CRT, the C Runtime Library
Standard C language functions. On Windows, this is implemented in msvcrt.dll (however, there also are version-specific msvcrXX.dlls, the Microsoft Visual C++ runtimes). On Linux, the C runtime is typically implemented by glibc. For DOS, FreeBASIC uses DJGPP, which provides a libc library.
Websites: http://msdn.microsoft.com/en-us/library/59ey50w6.aspx, https://www.gnu.org/software/libc/, http://www.delorie.com/djgpp/
Platforms supported: Win32, Linux, DOS
Headers to include: crt.bi
Function reference: C Runtime Functions
MSDN function reference: http://msdn.microsoft.com/en-us/library/634ca0c2.aspx