There are situations in runtime, where program runs in exceptional way. This could be handle in many different ways: using boolean return values, assertions or exceptions.

Handling exceptional situations:

struct record { ... };

record r;
extern int errno;

FILE *fp;

if ( (fp = fopen( "fname", "r")) != NULL )
{
    fprintf( stderr, "can't open file %s\n", "fname");
    errno = 1;
}
else if ( ! fseek( fp, 0L, n*sizeof(r)) )
{
    fprintf( stderr, "can't find record %d\n", n);
    errno = 2;
}
else if ( 1 != fread( &r, sizeof(r), 1, fp) )
{
    fprintf( stderr, "can't read record\n");
    errno = 3;
}
else ...


//============================================
//
//  asserts
//

#include <cassert>

int main()
{
    ...
    assert( ptr );
    ...
}