make shared memory exclusive

Author: NikolasK-source

src/main.cpp

@@ -107,6 +107,11 @@ int main(int argc, char **argv) {
                           "tcp timeout in seconds. Set to 0 to use the system defaults (not recommended).",
                           cxxopts::value<std::size_t>()->default_value("5"))
 #endif
+                         ("force",
+                          "Force the use of the shared memory even if it already exists. "
+                          "Do not use this option per default! "
+                          "It should only be used if the shared memory of an improperly terminated instance continues "
+                          "to exist as an orphan and is no longer used.")
                          ("h,help",
                           "print usage")
                          ("version",
@@ -181,18 +186,25 @@ int main(int argc, char **argv) {
     }
 
     // create shared memory object for modbus registers
-    Modbus::shm::Shm_Mapping mapping(args["do-registers"].as<std::size_t>(),
-                                     args["di-registers"].as<std::size_t>(),
-                                     args["ao-registers"].as<std::size_t>(),
-                                     args["ai-registers"].as<std::size_t>(),
-                                     args["name-prefix"].as<std::string>());
+    std::unique_ptr<Modbus::shm::Shm_Mapping> mapping;
+    try {
+        mapping = std::make_unique<Modbus::shm::Shm_Mapping>(args["do-registers"].as<std::size_t>(),
+                                                             args["di-registers"].as<std::size_t>(),
+                                                             args["ao-registers"].as<std::size_t>(),
+                                                             args["ai-registers"].as<std::size_t>(),
+                                                             args["name-prefix"].as<std::string>(),
+                                                             args.count("force") > 0);
+    } catch (const std::system_error &e) {
+        std::cerr << e.what() << std::endl;
+        exit(EX_OSERR);
+    }
 
     // create slave
     std::unique_ptr<Modbus::TCP::Slave> slave;
     try {
         slave = std::make_unique<Modbus::TCP::Slave>(args["ip"].as<std::string>(),
                                                      args["port"].as<uint16_t>(),
-                                                     mapping.get_mapping(),
+                                                     mapping->get_mapping(),
 #ifdef OS_LINUX
                                                      args["tcp-timeout"].as<std::size_t>());
 #else

src/modbus_shm.cpp

@@ -18,7 +18,8 @@ Shm_Mapping::Shm_Mapping(std::size_t        nb_bits,
                          std::size_t        nb_input_bits,
                          std::size_t        nb_registers,
                          std::size_t        nb_input_registers,
-                         const std::string &prefix) {
+                         const std::string &prefix,
+                         bool               force) {
     // check argument ranges
     if (nb_bits > 0x10000 || !nb_bits) throw std::invalid_argument("invalid number of digital output registers.");
     if (nb_input_bits > 0x10000 || !nb_input_bits)
@@ -50,8 +51,11 @@ Shm_Mapping::Shm_Mapping(std::size_t        nb_bits,
     for (std::size_t i = 0; i < reg_index_t::REG_COUNT; ++i) {
         auto &shm = shm_data[i];
 
+        int flags = O_RDWR | O_CREAT;
+        if (!force) flags |= O_EXCL;
+
         // create shm object
-        shm.fd = shm_open(shm.name.c_str(), O_RDWR | O_CREAT, 0660);
+        shm.fd = shm_open(shm.name.c_str(), flags, 0660);
         if (shm.fd < 0) {
             throw std::system_error(
                     errno, std::generic_category(), "Failed to create shared memory '" + shm.name + '\'');

src/modbus_shm.hpp

@@ -50,12 +50,14 @@ class Shm_Mapping {
      * @param nb_registers number of analog output registers (AO)
      * @param nb_input_registers number of analog input registers (AI)
      * @param shm_name_prefix name prefix of the created shared memory object
+     * @param force do not fail if the shared memory exist, but use the existing shared memory
      */
     Shm_Mapping(std::size_t        nb_bits,
                 std::size_t        nb_input_bits,
                 std::size_t        nb_registers,
                 std::size_t        nb_input_registers,
-                const std::string &shm_name_prefix = "modbus_");
+                const std::string &shm_name_prefix = "modbus_",
+                bool               force           = false);
 
     ~Shm_Mapping();