sprintf: makes pointer from integer without a cast`

[Diode]

So I'm messing with some code, trying to get more familiar with string functions, and I stumbled into an apparent error on my part. Here is my code:

#include <stdio.h>
#include <string.h>

int main(void)
{
  int len = 0;

  char *str = "This is a string";
  char ch = '\0';
  int i = 0;
  len = strlen(str);

  i = sprintf(ch, "%X", 0x0D);
  strcat(str, ch);
  i = sprintf(ch, "%X", 0x0A);
  strcat(str, ch);

  len = strlen(str);
  printf("\n%s", str);
  printf("str is %d chars long\n", len);

  return 0;
}

Those are hexadecimal values I'm trying to put in there. I understand that every ASCII character also has a decimal representation, but what else can I do to put in CR and LF to the end of strings? If I do what the error says, it would give the characters values I don't want, which would be 13, and 10, respectively.

I've read various articles on the web about this, and each one has said to use sprintf, so what am I doing wrong?

Thanks, I appreciate it.

[Salem]

Say char ch[10]; but make sure the array is big enough.

Also, you can't do this either strcat( str, ch ); str is a string constant (it may be in read-only memory). Modifying it will kill your program.

Use say char str[100] = "a string";

[Diode]

Say char ch[10]; but make sure the array is big enough.

In this example, 10 is the index of a char array ch, but I don't want to modify the tenth element. Also, ch is a single char. I don't quite understand you mean here.

[Salem]

> 10 is the index of a char array ch, but I don't want to modify the tenth element.
Who said that?
It's an array of 10 chars.

char ch[10];
sprintf( ch, "%x", value );
Simple.

[Diode]

I knew that you could just use a normal char array, but I didn't know you couldn't modify a char array pointed to by a pointer. If you type *ptr, you can access the value at the variable the pointer points do, but I guess it can't be used in strcat since I tried it earlier before I posted.

I'm practicing pointers, hence the purpose of this stupid program. I know they hold addresses to variables, and that they are useful to be passed as reference to functions so functions can modify the contents, but I'm still just a little foggy on what can be done with them.

So once a pointer to an array is declared as in the *str line, it can't be modified. I got it now.

Thanks Salem

[Aia]

>So once a pointer to an array is declared as in the *str line, it can't be modified. I got it now.

If you are referring to something like this char *str = "This is a string"; you are correct. It is better to think of it as a string in read only memory.

[Ancient Dragon]

>> but what else can I do to put in CR and LF to the end of strings?
simple

char str[100] = "a string";  // see Salem's post #2 for this line
strcat(str, "\r\n");

[Diode]

Gee, I totally forgot about that, thanks, Ancient Dragon. I was trying to make it too difficult.