libcamera: file_descriptor: Add a function to retrieve the inode

The inode is useful to check if two file descriptors refer to the same
file. Add a function to retrieve it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>

include/libcamera/file_descriptor.h

@@ -8,6 +8,7 @@
 #define __LIBCAMERA_FILE_DESCRIPTOR_H__
 
 #include <memory>
+#include <sys/types.h>
 
 namespace libcamera {
 
@@ -27,6 +28,8 @@ public:
 	int fd() const { return fd_ ? fd_->fd() : -1; }
 	FileDescriptor dup() const;
 
+	ino_t inode() const;
+
 private:
 	class Descriptor
 	{

src/libcamera/file_descriptor.cpp

@@ -8,6 +8,8 @@
 #include <libcamera/file_descriptor.h>
 
 #include <string.h>
+#include <sys/stat.h>
+#include <sys/types.h>
 #include <unistd.h>
 #include <utility>
 
@@ -221,6 +223,30 @@ FileDescriptor FileDescriptor::dup() const
 	return FileDescriptor(fd());
 }
 
+/**
+ * \brief Retrieve the file descriptor inode
+ *
+ * \todo Should this move to the File class ?
+ *
+ * \return The file descriptor inode on success, or 0 on error
+ */
+ino_t FileDescriptor::inode() const
+{
+	if (!isValid())
+		return 0;
+
+	struct stat st;
+	int ret = fstat(fd_->fd(), &st);
+	if (ret < 0) {
+		ret = -errno;
+		LOG(FileDescriptor, Fatal)
+			<< "Failed to fstat() fd: " << strerror(-ret);
+		return 0;
+	}
+
+	return st.st_ino;
+}
+
 FileDescriptor::Descriptor::Descriptor(int fd, bool duplicate)
 {
 	if (!duplicate) {