Printing pointers in C

Question

I was trying to understand something with pointers  so I wrote this code    include  lt stdio h gt   int main void        char s      asd       char   p    amp s       printf  The value of s is   p n   s       printf  The direction of s is   p n    amp s        printf  The value of p is   p n   p       printf  The direction of p is   p n    amp p        printf  The direction of s 0  is   p n    amp s 0        printf  The direction of s 1  is   p n    amp s 1        printf  The direction of s 2  is   p n    amp s 2         return 0      When compiling it with gcc I get these warnings     gcc main c -o main-bin -ansi -pedantic -Wall -lm main c  In function    main     main c 6  warning  initialization from incompatible pointer type main c 9  warning  format     p    expects type    void       but argument 2 has type    char     4     main c 11  warning  format     p    expects type    void       but argument 2 has type    char       main c 12  warning  format     p    expects type    void       but argument 2 has type    char           The flags for gcc are because I must be C89   Why incompatible types of pointer  Isn t the name of an array a pointer to it s first element  So if s is a pointer to  a    amp s must be a char     no  And why do I get the other warnings  Do I have to cast the pointers with  void    in order to print them    And when running I get something like this       main-bin The value of s is  0xbfb7c860 The direction of s is  0xbfb7c860 The value of p is  0xbfb7c860 The direction of p is  0xbfb7c85c The direction of s 0  is  0xbfb7c860 The direction of s 1  is  0xbfb7c861 The direction of s 2  is  0xbfb7c862   How can the value of s and it s direction  and of course the value of p  be the same

User · Answer

Normally  it s considered poor style to unnecessarily cast pointers to  void     Here  however  you need the casts to  void   on the printf arguments because printf is variadic  The prototype doesn t tell the compiler what type to convert the pointers to at the call site

User · Answer

If you pass the name of an array as an argument to a function  it is treated as if you had passed the address of the array  So  amp s and s are identical arguments  See K amp R 5 3   amp s 0  is the same as  amp s  since it takes the address of the first element of the array  which is the same as taking the address of the array itself   For all the others  although all pointers are essentially memory locations they are still typed  and the compiler will warn about assigning one type of pointer to another     void  p  says p is a memory address  but I don t know what s in the memory char  s  says s is a memory address  and the first byte contains a character char   ps  says ps is a memory address  and the four bytes there  for a 32-bit system  contain a pointer of type char     cf http   www oberon2005 ru paper kr c pdf  e-book version of K amp R

User · Answer

change line   char s      asd    to   char  s    asd    and things will get more clear

User · Answer

Normally  it s considered poor style to unnecessarily cast pointers to  void     Here  however  you need the casts to  void   on the printf arguments because printf is variadic  The prototype doesn t tell the compiler what type to convert the pointers to at the call site

User · Answer

You can t change the value  i e   address of  a static array  In technical terms  the lvalue of an array is the address of its first element  Hence s     amp s  It s just a quirk of the language

User · Answer

It s not a pointer to character char  but a pointer to array of 4 characters   char   4   With g   it doesn t compile      main cpp  In function    int main int  char        main cpp 126  error    cannot convert    char     4     to    char      in initialization   Moreover  the linux man pages says      p      The void   pointer argument is printed in hexadecimal  as if by   x or     lx     It shoud be pointer to void    You can change your code to   char  s    asd   char   p    amp s   printf  The value of s is   p n   s   printf  The address of s is   p n    amp s    printf  The value of p is   p n   p   printf  The address of p is   p n    amp p    printf  The address of s 0  is   p n    amp s 0    printf  The address of s 1  is   p n    amp s 1    printf  The address of s 2  is   p n    amp s 2      result   The value of s is  0x403f00    The address of s is  0x7fff2df9d588  The value of p is  0x7fff2df9d588   The address of p is  0x7fff2df9d580   The address of s 0  is  0x403f00   The address of s 1  is  0x403f01   The address of s 2  is  0x403f02

User · Answer

You have used   char s      asd     Here s actually points to the bytes  asd    The address of s  would also point to this location   If you used   char  s    asd     the value of s and  amp s would be different  as s would actually be a pointer to the bytes  asd    You used   char s      asd   char   p    amp s    Here s points to the bytes  asd    p is a pointer to a pointer to characters  and has been set to a the address of characters   In other words you have too many indirections in p   If you used char  s    asd   you could use this additional indirection

