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  1. #include<stdio.h>
  2. #include<string.h>
  3. static char* abc = "kj";
  4.  
  5. // What exactly does this function do?
  6. void fn(char**s) {
  7. 	printf("arg: %p\n", s);
  8. 	printf("arg deref before: %p\n", *s);
  9.  
  10. 	// This throws. *s is not a valid address.
  11. 	// printf("contents: ->%s<-\n", *s);
  12.  
  13. 	*s = abc;
  14. 	printf("arg deref after assigning abc: %p\n", *s);
  15.  
  16. 	// Now *s holds a valid address (that of "kj").
  17. 	printf("contents: ->%s<-\n", *s);
  18.  
  19. }
  20.  
  21. // Helper function to print a char pointer and the first bytes
  22. // it points to
  23. void printStr(const char *const caption, const char *const ptr)
  24. {
  25. 	int i=0;
  26. 	printf("%s: ->%s<-, i.e. {", caption, ptr);
  27. 	for( i=0; i<sizeof(char *)-1; ++i)
  28. 	{
  29. 	    printf("0x%x,", ptr[i]);
  30. 	}
  31. 	printf( "0x%x ...}\n", ptr[sizeof(char *)-1] );
  32. }
  33.  
  34. int main() {
  35.    char str[256];
  36.  
  37.    printf("size of ptr: %zu\n", sizeof(void *));
  38.    strcpy(str, "AAAAAAAA"); // 9 defined bytes
  39.    printStr("str", str);
  40.    printf("arr addr: %p\n", &str);
  41.    printf("addr of abc: %p\n", abc);
  42.  
  43.    fn(&str);
  44.  
  45.  
  46.    printStr("str after fn (a pointer value, only accidentally printable): ", str);
  47.    printf("arr addr after fn: %p\n", &str);
  48.  
  49.    return 0;
  50. }
Success #stdin #stdout 0s 2296KB
comments (?)
stdin 
Standard input is empty
stdout 
size of ptr: 4
str: ->AAAAAAAA<-, i.e. {0x41,0x41,0x41,0x41 ...}
arr addr: 0xbfa9bc40
addr of abc: 0x804868f
arg: 0xbfa9bc40
arg deref before: 0x41414141
arg deref after assigning abc: 0x804868f
contents: ->kj<-
str after fn (a pointer value, only accidentally printable): : ->��AAAA<-, i.e. {0xffffff8f,0xffffff86,0x4,0x8 ...}
arr addr after fn: 0xbfa9bc40
https://ideone.com/aL407L
language:
C (gcc 8.3)
created:
9 years ago
visibility:
public

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