CODEWITHSUNDEEP
c
Dewan Ahmed
Dewan Ahmed

Reputation: 305

Please explain this declaration of the POSIX getline() function

I see this line right after include and before main function.

size_t getline(char **lineptr, size_t *n, FILE *stream);

What I can deduce

Where I'm stuck

Upvotes: 0

Views: 534

Answers (2)

WangYang
WangYang

Reputation: 504

Pointer variable needs memory to store value that is a address of memory. If you want to define a pointer variable to store address of a int type variable, you can define the pointer like this: 

int *mypointer = NULL;
int x = 10;
//store the address of variable x
mypointer = &x;

As we know, the pointer variable is just to store someone's address that is just a number. Thus that pointer variable also has address in the memory space. How to store the pointer's address? We can do that like this:

int *mypointer = NULL;
int x = 10;
//the pointer store the x's address
mypointer = &x;

//define a pointer to store the pointer variable mypointer's address
int **newpointer = &mypointer;

So, char ** is similar with int **.

The type size_t is unsigned int or other type. Note size_t is unsigned type whatever size_t is type of int or someone else. Type ssize_t is signed type in C. For simplicity, size_t maybe define as follow:

typedef  unsigned int size_t;
typedef int ssize_t;

Upvotes: 2

Dave Costa
Dave Costa

Reputation: 48121

size_t is a type. It is probably defined in one of the header files that are included. The function returns a value of that type, and its second parameter is a pointer to a value of that type.

You are correct that * indicates a pointer. So ** is a pointer to a pointer. So the function parameter lineptr is the address of a location (A) in memory, which contains the address of another location (B) in memory; the data at the second location should be interpreted as char values. This implies that the function could change the value stored at location A to point to some location other than B.

Upvotes: 3

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