CheckVariableExists

This module provides a command to check whether a C variable exists.

在 CMake 專案中使用以下命令載入該模組：

include(CheckVariableExists)

命令

This module provides the following command:

check_variable_exists

Checks once if a C variable exists:

check_variable_exists(<var> <variable>)

This command attempts to compile and link a test C program that references the specified C variable <var>. A boolean result of whether the check was successful is stored in an internal cache variable <variable>.

備註

Prefer using CheckSymbolExists or CheckSourceCompiles instead of this command for more robust detection. This command performs a link-only check and doesn't detect whether a variable is also declared in system or library headers. Neither can it detect variables that might be defined as preprocessor macros.

影響檢查的變數

可以在呼叫此命令之前設定以下變數，以修改檢查的執行方式：

CMAKE_REQUIRED_FLAGS

一個以空格分隔的字串，包含要傳遞給編譯器的額外旗標。分號分隔列表將無法運作。CMAKE_<LANG>_FLAGS 及其關聯的特定組態 CMAKE_<LANG>_FLAGS_<CONFIG> 變數的內容會在該變數的內容之前自動預置到編譯器命令中。

CMAKE_REQUIRED_DEFINITIONS

一個編譯器定義的分號分隔列表，每個定義的形式為 -DFOO 或 -DFOO=bar。由檢查命令的結果變數引數所指定的名稱的定義也會自動新增。

CMAKE_REQUIRED_LINK_OPTIONS

在 3.14 版被加入.

一個要新增到連結命令的選項的分號分隔列表（詳情請參見 try_compile()）。

CMAKE_REQUIRED_LIBRARIES

一個要新增到連結命令的程式庫的分號分隔列表。這些可以是系統程式庫的名稱，也可以是 Imported Targets`（詳情請參見 :command:`try_compile）。

CMAKE_REQUIRED_LINK_DIRECTORIES

在 3.31 版被加入.

一個要傳遞給連結器的程式庫搜尋路徑的分號分隔列表（詳情請參見 try_compile()）。

CMAKE_REQUIRED_QUIET

在 3.1 版被加入.

如果此變數的值為布林真值，則與檢查相關的所有狀態訊息都將被抑制。

範例

Example: Basic Usage

In the following example, a check is performed whether the linker sees the C variable tzname and stores the check result in the PROJECT_HAVE_TZNAME internal cache variable:

include(CheckVariableExists)

check_variable_exists(tzname PROJECT_HAVE_TZNAME)

Example: Isolated Check With Linked Libraries

In the following example, this module is used in combination with the CMakePushCheckState module to link additional required library using the CMAKE_REQUIRED_LIBRARIES variable. For example, in a find module, to check whether the Net-SNMP library has the usmHMAC192SHA256AuthProtocol array:

include(CheckVariableExists)
include(CMakePushCheckState)

find_library(SNMP_LIBRARY NAMES netsnmp)

if(SNMP_LIBRARY)
  cmake_push_check_state(RESET)

  set(CMAKE_REQUIRED_LIBRARIES ${SNMP_LIBRARY})

  check_variable_exists(usmHMAC192SHA256AuthProtocol SNMP_HAVE_SHA256)

  cmake_pop_check_state()
endif()

另請參見

• The CheckSymbolExists module to check whether a C symbol exists.