Declarations and Definitions

Programs must be broken up into units which are compiled separately

• standard functions are compiled and put in libraries

• program code is compiled and linked to libraries

• other program code is compiled and linked to libraries

Functions and other externals must be declared before their use

      extern double sqrt (double);

      double x, y, z, r;
      /* initialization, etc. */
      r = sqrt (x*x + y*y + z*z);

• sqrt (double) and sqrt (double x) are equivalent in the declaration statement

What would happen if the declaration we used did not correspond to the function definition in the library?

To ensure consistency, we must force the library function and the function we use in a program to share the same declaration