What is the difference between char array and char pointer in C

Question

I am trying to understand pointers in C but I am currently confused with the following    char  p    hello    This is a char pointer pointing at the character array  starting at h  char p      hello    This is an array that stores hello    What is the difference when I pass both these variables into this function   void printSomething char  p        printf  p   s  p

User · Accepted Answer

char  and char   are different types  but it s not immediately apparent in all cases  This is because arrays decay into pointers  meaning that if an expression of type char   is provided where one of type char  is expected  the compiler automatically converts the array into a pointer to its first element   Your example function printSomething expects a pointer  so if you try to pass an array to it like this   char s 10     hello   printSomething s     The compiler pretends that you wrote this   char s 10     hello   printSomething  amp s 0

User · Answer

For cases like this  the effect is the same  You end up passing the address of the first character in a string of characters   The declarations are obviously not the same though   The following sets aside memory for a string and also a character pointer  and then initializes the pointer to point to the first character in the string   char  p    hello     While the following sets aside memory just for the string  So it can actually use less memory   char p 10     hello

User · Answer

char p 3     hello    should be char p 6     hello  remember there is a   0  char in the end of a  string  in C   anyway  array in C is just a pointer to the first object of an adjust objects in the memory  the only different s are in semantics  while you can change the value of a pointer to point to a different location in the memory an array  after created  will always point to the same location  also when using array the  new  and  delete  is automatically done for you

User · Answer

You re not allowed to change the contents of a string constant  which is what the first p points to  The second p is an array initialized with a string constant  and you can change its contents

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Let s see    include  lt stdio h gt   include  lt string h gt   int main         char  p    hello       char q      hello      no need to count this      printf   zu n   sizeof p         gt  size of pointer to char -- 4 on x86  8 on x86-64     printf   zu n   sizeof q         gt  size of char array in memory -- 6 on both         size t strlen const char  s  and we don t get any warnings here      printf   zu n   strlen p         gt  5     printf   zu n   strlen q         gt  5      return 0      foo  and foo   are different types and they are handled differently by the compiler  pointer   address   representation of the pointer s type  array   pointer   optional length of the array  if known  for example  if the array is statically allocated   the details can be found in the standard  And at the level of runtime no difference between them  in assembler  well  almost  see below    Also  there is a related question in the C FAQ      Q  What is the difference between these initializations   char a      string literal      char  p     string literal            My program crashes if I try to assign a new value to p i        A  A string literal  the formal term for a double-quoted string in C source  can be used in two slightly different ways          As the initializer for an array of char  as in the declaration of char a     it specifies the initial values of the characters in that array  and  if necessary  its size     Anywhere else  it turns into an unnamed  static array of characters  and this unnamed array may be stored in read-only memory  and which therefore cannot necessarily be modified  In an expression context  the array is converted at once to a pointer  as usual  see section 6   so the second declaration initializes p to point to the unnamed array s first element           Some compilers have a switch controlling whether string literals are writable or not  for compiling old code   and some may have options to cause string literals to be formally treated as arrays of const char  for better error catching        See also questions 1 31  6 1  6 2  6 8  and 11 8b       References  K amp R2 Sec  5 5 p  104      ISO Sec  6 1 4  Sec  6 5 7       Rationale Sec  3 1 4       H amp S Sec  2 7 4 pp  31-2

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What is the difference between char array vs char pointer in C    C99 N1256 draft  There are two different uses of character string literals    Initialize char     char c      abc           This is  more magic   and described at 6 7 8 14  Initialization       An array of character type may be initialized by a character string literal  optionally   enclosed in braces  Successive characters of the character string literal  including the   terminating null character if there is room or if the array is of unknown size  initialize the   elements of the array    So this is just a shortcut for   char c       a    b    c     0      Like any other regular array  c can be modified  Everywhere else  it generates an    unnamed array of char What is the type of string literals in C and C    with static storage that gives UB  undefined behavior  if modified   So when you write   char  c    abc     This is similar to        unnamed is magic because modifying it gives UB     static char   unnamed      abc   char  c     unnamed    Note the implicit cast from char   to char    which is always legal   Then if you modify c 0   you also modify   unnamed  which is UB   This is documented at 6 4 5  String literals       5 In translation phase 7  a byte or code of value zero is appended to each multibyte   character sequence that results from a string literal or literals  The multibyte character   sequence is then used to initialize an array of static storage duration and length just   sufficient to contain the sequence  For character string literals  the array elements have   type char  and are initialized with the individual bytes of the multibyte character   sequence            6 It is unspecified whether these arrays are distinct provided their elements have the   appropriate values  If the program attempts to modify such an array  the behavior is   undefined     6 7 8 32  Initialization  gives a direct example      EXAMPLE 8  The declaration  char s      abc   t 3     abc         defines  plain  char array objects s and t whose elements are initialized with character string literals       This declaration is identical to  char s        a    b    c     0     t        a    b    c           The contents of the arrays are modifiable  On the other hand  the declaration  char  p    abc         defines p with type  pointer to char  and initializes it to point to an object with type  array of char  with length 4 whose elements are initialized with a character string literal  If an attempt is made to use p to modify the contents of the array  the behavior is undefined    GCC 4 8 x86-64 ELF implementation  Program    include  lt stdio h gt   int main void        char  s    abc       printf   s n   s       return 0      Compile and decompile   gcc -ggdb -std c99 -c main c objdump -Sr main o   Output contains    char  s    abc   8   48 c7 45 f8 00 00 00    movq    0x0 -0x8  rbp  f   00          c  R X86 64 32S  rodata   Conclusion  GCC stores char  it in  rodata section  not in  text   If we do the same for char      char s      abc     we obtain   17    c7 45 f0 61 62 63 00    movl    0x636261 -0x10  rbp    so it gets stored in the stack  relative to  rbp    Note however that the default linker script puts  rodata and  text in the same segment  which has execute but no write permission  This can be observed with   readelf -l a out   which contains    Section to Segment mapping    Segment Sections       02      text  rodata

User · Answer

As far as I can remember  an array is actually a group of pointers  For example   p 1       amp p 1    is a true statement

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From APUE  Section 5 14     char    good template       tmp dirXXXXXX      right way    char     bad template     tmp dirXXXXXX        wrong way            For the first template  the name is allocated on the stack  because we use an   array variable  For the second name  however  we use a pointer  In this case  only the   memory for the pointer itself resides on the stack  the compiler arranges for the string to   be stored in the read-only segment of the executable  When the mkstemp function tries   to modify the string  a segmentation fault occurs    The quoted text matches  Ciro Santilli  s explanation

[c] What is the difference between char array and char pointer in C?

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