NULL, pointer to nothing, is a form of $0,$ meaning a NULL pointer is false and all other pointer values are true.

Space is allocated by calling malloc with the number of bytes needed $($for strings this is always one more than the maximum length of the string to be stored$)$:

char $*$pc $=$ malloc$($MAXSTR $+1)$ ; $//$ can hold a string of up to MAXSTR characters.

pc $=$ malloc$($strlen$("$Hello$!")+1)$ ; $//$ can hold a copy of "Hello" with the terminating null$$character $\text{'}\backslash 0\text{'}.$

The size given to malloc is always in bytes; the sizeof operator allows us to get the size in bytes of any type:

double $*$pd ;

pd $=$ malloc$(100*$ sizeof$($double$))$ ; $//$ can hold up to $100$ double precision numbers.double pi $=3\mathrm{.}14159$ ;

pd $=$ malloc$(100*$ sizeof$($pi$))$ ;$//$ can hold up to $100$ double precision numbers.

BE CAREFUL: If p is a pointer, then sizeof$($p$)$ is the number of bytes to hold a pointer, not what p points to$!$:

int $*$ip ;

ip $=$ malloc$($ sizeof$($ip$))$ ;$//$ WRONG! allocate space for an integer pointer.

ip $=$ malloc$($ sizeof$(*$ip$))$ ;$//$ CORRECT! allocate space for an integer.

ip $=$ malloc$($ sizeof$($int$))$ ;$//$ ALSO CORRECT: take the size of the type.

Space no longer needed must be returned to the memory allocator using free$()$:

pc $=$ malloc$($MAXSTR$+1)$ ;$//$ get space for a string

$//...$ use the allocated space $...$

free$($pc$)$ ;$//$ free up the space associated with pc

Orphaned storage:

char buf$[$MAXSTR$+1]$ ;

pc $=$ malloc$($MAXSTR $+1)$ ;

pc $=$ buf ;$$

Dangling references:

pc $=$ malloc$($MAXSTR $+1)$ ;

char $*$pc$1=$ pc ;

$//...$ use the allocated space $...$

free$($pc$)$ ;

$//...$ work without changing pc or pc$1...$

$*$pc $=\text{'}$X$\text{'}$ ;$//$ PROBLEM: space was freed $-$ may have been reallocated

$*$pc$1=\text{'}$Z$\text{'}$ ;$//$ PROBLEM: pc$1$ aliased pc$,$ so its space was also freed