# CheckSymbolExists

Provides a macro to check if a symbol exists as a function, variable, or macro in `C`.

check_symbol_exists

      check_symbol_exists(<symbol> <files> <variable>)

  Check that the `<symbol>` is available after including given header `<files>` and store the result in a `<variable>`. Specify the list of files in one argument as a semicolon-separated list. `<variable>` will be created as an internal cache variable.

If the header files define the symbol as a macro it is considered available and assumed to work. If the header files declare the symbol as a function or variable then the symbol must also be available for linking (so intrinsics may not be detected). If the symbol is a type, enum value, or intrinsic it will not be recognized (consider using `CheckTypeSize` or `CheckSourceCompiles`). If the check needs to be done in C++, consider using `CheckCXXSymbolExists` instead.

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_INCLUDES`

>   A ;-list of header search paths to pass to the compiler. These will be the only header search paths used--the contents of the `INCLUDE_DIRECTORIES` directory property will be ignored.

> `CMAKE_REQUIRED_LINK_OPTIONS`

>   New 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_QUIET`

>   New in version 3.1.

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

For example:

    include(CheckSymbolExists)

    # Check for macro SEEK_SET
    check_symbol_exists(SEEK_SET "stdio.h" HAVE_SEEK_SET)
    # Check for function fopen
    check_symbol_exists(fopen "stdio.h" HAVE_FOPEN)