User · Answer

If you pass the name of an array as an argument to a function  it is treated as if you had passed the address of the array  So  amp s and s are identical arguments  See K amp R 5 3   amp s 0  is the same as  amp s  since it takes the address of the first element of the array  which is the same as taking the address of the array itself   For all the others  although all pointers are essentially memory locations they are still typed  and the compiler will warn about assigning one type of pointer to another     void  p  says p is a memory address  but I don t know what s in the memory char  s  says s is a memory address  and the first byte contains a character char   ps  says ps is a memory address  and the four bytes there  for a 32-bit system  contain a pointer of type char     cf http   www oberon2005 ru paper kr c pdf  e-book version of K amp R

User · Answer

Yes  your compiler is expecting void     Just cast them to void        for instance       printf  The value of s is   p n    void    s   printf  The direction of s is   p n    void     amp s

User · Answer

You can t change the value  i e   address of  a static array  In technical terms  the lvalue of an array is the address of its first element  Hence s     amp s  It s just a quirk of the language

User · Answer

s  is not a  char    it s a  char 4     And so    amp s  is not a  char     but actually  a pointer to an array of 4 characater    Your compiler may treat   amp s  as if you had written   amp s 0    which is roughly the same thing  but is a  char     When you write  char   p    s   you are trying to say  I want p to be set to the address of the thing which currently points to  asd    But currently there is nothing which points to  asd    There is just an array which holds  asd    char s      asd   char  p    amp s 0       alternately you could use the shorthand char p   s  char   pp    amp p

User · Answer

Yes  your compiler is expecting void     Just cast them to void        for instance       printf  The value of s is   p n    void    s   printf  The direction of s is   p n    void     amp s

User · Answer

If you pass the name of an array as an argument to a function  it is treated as if you had passed the address of the array  So  amp s and s are identical arguments  See K amp R 5 3   amp s 0  is the same as  amp s  since it takes the address of the first element of the array  which is the same as taking the address of the array itself   For all the others  although all pointers are essentially memory locations they are still typed  and the compiler will warn about assigning one type of pointer to another     void  p  says p is a memory address  but I don t know what s in the memory char  s  says s is a memory address  and the first byte contains a character char   ps  says ps is a memory address  and the four bytes there  for a 32-bit system  contain a pointer of type char     cf http   www oberon2005 ru paper kr c pdf  e-book version of K amp R

User · Answer

Yes  your compiler is expecting void     Just cast them to void        for instance       printf  The value of s is   p n    void    s   printf  The direction of s is   p n    void     amp s

User · Answer

Normally  it s considered poor style to unnecessarily cast pointers to  void     Here  however  you need the casts to  void   on the printf arguments because printf is variadic  The prototype doesn t tell the compiler what type to convert the pointers to at the call site

User · Answer

s  is not a  char    it s a  char 4     And so    amp s  is not a  char     but actually  a pointer to an array of 4 characater    Your compiler may treat   amp s  as if you had written   amp s 0    which is roughly the same thing  but is a  char     When you write  char   p    s   you are trying to say  I want p to be set to the address of the thing which currently points to  asd    But currently there is nothing which points to  asd    There is just an array which holds  asd    char s      asd   char  p    amp s 0       alternately you could use the shorthand char p   s  char   pp    amp p

User · Answer

You have used   char s      asd     Here s actually points to the bytes  asd    The address of s  would also point to this location   If you used   char  s    asd     the value of s and  amp s would be different  as s would actually be a pointer to the bytes  asd    You used   char s      asd   char   p    amp s    Here s points to the bytes  asd    p is a pointer to a pointer to characters  and has been set to a the address of characters   In other words you have too many indirections in p   If you used char  s    asd   you could use this additional indirection

