The header for the C programming language’s General Purpose Standard Library, stdlib.h, declares a variety of utility functions for type conversions, memory allocation, process control, and other similar tasks. The header also defines a number of data types and macros.
Prototypes for standard input/output functions such as scanf/printf can be found in stdio. h. Without this file, it will be impossible to read keyboard input or write output to the screen. The name std(Standard)-i(input)-o indicates that it is a header file or library for Standard Input and Output (output).