
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105 | |
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106 | Status Codes |
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107 | ============ |
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108 | |
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109 | All classes which form the C++ API are derived from the StatusCode |
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110 | class. This class provides a common method for handling the status |
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111 | code returned by RTEMS. |
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112 | |
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113 | The last returned status code is held in the StatusCode object. It |
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114 | can be queried directly, or as a boolean. You can also obtain an |
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115 | error string for the status code. |
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116 | |
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117 | The setting of a status code is restricted to derived classes. |
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118 | |
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119 | The last status code attribute of the class is only ever set to an |
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120 | RTEMS defined status code. |
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121 | |
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122 | Event Class |
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123 | =========== |
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124 | |
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125 | The event class allows users to send and receive events to and from |
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126 | tasks. |
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127 | |
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128 | Events objects are by default connected the RTEMS_SELF task. A send |
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129 | or receive will operate on the task currently executing. |
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130 | |
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131 | An Event object can be connected to a task using the connect method. |
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132 | The name is the name of the task. Connection can also be achieved by |
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133 | using the copy constructor or assignment operator. |
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134 | |
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135 | Events can be sent to a task by specifying an RTEMS task id, or by |
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136 | passing a reference to a Task object. |
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137 | |
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138 | Interrupt Class |
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139 | =============== |
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140 | |
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141 | The interrupt class allows a protected virtual method of a derived |
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142 | class to be an interrupt handler. |
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143 | |
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144 | You derive from this class and provide the handler method. The next |
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145 | interrupt after the vector is caught will cause the handler method to |
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146 | be entered. |
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147 | |
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148 | You can chain the interrupt by calling the chain method. If the old |
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149 | handler is not an instance of this class the chain is passed as "void |
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150 | (*)(void)". If it is an instance of this class, the handler method is |
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151 | directly called. (Chaining has not been tested) |
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152 | |
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153 | This class implements a table of pointers to the last instance to |
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154 | catch the interrupt. A static method of the class catches the |
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155 | interrupt and re-directs the interrupt to the instance in the table. |
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156 | The re-direct adds a additional virtual function call and return to |
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157 | the overhead of the interrupt. For a i386 type processor this is |
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158 | about 12 instructions including the function call entry. |
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159 | |
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160 | Message Queue Class |
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161 | =================== |
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162 | |
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163 | The MessageQueue class allows message queue's to be created, or |
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164 | connected too. Only the creator can destroy a message queue. |
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165 | |
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166 | The class implements, sending, urgent sending, broadcast, flushing, |
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167 | and receiving. |
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168 | |
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169 | Semaphore Class |
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170 | =============== |
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171 | |
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172 | The Semaphore class allows semaphores to be created, or connected |
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173 | too. Only the creator can destroy a semaphore. |
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174 | |
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175 | All types of semaphores can be created. |
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176 | |
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177 | (Not tested in the test code) |
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178 | |
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179 | Task Class |
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180 | ========== |
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181 | |
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182 | The Task class allows tasks to be created, or connected too. Only the |
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183 | creator can destroy a task. |
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184 | |
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185 | If creating a task, derive from the Task class and provide the body |
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186 | method. The body method is the entry point for a task. When |
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187 | connecting to an existing task, no body method is required to be |
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188 | provided. It is how-ever required if you create a task. This is not |
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189 | enforced by the compiler, how-ever the default body will be entered, |
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190 | and it contains no code. The RTEMS default behaviour for a task that |
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191 | returns occurs. |
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192 | |
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193 | The mode of a task is controlled using the TaskMode class. |
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194 | |
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195 | The Task class allows you to start, restart, suspend, and resume a |
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196 | task. You can control the priority, and access the note-pad |
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197 | registers. The task can also be slept using the wake_after and |
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198 | wake_when methods. |
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199 | |
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200 | Currently the task argument is used to pass the 'this' pointer to the |
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201 | libraries default task body. The actual argument is held in the class |
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202 | instance and passed to the virtual body method. This means of passing |
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203 | the 'this' pointer through RTEMS to the default task body requires the |
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204 | actual task object to perform a restart call. This is not really the |
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205 | best solution to the problem. Another solution is to remove a notpad |
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206 | register, say 31 from the task and use it. This would mean any Task |
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207 | object could stop and restart a task how-ever a notpad register is |
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208 | lost. Any other ideas are welcome. |
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209 | |
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210 | Task Mode Class |
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211 | =============== |
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212 | |
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213 | The TaskMode class allows you to query or change the mode of a task. |
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214 | The object only operates on the currently executing task. |
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215 | |
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216 | The standard flags defined in RTEMS are used. |
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217 | |
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218 | Methods are provided to operate on a group of modes which are required |
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219 | to be changed in a single operation. The mode and mask is specified |
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220 | by ORing the required flags as documented in the RTEMS manual. |
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221 | |
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222 | Methods are provided for accessing and controlling a specific mode. |
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223 | The returned value will only contain the requested mode's flags, and |
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224 | only the that mode will be changed when setting a mode. |
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225 | |
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226 | Timer Class |
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227 | =========== |
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228 | |
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229 | The Timer class allows timers to be created. You cannot connect to an |
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230 | existing timer. |
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231 | |
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232 | You derive from the Timer class and provide the trigger method. This |
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233 | method is called when the timer triggers or times out. |
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234 | |
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235 | You can request a single shot timer using the fire_after or fire_when |
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236 | methods, or a periodic timer by calling the repeat_file_at method. |
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237 | |
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238 | You cannot copy timer objects. |
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239 | |
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240 | Contact |
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241 | ======= |
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242 | Send any question to me Chris Johns at cjohns@plessey.com.au, or the RTEMS |
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243 | mailing list. |
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244 | |
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245 | To Do |
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246 | ===== |
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247 | |
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248 | 1) Develop a complete test suite (under way, cjohns@plessey.com.au). |
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249 | |
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250 | 2) Complete wrapping the remaining RTEMS C API. |
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251 | |
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252 | 3) Provide light weight cout/cerr/clog classes based on printf for |
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253 | embedded systems. |
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254 | |
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255 | 4) Provide a memory serial class which maps the <</>> operators onto |
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256 | raw memory in network byte order independent of CPU byte order. |
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257 | |
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258 | 5) Fix the Task class so any Task object can restart a task. |
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259 | |
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260 | 6) Provide some frame work classes which allow actor type objects that |
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261 | start in an ordered manner. |
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262 | |
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