# 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 `CheckCSourceCompiles`). 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 compile command line flags.

`CMAKE_REQUIRED_DEFINITIONS`

  a ;-list of macros to define (-DFOO=bar).

`CMAKE_REQUIRED_INCLUDES`

  a ;-list of header search paths to pass to the compiler.

`CMAKE_REQUIRED_LINK_OPTIONS`

  New in version 3.14: a ;-list of options to add to the link command.

`CMAKE_REQUIRED_LIBRARIES`

  a ;-list of libraries to add to the link command. See policy `CMP0075`.

`CMAKE_REQUIRED_QUIET`

  New in version 3.1: execute quietly without messages.

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)