Is it possible to modify a string of char in C

Question

I have been struggling for a few hours with all sorts of C tutorials and books related to pointers but what I really want to know is if it s possible to change a char pointer once it s been created   This is what I have tried   char  a    This is a string   char  b    new string    a 2    b 1      Causes a segment fault   b 2    b 1      This almost seems like it would work but the compiler throws an error    So is there any way to change the values inside the strings rather than the pointer addresses

User · Answer

It seems like your question has been answered but now you might wonder why char *a = "String" is stored in read-only memory. Well, it is actually left undefined by the c99 standard but most compilers choose to it this way for instances like:

printf("Hello, World\n");

c99 standard(pdf) [page 130, section 6.7.8]:

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.

User · Answer

All are good answers explaining why you cannot modify string literals because they are placed in read-only memory  However  when push comes to shove  there is a way to do this  Check out this example    include  lt sys mman h gt   include  lt unistd h gt   include  lt stddef h gt   include  lt string h gt   include  lt stdlib h gt   include  lt stdio h gt   int take me back to DOS times const void  ptr  size t len    int main         const  data    Bender is always sober        printf  Before   s n   data       if  take me back to DOS times data  sizeof data      0          perror  Time machine appears to be broken         memcpy  char   data   17   drunk    6       printf  After   s n   data        return 0     int take me back to DOS times const void  ptr  size t len        int pagesize      unsigned long long pg off      void  page       pagesize   sysconf  SC PAGE SIZE       if  pagesize  lt  0          return -1      pg off    unsigned long long ptr    unsigned long long pagesize      page     char   ptr - pg off       if  mprotect page  len   pg off  PROT READ   PROT WRITE   PROT EXEC     -1          return -1      return 0      I have written this as part of my somewhat deeper thoughts on const-correctness  which you might find interesting  I hope       Hope it helps  Good Luck

User · Answer

No  you cannot modify it  as the string can be stored in read-only memory  If you want to modify it  you can use an array instead e g   char a      This is a string     Or alternately  you could allocate memory using malloc e g   char  a   malloc 100   strcpy a   This is a string    free a      deallocate memory once you ve done

User · Answer

You could also use strdup      The strdup   function returns a pointer to a new string which is a duplicate of the string  s     Memory for the new string is obtained with malloc 3   and can be freed with free 3     For you example   char  a   strdup  stack overflow

User · Answer

When you write a  string  in your source code  it gets written directly into the executable because that value needs to be known at compile time  there are tools available to pull software apart and find all the plain text strings in them   When you write char  a    This is a string   the location of  This is a string  is in the executable  and the location a points to  is in the executable  The data in the executable image is read-only   What you need to do  as the other answers have pointed out  is create that memory in a location that is not read only--on the heap  or in the stack frame  If you declare a local array  then space is made on the stack for each element of that array  and the string literal  which is stored in the executable  is copied to that space in the stack   char a      This is a string     you can also copy that data manually by allocating some memory on the heap  and then using strcpy   to copy a string literal into that space   char  a   malloc 256   strcpy a   This is a string      Whenever you allocate space using malloc   remember to call free   when you are finished with it  read  memory leak    Basically  you have to keep track of where your data is  Whenever you write a string in your source  that string is read only  otherwise you would be potentially changing the behavior of the executable--imagine if you wrote char  a    hello   and then changed a 0  to  c   Then somewhere else wrote printf  hello     If you were allowed to change the first character of  hello   and your compiler only stored it once  it should   then printf  hello    would output cello

User · Answer

You need to copy the string into another  not read-only memory buffer and modify it there  Use strncpy   for copying the string  strlen   for detecting string length  malloc   and free   for dynamically allocating a buffer for the new string   For example  C   like pseudocode    int stringLength   strlen  sourceString    char  newBuffer   malloc  stringLength   1        you should check if newBuffer is 0 here to test for memory allocaton failure - omitted  strncpy  newBuffer  sourceString  stringLength    newBuffer stringLength    0      you can now modify the contents of newBuffer freely  free  newBuffer    newBuffer   0

User · Answer

The memory for a  amp  b is not allocated by you  The compiler is free to choose a read-only memory location to store the characters  So if you try to change it may result in seg fault  So I suggest you to create a character array yourself  Something like  char a 10   strcpy a   Hello

User · Answer

A lot of folks get confused about the difference between char  and char   in conjunction with string literals in C  When you write   char  foo    hello world        you are actually pointing foo to a constant block of memory  in fact  what the compiler does with  hello world  in this instance is implementation-dependent    Using char   instead tells the compiler that you want to create an array and fill it with the contents   hello world   foo is the a pointer to the first index of the char array  They both are char pointers  but only char   will point to a locally allocated and mutable block of memory

User · Answer

char  a    stack overflow   char  b    new string  it s real   int d   strlen a    b   malloc d   sizeof char    b   strcpy b a   printf   s  s n   a  b

[c] Is it possible to modify a string of char in C?

Examples related to c

Examples related to string

Examples related to debugging