Need help identifying C variable types.

c25fpdQ · 09-15-2012, 11:31 PM

Hi guys, I need help with identifying type declaration of variable p.

I've attempted, but I am not certain of my answers.

Can someone help me out please ?

1)

Code:

int *vals[5] = [1,3,5,3,5];
p = *vals[2];

p is of type of third pointer of array of 5 pointer to int ?

2)

Code:

char *foo(int x) 
   { 
     ... 
      .
     ...
   }

foo is a type of pointer to function returing char ?

Code:

p = foo(5);

p is of type char ?

3)

Code:

typedef unsigned short u_int16_t;
u_int16_t *val = ...;
p = *val;

val is of type pointer to u_int16_t of type unsigned short ?
p is type pointer to u_int16_t of type unsigned short ?

4)

Code:

p = printf("Hello!")

p is of type int; returns the number of characters printed, ?

5)

Code:

struct trapframe 
{
  u_int32_t tf_vaddr;	/* coprocessor 0 vaddr register */
  u_int32_t tf_status;	/* coprocessor 0 status register */
  ...
  u_int32_t tf_v0;      /* Saved register 2 (v0) */
  ...
};


struct trapframe *tf = ...;

tf is a type of pointer to struct trapframe ?

Code:

p = tf->tf_v0;

p is of type u_int32_t ?

6)

Code:

struct trapframe tf = ...;
p = &tf.tf_v0;

p is of type u_int32_t as well ?

Thank you very much in advance !

Celyr · 09-16-2012, 02:55 AM

Quote:

Originally Posted by c25fpdQ

1)

Code:

int *vals[5] = [1,3,5,3,5];
p = *vals[2];

p is of type of third pointer of array of 5 pointer to int ?

in this case p is int
read it as int **vals
and then p = **(vals+2)

Quote:

Originally Posted by c25fpdQ

2)

Code:

char *foo(int x) 
   { 
     ... 
      .
     ...
   }

foo is a type of pointer to function returing char ?

Code:

p = foo(5);

p is of type char ?

foo return a pointer to a char
and also p is char* thus is _maybe_ a pointer to a sequence of char null terminated

3)

Quote:

Originally Posted by c25fpdQ

Code:

typedef unsigned short u_int16_t;
u_int16_t *val = ...;
p = *val;

val is of type pointer to u_int16_t of type unsigned short ?
p is type pointer to u_int16_t of type unsigned short ?

val is a pointer, and p is a u_int16_t, I think that the format used by most of C developers can be misleading for students, read it this way:
u_int16_t* val = ...;
p = *val;

in this way it should be clear that val is a pointer type and p is a u_int_16_t (well it isn't defined but it should be like that)

Quote:

Originally Posted by c25fpdQ

4)

Code:

p = printf("Hello!")

p is of type int; returns the number of characters printed, ?

yes.

Quote:

Originally Posted by c25fpdQ

5)

Code:

struct trapframe 
{
  u_int32_t tf_vaddr;	/* coprocessor 0 vaddr register */
  u_int32_t tf_status;	/* coprocessor 0 status register */
  ...
  u_int32_t tf_v0;      /* Saved register 2 (v0) */
  ...
};


struct trapframe *tf = ...;

tf is a type of pointer to struct trapframe ?

yep

Quote:

Originally Posted by c25fpdQ

Code:

p = tf->tf_v0;

p is of type u_int32_t ?

yep.

Quote:

Originally Posted by c25fpdQ

6)

Code:

struct trapframe tf = ...;
p = &tf.tf_v0;

p is of type u_int32_t as well ?

No, when you use & you are getting the memory address of that variable. So p is u_int32_t*, a pointer

Quote:

Originally Posted by c25fpdQ

Thank you very much in advance !

you are welcome