sizes and arrays in c

Question

i'm following cLearnthehardway at exercise 8 the following code was written , i have 2 questions

• while printing and integers it used %ld to print them not %d !!
• while printing areas[10]//out of range it printed 0! why didn't it show me an error while it does by valgrind(6erros)

---

#include<stdio.h>

int main(int argc,char *argv[])
{

int areas[]={10,12,13,14,20};
char name[]="Zed";
char full_name[]={'Z','e','d',' ','A','.',' ','S','h','a','w','\0'};

printf("The size of an int: %ld\n", sizeof(int));//why didn't we use %d instead of %ld(*)
printf("The size of areas (int[]):%ld\n",sizeof(areas));//*
printf("The number of ints in areas : %ld\n",sizeof(areas)/sizeof(int));//*
printf("The first area is %d, the second area is %d\n",areas[0],areas[1]);//Printed areas[10]=0!!
printf("The size of a char is: %ld\n",sizeof(char));//*
printf("The size of name(char[]) is:%ld\n",sizeof(name));//*
printf("The number of chars is : %ld\n",sizeof(name)/sizeof(char));//*
printf("the size of FULL NAME  (char[]) is:%ld\n",sizeof(full_name));//*
printf("The number of chars in full name is %ld\n",sizeof(full_name)/sizeof(char));//*
printf("name=\"%s\" and full name =\"%s\"\n",name,full_name);// what is \"%s\" \ is an    ESCAPE


return 0;
}

Answer 1

Operator sizeof returns value of type size_t. Usually size_t is defined as unsigned long (though it can be any unsigned integral type). According to the C Standard sizeof( long ) is greater than or equal to sizeof( int ). For example sizeof( long ) can be equal to 8 while sizeof( int ) can be equal to 4. So in the code you showed format specifier %ld is used to output an object of type long int though it would be better to use %zu where flag z means that an object of type size_t will be outputed.

As for arrays then the compiler does not check boundaries of arrays. It is the programmer who shall correctly specify indices of array elements.

Answer 2

About printing sizes: sizeof(int) is of integral type size_t. On some machines that type is the same as unsigned int, on other machines it is the same as unsigned long. In practice the size is a small integer, so you can code

 printf("The size of an int: %d\n", (int) sizeof(int));

Pedantically you could #include <inttypes.h> and use some formats (e.g. %zu) there.

About out-of-range indexes, they cause a buffer overflow at runtime (which may SEGV). This an example of undefined behavior. Always avoid it. Here are examples of horrors which could happen on UB.