CheckFunctionExists

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

在 CMake 專案中使用以下命令載入該模組：

include(CheckFunctionExists)

命令

This module provides the following command:

check_function_exists

Checks once whether a C function can be linked from system libraries:

check_function_exists(<function> <variable>)

This command checks whether the <function> is provided by libraries on the system, and stores the result in an internal cache variable <variable>.

備註

Prefer using CheckSymbolExists or CheckSourceCompiles instead of this command, for the following reasons:

  • check_function_exists() can't detect functions that are inlined in headers or defined as preprocessor macros.

  • check_function_exists() can't detect anything in the 32-bit versions of the Win32 API, because of a mismatch in calling conventions.

  • check_function_exists() only verifies linking, it does not verify that the function is declared in system headers.

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_INCLUDES

一個要傳遞給編譯器的標頭搜尋路徑的分號分隔列表。這些將是唯一使用的標頭搜尋路徑；INCLUDE_DIRECTORIES 目錄屬性的內容將被忽略。

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 to determine whether the linker sees the C function fopen(), and the result is stored in the HAVE_FOPEN internal cache variable:

include(CheckFunctionExists)

check_function_exists(fopen HAVE_FOPEN)

Example: Missing Declaration

As noted above, the CheckSymbolExists module is preferred for checking C functions, since it also verifies whether the function is declared or defined as a macro. In the following example, this module is used to check an edge case where a function may not be declared in system headers. For instance, on macOS, the fdatasync() function may be available in the C library, but its declaration is not provided in the unistd.h system header.

CMakeLists.txt
include(CheckFunctionExists)
include(CheckSymbolExists)

check_symbol_exists(fdatasync "unistd.h" HAVE_FDATASYNC)

# Check if fdatasync() is available in the C library.
if(NOT HAVE_FDATASYNC)
  check_function_exists(fdatasync HAVE_FDATASYNC_WITHOUT_DECL)
endif()

In such a case, the project can provide its own declaration if missing:

example.c
#ifdef HAVE_FDATASYNC_WITHOUT_DECL
  extern int fdatasync(int);
#endif

另請參見