15 213 Malloc Recitation Allocate The Right Amount of Space Which of these are correct incorrect or incorrect but will still behave as expected int x malloc sizeof int int x malloc sizeof int int x malloc sizeof x int x malloc sizeof x char orig some string char copy malloc orig char copy malloc strlen orig char copy malloc strlen orig 1 char copy malloc strlen orig 1 struct player int health char name 4 struct player p malloc sizeof struct player struct player p malloc sizeof struct player What s wrong with this code define SUCCESS 0 define ERROR 1 typedef struct char name unsigned age double height person t int person init person t p p malloc sizeof person t p name NULL p age 18 p height 5 5 return SUCCESS What s wrong with this code person t new person person t p malloc sizeof person t if person init p SUCCESS return NULL return p What s wrong with this code static person t people int init people int n people malloc sizeof person t n if people return ERROR for int i 0 i n i person init people i return SUCCESS int clear person int i return person init people i What s wrong with this code int main pid t pid if pid fork 0 int status waitpid pid status 0 printf Child d is done n pid else Really long computation return 0 What s wrong with this code int main int counts malloc MAX sizeof int while 1 int n printf Enter a number n scanf d n if n 0 n MAX counts n else if n 1 break int i for i 0 i MAX i printf d appeared d times n i counts i return 0 Things to Remember Allocate the right amount of space if you are allocating something to put in a person allocate sizeof person etc When a function takes in a pointer to an object it s wrong to allocate space for that object inside the function ALWAYS check the return value of malloc You should call free on every pointer you get back from malloc exactly one once you re done using it Don t overstep your bounds Pointers don t magically point to something you have to allocate space for an object to point to malloc doesn t initialize the returned memory use calloc if appropriate See the m c re alloc manpages for exact behavior Macros in C A macro is a code fragment that has been given a name The preprocessor will go through your source and replace every occurrence of that name with the fragment of code Macros can make your code cleaner and yet not incur the overhead of a function call How not to use macros What s wrong here define twice x 2 x twice x 1 2x 2 define twice x x x define min X Y X Y X Y twice x 2x min a b min foo a foo b What s wrong here define debug printf is debug str if is debug printf s n str if x 0 debug printf debug on Negative input else debug printf debug on Non negative input if x 0 if debugon printf s n Negative input else if debugon printf s n OK input OK input never prints Things to Remember Surround names in macros with parentheses Don t pass code with side effects to macros you have no idea how many times they re evaluated Try not to evaluate macro arguments more than once in your macros When using macros in conditionals put braces around them
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