postgresql FRONTEND、EXEC_BACKEND宏定义的作用
FONTEND宏主要用于标记某些可能会在libpq客户端使用的场景，虽然他们主要用于服务端。如下：
/* * Although this header file is nominally backend-only, certain frontend * programs like pg_controldata include it via postgres.h. For some compilers * it's necessary to hide the inline definition of MemoryContextSwitchTo in * this scenario; hence the #ifndef FRONTEND. */ #ifndef FRONTEND static inline MemoryContext MemoryContextSwitchTo(MemoryContext context) { MemoryContext old = CurrentMemoryContext; CurrentMemoryContext = context; return old; } #endif /* FRONTEND */
典型的一处用途是pg_waldump获取下一个可读取的wal record位置，如下：
#ifdef FRONTEND extern XLogRecPtr XLogFindNextRecord(XLogReaderState *state, XLogRecPtr RecPtr); #endif /* FRONTEND */
并且看起来，主要用于windows下为主。
============================
除了FRONTEND宏定义外，还有一个EXEC_BACKEND宏定义。主要使用在PostgreSQL源代码 postmaster.c 的BackendStartup 函数中，如下：
#ifdef EXEC_BACKEND pid = backend_forkexec(port); #else /* !EXEC_BACKEND */ pid = fork_process(); .... #endif /* EXEC_BACKEND */
经相关上下文搜索以及网上资料参考，看起来也是windows使用为主。因为我们不支持windows版，所以编译日志选项中确实是没有出现过EXEC_BACKEND这个宏标志。
https://my.oschina.net/ashnah/blog/3132343
https://www.cnblogs.com/gaojian/archive/2012/07/19/2598959.html
通过grep可以搜索到pg中的所有宏开关，如下：
[zjh@hs-10-20-30-193 postgresql-13.3]$ grep -r "#ifdef HAVE_" | awk '{print $NF}' | grep HAVE | sort | uniq HAVE_ASN1_STRING_GET0_DATA HAVE_ATOMIC_H HAVE_ATOMICS HAVE_BACKTRACE_SYMBOLS HAVE_BE_TLS_GET_CERTIFICATE_HASH HAVE_BIO_METH_NEW HAVE_BUGGY_SOLARIS_STRTOD HAVE_BUGGY_STRTOF HAVE__BUILTIN_CLZ HAVE__BUILTIN_CONSTANT_P HAVE__BUILTIN_CTZ HAVE__BUILTIN_FRAME_ADDRESS HAVE__BUILTIN_POPCOUNT HAVE__BUILTIN_TYPES_COMPATIBLE_P HAVE_CLOCK_GETTIME HAVE_COMPUTED_GOTO HAVE__CONFIGTHREADLOCALE HAVE_COPYFILE_H HAVE__CPUID HAVE_CRYPTO_LOCK HAVE_DLOPEN HAVE_EXECINFO_H HAVE_FDATASYNC HAVE_FLOCK HAVE_FSYNC_WRITETHROUGH HAVE_FUNCNAME__FUNC HAVE_FUNCNAME__FUNCTION HAVE_GCC__SYNC_INT32_TAS HAVE_GETADDRINFO HAVE__GET_CPUID HAVE_GETOPT_H HAVE_GETRLIMIT HAVE_GETRUSAGE HAVE_HSTRERROR HAVE_I_CONSTRAINT__BUILTIN_CONSTANT_P HAVE_IFADDRS_H HAVE_INT128 HAVE_INT64_TIMESTAMP HAVE_INT_OPTERR HAVE_INT_OPTRESET HAVE_INTTYPES_H HAVE_IPSET_ATTR_IFACE HAVE_IPV6 HAVE_KERNEL_OS_H HAVE_LANGINFO_H HAVE_LDAP_INITIALIZE HAVE_LIBCURL HAVE_LIBREADLINE HAVE_LIBZ HAVE_LIMITS_H HAVE_LINUX_EIDRM_BUG HAVE_LINUX_IP_VS_H HAVE_LINUX_NETFILTER_X_TABLES_H HAVE_LOCALE_T HAVE_LONG_INT_64 HAVE_LONG_LONG_INT_64 HAVE_MBARRIER_H HAVE_MBSTOWCS_L HAVE_NET_IF_H HAVE_NETINET_TCP_H HAVE_OPENSSL_INIT_SSL HAVE_PAM_PAM_APPL_H HAVE_PG_ATTRIBUTE_NORETURN HAVE_PGTLS_GET_PEER_CERTIFICATE_HASH HAVE_PIPE2 HAVE_POLL HAVE_POLL_H HAVE_PREAD HAVE_PS_STRINGS HAVE_PTHREAD_IS_THREADED_NP HAVE_PWRITE HAVE_READLINK HAVE_REPLICATION_ORIGINS HAVE_RL_COMPLETION_APPEND_CHARACTER HAVE_RL_COMPLETION_SUPPRESS_QUOTE HAVE_RL_FILENAME_QUOTE_CHARACTERS HAVE_RL_FILENAME_QUOTING_FUNCTION HAVE_RL_RESET_SCREEN_SIZE HAVE_SECURITY_PAM_APPL_H HAVE_SETSID HAVE_SHM_OPEN HAVE__STATIC_ASSERT HAVE_STDBOOL_H HAVE_STDINT_H HAVE_STRERROR_R HAVE_STRING_H HAVE_STRINGS_H HAVE_STRSIGNAL HAVE_STRTOLL HAVE_STRTOULL HAVE_STRUCT_CMSGCRED HAVE_STRUCT_SOCKADDR_SA_LEN HAVE_STRUCT_SOCKADDR_STORAGE HAVE_STRUCT_SOCKADDR_STORAGE___SS_FAMILY HAVE_STRUCT_SOCKADDR_STORAGE___SS_LEN HAVE_STRUCT_SOCKADDR_STORAGE_SS_LEN HAVE_STRUCT_SOCKADDR_UN HAVE_SYMLINK HAVE_SYNC_FILE_RANGE HAVE_SYS_EPOLL_H HAVE_SYS_EVENT_H HAVE_SYS_IPC_H HAVE_SYSLOG HAVE_SYS_PRCTL_H HAVE_SYS_PROCCTL_H HAVE_SYS_PSTAT_H HAVE_SYS_RESOURCE_H HAVE_SYS_SELECT_H HAVE_SYS_SEM_H HAVE_SYS_SHM_H HAVE_SYS_SOCKIO_H HAVE_SYS_STAT_H HAVE_SYS_TYPES_H HAVE_SYS_UCRED_H HAVE_SYS_UN_H HAVE_TERMIOS_H HAVE_TYPEOF HAVE_UCOL_STRCOLLUTF8 HAVE_UCRED_H HAVE_UNISTD_H HAVE_UNIX_SOCKETS HAVE_USELOCALE HAVE_UUID_BSD HAVE_UUID_E2FS HAVE_UUID_OSSP HAVE_VFORK_H HAVE_WCSTOMBS_L HAVE_WCTYPE_H HAVE_WORKING_LINK HAVE_X509_GET_SIGNATURE_NID HAVE_X86_64_POPCNTQ HAVE_XMLSTRUCTUREDERRORCONTEXT HAVE_XT_SET_INFO_MATCH_V4
