C traps and defects Chapter 3 semantic "traps" 3.2 pointers to non arrays

Pointer to a non array
stay C In language , A string constant represents a block that includes all characters in the string and an empty character ('\0') Of memory
The address of the area . because C Language requires string constants to end with empty characters , For other strings ,C Programmers usually
This practice is also followed .
character string s and t, Connect two strings into a single string r. Do that , We can use common library functions strcpy
and strcat. The following method seems to be clear at a glance , But it can't meet our goal ： 
char *r;
strcpy(r, s);
strcat(r, t);
Why not , Because I'm not sure r Where . We should also see , Not just let r Point to an address , and r The address pointed to should also have memory space to hold the string , This memory space should have been allocated in some way .
to r Allocate a certain amount of memory space ： 
char r[100];
strcpy(r, s);
strcat(r, t);
as long as s and t The string pointed to is not too large , Then the method we use now can work normally . Unfortunately ,C The language forces us to declare the array size as a constant , So we dare not ensure r Large enough . However , majority C Language implementation provides us with a library function malloc, This function accepts an integer , Then allocate a block of memory that can hold the same number of characters . majority C The language implementation also provides library functions strlen, This function returns the number of characters included in a string . 
char *r, *malloc();
r = malloc(strlen(s) + strlen(t));
strcpy(r, s);
strcat(r, t);
This example is wrong , There are three reasons .
The first reason ,malloc Function may not be able to provide the requested memory , In this case malloc The function returns a null pointer as “ memory allocation failed ” Signal of time .
The second reason , to r The allocated memory should be released in time after use , This must be kept in mind . Because in the previous program example r Is declared as a local variable , So when you leave r When scoped ,r Automatically released . The modified program is displayed to r Allocated memory . The revised procedure is shown to r Allocated memory , To do this, you must explicitly free memory .
The third reason , And the most important reason , It is the previous routine that calls malloc Function did not allocate enough memory . Because of the convention that strings use empty characters as terminators . Library function strlen Returns the number of characters contained in the string in the parameter , The empty character as the end sign is not counted .
therefore , If strlen(s) The value of is n, Then the string actually needs n+1 Space of characters . therefore , We have to r Allocate one more character space . 
char *r, *malloc();
r = malloc(strlen(s) + strlen(t) + 1);
if (!r) {
    complain();
    exit();
}
strcpy(r, s);
strcat(r, t);
/* Use it after a period of time */
free(r);