everest% pr -t -n2 assertions.c
 1      /* File: assertions.c
 2         Using the assert function.
 3      */
 4      
 5      #include <stdio.h>
 6      #include <assert.h>
 7      
 8      main() {
 9         int a = 3, b =15 ;
10      
11         printf("The first assertion is true\n") ;
12         assert(a < b) ;
13      
14         printf("The second assertion is false\n") ;
15         assert(b < a) ;
16      
17         printf("This line never gets printed\n") ;
18      }
everest% 

everest% cc assertions.c
everest% 

everest% a.out
The first assertion is true
The second assertion is false
Assertion failed: b < a, file assertions.c, line 15
Abort (core dumped)
everest% 

everest% cc -DNDEBUG assertions.c
everest% 

everest% a.out
The first assertion is true
The second assertion is false
This line never gets printed
everest%