# CheckLibraryExists

Check once if the function exists in system or specified library.

check_library_exists

      check_library_exists(<library> <function> <location> <variable>)

  Check that the library `<library>` exists in the given location `<location>` and has the specified `<function>`. The result is stored in an internal cache variable `<variable>`. If `<location>` is empty string, default directories are searched.

Prefer using `CheckSymbolExists` or `CheckSourceCompiles` instead of this module for more robust detection if a function is available in a library.

The following variables may be set before calling this macro to modify the way the check is run:

> `CMAKE_REQUIRED_FLAGS`

>   String of additional flags to pass to the compiler. The string must be space-delimited--a ;-list will not work. The contents of `CMAKE_<LANG>_FLAGS` and its associated configuration-specific variable are automatically added to the compiler command before the contents of `CMAKE_REQUIRED_FLAGS`.

> `CMAKE_REQUIRED_DEFINITIONS`

>   A ;-list of compiler definitions of the form `-DFOO` or `-DFOO=bar`. A definition for the name specified by `<resultVar>` will also be added automatically.

> `CMAKE_REQUIRED_LINK_OPTIONS`

>   Added in version 3.14.

>   A ;-list of options to add to the link command (see `try_compile()` for further details).

> `CMAKE_REQUIRED_LIBRARIES`

>   A ;-list of libraries to add to the link command. These can be the name of system libraries or they can be Imported Targets (see `try_compile()` for further details).

> `CMAKE_REQUIRED_LINK_DIRECTORIES`

>   Added in version 3.31.

>   A ;-list of libraries search paths to pass to the linker (see `try_compile()` for further details).

> `CMAKE_REQUIRED_QUIET`

>   Added in version 3.1.

>   If this variable evaluates to a boolean true value, all status messages associated with the check will be suppressed.

## Examples

This module can be useful for performing so-called sanity checks to verify that the specified library provides the expected functionality and is indeed the correct one being located.

For example, to check if the `curl` library exists in the default paths and has the `curl_easy_perform` function:

    include(CheckLibraryExists)
    check_library_exists(curl curl_easy_perform "" HAVE_LIBRARY_CURL)

To check if library exists in specific non-standard location and has a specified function:

    include(CheckLibraryExists)
    check_library_exists(curl curl_easy_perform "/opt/curl/lib" HAVE_LIBRARY_CURL)

Also IMPORTED library (for example, from the `find_package()` call) can be used:

    find_package(CURL)

    # ...

    if(TARGET CURL::libcurl)
      include(CheckLibraryExists)
      check_library_exists(CURL::libcurl curl_easy_perform "" HAVE_LIBRARY_CURL)
    endif()