What does “return p ? memcpy(p, s, len) : NULL;” mean?

Question

What does mean of " return p ? memcpy(p, s, len) : NULL; " in below code? (More generally, what is the conditional operator, a ? b : c ?)

char * strdup(const char * s)
{
  size_t len = 1+strlen(s);
  char *p = malloc(len);

  return p ? memcpy(p, s, len) : NULL;
}

Answer 1

This syntax is called a ternary operator and you can think of it as of simplified if statement. return p ? memcpy(p, s, len) : NULL; is the same as:

if(p)
    return memcpy(p, s, len);
else
    return NULL;

memcpy() function returns a pointer to dest, which is a first argument of memcpy and in your case this is p . So, if p has value different than 0 (pointer is not NULL) then return that pointer. Otherwise, return NULL.

Answer 2

It means execute and return memcpy(p, s, len) , unless p==0 . If p==0 , it will return NULL , and not execute memcpy(p, s, len) .

Read https://en.wikipedia.org/wiki/%3F:#C for more.

Also, to paraphrase http://man7.org/linux/man-pages/man3/memcpy.3.html : The memcpy() function copies len bytes from memory area p to memory area s. The memory areas must not overlap.

That is, if we have the below memory:

   p            s
[][1][2][3][][][4][5][6][]

and len == 3, then when memcpy is called we get:

   p            s
[][1][2][3][][][1][2][3][]

Finally, the value a function returns is the value it evaluates to if you then use it in an expression; if foo() returns 5, print(foo()); prints 5.

Answer 3

This is a ternary operator in C.

p ? memcpy(p, s, len) : NULL;

It means that if the first condition is true ie, p then return the value of memcpy(p, s, len) else return NULL .