support for specified shm modes

include/shm.h

@@ -9,7 +9,12 @@ namespace shm {
 
 using id_t = void*;
 
-IPC_EXPORT id_t   acquire(char const * name, std::size_t size);
+enum : unsigned {
+    create = 0x01,
+    open   = 0x02
+};
+
+IPC_EXPORT id_t   acquire(char const * name, std::size_t size, unsigned mode = create | open);
 IPC_EXPORT void * to_mem (id_t id);
 IPC_EXPORT void   release(id_t id, void * mem, std::size_t size);
 IPC_EXPORT void   remove (char const * name);
@@ -17,7 +22,7 @@ IPC_EXPORT void   remove (char const * name);
 class IPC_EXPORT handle {
 public:
     handle();
-    handle(char const * name, std::size_t size);
+    handle(char const * name, std::size_t size, unsigned mode = create | open);
     handle(handle&& rhs);
 
     ~handle();
@@ -29,7 +34,7 @@ public:
     std::size_t  size () const;
     char const * name () const;
 
-    bool acquire(char const * name, std::size_t size);
+    bool acquire(char const * name, std::size_t size, unsigned mode = create | open);
     void release();
 
     void* get() const;

src/platform/shm_linux.cpp

@@ -40,7 +40,7 @@ inline auto& str_of(void* mem, std::size_t size) {
 namespace ipc {
 namespace shm {
 
-id_t acquire(char const * name, std::size_t size) {
+id_t acquire(char const * name, std::size_t size, unsigned mode) {
     if (name == nullptr || name[0] == '\0' || size == 0) {
         return nullptr;
     }
@@ -49,8 +49,22 @@ id_t acquire(char const * name, std::size_t size) {
         ipc::error("name is too long!: [%d]%s\n", static_cast<int>(op_name.size()), op_name.c_str());
         return nullptr;
     }
-    int fd = ::shm_open(op_name.c_str(), O_CREAT | O_RDWR,
-                                         S_IRUSR | S_IWUSR |
+    // Open the object for read-write access.
+    int flag = O_RDWR;
+    switch (mode) {
+    case open:
+        break;
+    // The check for the existence of the object, 
+    // and its creation if it does not exist, are performed atomically.
+    case create:
+        flag |= O_CREAT | O_EXCL;
+        break;
+    // Create the shared memory object if it does not exist.
+    default:
+        flag |= O_CREAT;
+        break;
+    }
+    int fd = ::shm_open(op_name.c_str(), flag, S_IRUSR | S_IWUSR |
                                                S_IRGRP | S_IWGRP |
                                                S_IROTH | S_IWOTH);
     if (fd == -1) {

src/platform/shm_win.cpp

@@ -13,16 +13,29 @@
 namespace ipc {
 namespace shm {
 
-id_t acquire(char const * name, std::size_t size) {
+id_t acquire(char const * name, std::size_t size, unsigned mode) {
     if (name == nullptr || name[0] == '\0' || size == 0) {
         return nullptr;
     }
-    HANDLE h = ::CreateFileMapping(INVALID_HANDLE_VALUE, NULL,
-                                   PAGE_READWRITE | SEC_COMMIT,
-                                   0, static_cast<DWORD>(size),
-                                   ipc::detail::to_tchar(std::string{"__IPC_SHM__"} + name).c_str());
+    HANDLE h;
+    auto fmt_name = ipc::detail::to_tchar(std::string{"__IPC_SHM__"} + name);
+    // Opens a named file mapping object.
+    if (mode == open) {
+        h = ::OpenFileMapping(FILE_MAP_ALL_ACCESS, FALSE, fmt_name.c_str());
+    }
+    // Creates or opens a named file mapping object for a specified file.
+    else {
+        h = ::CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE | SEC_COMMIT,
+                                0, static_cast<DWORD>(size), fmt_name.c_str());
+        // If the object exists before the function call, the function returns a handle to the existing object 
+        // (with its current size, not the specified size), and GetLastError returns ERROR_ALREADY_EXISTS.
+        if ((mode == create) && (::GetLastError() == ERROR_ALREADY_EXISTS)) {
+            ::CloseHandle(h);
+            h = NULL;
+        }
+    }
     if (h == NULL) {
-        ipc::error("fail CreateFileMapping[%d]: %s\n", static_cast<int>(::GetLastError()), name);
+        ipc::error("fail CreateFileMapping/OpenFileMapping[%d]: %s\n", static_cast<int>(::GetLastError()), name);
         return nullptr;
     }
     return static_cast<id_t>(h);

src/shm.cpp

@@ -21,9 +21,9 @@ handle::handle()
     : p_(p_->make()) {
 }
 
-handle::handle(char const * name, std::size_t size)
+handle::handle(char const * name, std::size_t size, unsigned mode)
     : handle() {
-    acquire(name, size);
+    acquire(name, size, mode);
 }
 
 handle::handle(handle&& rhs)
@@ -57,10 +57,10 @@ char const * handle::name() const {
     return impl(p_)->n_.c_str();
 }
 
-bool handle::acquire(char const * name, std::size_t size) {
+bool handle::acquire(char const * name, std::size_t size, unsigned mode) {
     release();
     impl(p_)->id_ = shm::acquire((impl(p_)->n_ = name).c_str(),
-                                  impl(p_)->s_ = size);
+                                  impl(p_)->s_ = size, mode);
     impl(p_)->m_  = shm::to_mem  (impl(p_)->id_);
     return valid();
 }