fix arithmetic on a pointer to an incomplete type

When including `bcc/BPF.h` there's a compilation error regarding arithmetic on a pointer to an incomplete type 'ebpf::USDT'.

The fix is straightforward: swap the order of the class declarations in the `bcc/BPF.h` file, after which things compile successfully.

More details are described in the issue iovisor#4160

src/cc/api/BPF.h

@@ -40,7 +40,74 @@ struct open_probe_t {
  std::vector<std::pair<int, int>>* per_cpu_fd;
 };
 
-class USDT;
+class BPF;
+
+class USDT {
+ public:
+ USDT(const std::string& binary_path, const std::string& provider,
+ const std::string& name, const std::string& probe_func);
+ USDT(pid_t pid, const std::string& provider, const std::string& name,
+ const std::string& probe_func);
+ USDT(const std::string& binary_path, pid_t pid, const std::string& provider,
+ const std::string& name, const std::string& probe_func);
+ USDT(const USDT& usdt);
+ USDT(USDT&& usdt) noexcept;
+
+ const std::string &binary_path() const { return binary_path_; }
+ pid_t pid() const { return pid_; }
+ const std::string &provider() const { return provider_; }
+ const std::string &name() const { return name_; }
+ const std::string &probe_func() const { return probe_func_; }
+
+ StatusTuple init();
+
+ bool operator==(const USDT& other) const;
+
+ std::string print_name() const {
+ return provider_ + ":" + name_ + " from binary " + binary_path_ + " PID " +
+ std::to_string(pid_) + " for probe " + probe_func_;
+ }
+
+ friend std::ostream& operator<<(std::ostream& out, const USDT& usdt) {
+ return out << usdt.provider_ << ":" << usdt.name_ << " from binary "
+ << usdt.binary_path_ << " PID " << usdt.pid_ << " for probe "
+ << usdt.probe_func_;
+ }
+
+ // When the kludge flag is set to 1 (default), we will only match on inode
+ // when searching for modules in /proc/PID/maps that might contain the
+ // tracepoint we're looking for.
+ // By setting this to 0, we will match on both inode and
+ // (dev_major, dev_minor), which is a more accurate way to uniquely
+ // identify a file, but may fail depending on the filesystem backing the
+ // target file (see bcc#2715)
+ //
+ // This hack exists because btrfs and overlayfs report different device
+ // numbers for files in /proc/PID/maps vs stat syscall. Don't use it unless
+ // you've had issues with inode collisions. Both btrfs and overlayfs are
+ // known to require inode-only resolution to accurately match a file.
+ //
+ // set_probe_matching_kludge(0) must be called before USDTs are submitted to
+ // BPF::init()
+ int set_probe_matching_kludge(uint8_t kludge);
+
+ private:
+ bool initialized_;
+
+ std::string binary_path_;
+ pid_t pid_;
+
+ std::string provider_;
+ std::string name_;
+ std::string probe_func_;
+
+ std::unique_ptr<void, std::function<void(void*)>> probe_;
+ std::string program_text_;
+
+ uint8_t mod_match_inode_only_;
+
+ friend class BPF;
+};
 
 class BPF {
  public:
@@ -349,71 +416,4 @@ class BPF {
  std::map<std::pair<uint32_t, uint32_t>, open_probe_t> perf_events_;
 };
 
-class USDT {
- public:
- USDT(const std::string& binary_path, const std::string& provider,
- const std::string& name, const std::string& probe_func);
- USDT(pid_t pid, const std::string& provider, const std::string& name,
- const std::string& probe_func);
- USDT(const std::string& binary_path, pid_t pid, const std::string& provider,
- const std::string& name, const std::string& probe_func);
- USDT(const USDT& usdt);
- USDT(USDT&& usdt) noexcept;
-
- const std::string &binary_path() const { return binary_path_; }
- pid_t pid() const { return pid_; }
- const std::string &provider() const { return provider_; }
- const std::string &name() const { return name_; }
- const std::string &probe_func() const { return probe_func_; }
-
- StatusTuple init();
-
- bool operator==(const USDT& other) const;
-
- std::string print_name() const {
- return provider_ + ":" + name_ + " from binary " + binary_path_ + " PID " +
- std::to_string(pid_) + " for probe " + probe_func_;
- }
-
- friend std::ostream& operator<<(std::ostream& out, const USDT& usdt) {
- return out << usdt.provider_ << ":" << usdt.name_ << " from binary "
- << usdt.binary_path_ << " PID " << usdt.pid_ << " for probe "
- << usdt.probe_func_;
- }
-
- // When the kludge flag is set to 1 (default), we will only match on inode
- // when searching for modules in /proc/PID/maps that might contain the
- // tracepoint we're looking for.
- // By setting this to 0, we will match on both inode and
- // (dev_major, dev_minor), which is a more accurate way to uniquely
- // identify a file, but may fail depending on the filesystem backing the
- // target file (see bcc#2715)
- //
- // This hack exists because btrfs and overlayfs report different device
- // numbers for files in /proc/PID/maps vs stat syscall. Don't use it unless
- // you've had issues with inode collisions. Both btrfs and overlayfs are
- // known to require inode-only resolution to accurately match a file.
- //
- // set_probe_matching_kludge(0) must be called before USDTs are submitted to
- // BPF::init()
- int set_probe_matching_kludge(uint8_t kludge);
-
- private:
- bool initialized_;
-
- std::string binary_path_;
- pid_t pid_;
-
- std::string provider_;
- std::string name_;
- std::string probe_func_;
-
- std::unique_ptr<void, std::function<void(void*)>> probe_;
- std::string program_text_;
-
- uint8_t mod_match_inode_only_;
-
- friend class BPF;
-};
-
 } // namespace ebpf