User · Answer

It s not a pointer to character char  but a pointer to array of 4 characters   char   4   With g   it doesn t compile      main cpp  In function    int main int  char        main cpp 126  error    cannot convert    char     4     to    char      in initialization   Moreover  the linux man pages says      p      The void   pointer argument is printed in hexadecimal  as if by   x or     lx     It shoud be pointer to void    You can change your code to   char  s    asd   char   p    amp s   printf  The value of s is   p n   s   printf  The address of s is   p n    amp s    printf  The value of p is   p n   p   printf  The address of p is   p n    amp p    printf  The address of s 0  is   p n    amp s 0    printf  The address of s 1  is   p n    amp s 1    printf  The address of s 2  is   p n    amp s 2      result   The value of s is  0x403f00    The address of s is  0x7fff2df9d588  The value of p is  0x7fff2df9d588   The address of p is  0x7fff2df9d580   The address of s 0  is  0x403f00   The address of s 1  is  0x403f01   The address of s 2  is  0x403f02

User · Answer

change line   char s      asd    to   char  s    asd    and things will get more clear

User · Answer

You can t change the value  i e   address of  a static array  In technical terms  the lvalue of an array is the address of its first element  Hence s     amp s  It s just a quirk of the language

User · Answer

You have used   char s      asd     Here s actually points to the bytes  asd    The address of s  would also point to this location   If you used   char  s    asd     the value of s and  amp s would be different  as s would actually be a pointer to the bytes  asd    You used   char s      asd   char   p    amp s    Here s points to the bytes  asd    p is a pointer to a pointer to characters  and has been set to a the address of characters   In other words you have too many indirections in p   If you used char  s    asd   you could use this additional indirection

User · Answer

change line   char s      asd    to   char  s    asd    and things will get more clear

User · Answer

You have used   char s      asd     Here s actually points to the bytes  asd    The address of s  would also point to this location   If you used   char  s    asd     the value of s and  amp s would be different  as s would actually be a pointer to the bytes  asd    You used   char s      asd   char   p    amp s    Here s points to the bytes  asd    p is a pointer to a pointer to characters  and has been set to a the address of characters   In other words you have too many indirections in p   If you used char  s    asd   you could use this additional indirection

User · Answer

change line   char s      asd    to   char  s    asd    and things will get more clear

User · Answer

Normally  it s considered poor style to unnecessarily cast pointers to  void     Here  however  you need the casts to  void   on the printf arguments because printf is variadic  The prototype doesn t tell the compiler what type to convert the pointers to at the call site

User · Answer

Yes  your compiler is expecting void     Just cast them to void        for instance       printf  The value of s is   p n    void    s   printf  The direction of s is   p n    void     amp s

User · Answer

You can t change the value  i e   address of  a static array  In technical terms  the lvalue of an array is the address of its first element  Hence s     amp s  It s just a quirk of the language

User · Answer

s  is not a  char    it s a  char 4     And so    amp s  is not a  char     but actually  a pointer to an array of 4 characater    Your compiler may treat   amp s  as if you had written   amp s 0    which is roughly the same thing  but is a  char     When you write  char   p    s   you are trying to say  I want p to be set to the address of the thing which currently points to  asd    But currently there is nothing which points to  asd    There is just an array which holds  asd    char s      asd   char  p    amp s 0       alternately you could use the shorthand char p   s  char   pp    amp p

User · Answer

s  is not a  char    it s a  char 4     And so    amp s  is not a  char     but actually  a pointer to an array of 4 characater    Your compiler may treat   amp s  as if you had written   amp s 0    which is roughly the same thing  but is a  char     When you write  char   p    s   you are trying to say  I want p to be set to the address of the thing which currently points to  asd    But currently there is nothing which points to  asd    There is just an array which holds  asd    char s      asd   char  p    amp s 0       alternately you could use the shorthand char p   s  char   pp    amp p

User · Answer

If you pass the name of an array as an argument to a function  it is treated as if you had passed the address of the array  So  amp s and s are identical arguments  See K amp R 5 3   amp s 0  is the same as  amp s  since it takes the address of the first element of the array  which is the same as taking the address of the array itself   For all the others  although all pointers are essentially memory locations they are still typed  and the compiler will warn about assigning one type of pointer to another     void  p  says p is a memory address  but I don t know what s in the memory char  s  says s is a memory address  and the first byte contains a character char   ps  says ps is a memory address  and the four bytes there  for a 32-bit system  contain a pointer of type char     cf http   www oberon2005 ru paper kr c pdf  e-book version of K amp R

[c] Printing pointers in C

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