C/Pointers
From Attie's Wiki
(Difference between revisions)
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int main(void) { | int main(void) { | ||
| − | char *s = " | + | char *s = "cba"; |
char **p = &s; | char **p = &s; | ||
| Line 38: | Line 38: | ||
return 0; | return 0; | ||
} | } | ||
| + | </source> | ||
| + | |||
| + | Output: | ||
| + | <source lang="text"> | ||
| + | $ gcc test.c -o test && ./test | ||
| + | s: 0x8048734 <-- the address of the 'c' | ||
| + | &s: 0xBF893E8C <-- the address of the variable s | ||
| + | s[0]: 0x63 <-- the 'c' | ||
| + | *s: 0x63 <-- the 'c' | ||
| + | p: 0xBF893E8C <-- the address of the variable s | ||
| + | &p: 0xBF893E88 <-- the address of the variable p | ||
| + | p[0]: 0x8048734 <-- the address of the 'c' | ||
| + | *p: 0x8048734 <-- the address of the 'c' | ||
| + | p[1]: 0x8048680 <-- INVALID - the value stored just after the variable p (the program may segfault here) | ||
| + | *p[1]: 0x55 <-- INVALID - the value stored at location 0x8048680 (again, the program may segfault here) | ||
| + | (*p)[1]: 0x62 <-- the 'b' | ||
| + | *(p[1]): 0x55 <-- INVALID - the same as *p[1] | ||
| + | (*p): 0x8048734 <-- the address of the 'c' | ||
| + | (*p)++: 0x8048734 <-- the address of the 'c' - the pointer is POST incremented | ||
| + | (*p)[0]: 0x62 <-- the 'b' | ||
| + | (*p): 0x8048735 <-- the address of the 'b' | ||
| + | ++(*p): 0x8048736 <-- the address of the 'a' - the pointer is PRE incremented | ||
| + | (*p)[0]: 0x61 <-- the 'a' | ||
| + | (*p): 0x8048736 <-- the address of the 'a' | ||
</source> | </source> | ||
Revision as of 23:22, 8 March 2012
A helper for people starting out with pointers:
#include <stdio.h> #include <stdlib.h> #include <ctype.h> #define P(expr) printf("%10s: 0x%02X\n", #expr, expr) int main(void) { char *s = "cba"; char **p = &s; P( s ); P( &s ); P( s[0] ); P( *s ); P( p ); P( &p ); P( p[0] ); P( *p ); P( p[1] ); P( *p[1] ); P( (*p)[1] ); P( *(p[1]) ); P( (*p) ); P( (*p)++ ); P( (*p)[0] ); P( (*p) ); P( ++(*p) ); P( (*p)[0] ); P( (*p) ); return 0; }
Output:
$ gcc test.c -o test && ./test
s: 0x8048734 <-- the address of the 'c'
&s: 0xBF893E8C <-- the address of the variable s
s[0]: 0x63 <-- the 'c'
*s: 0x63 <-- the 'c'
p: 0xBF893E8C <-- the address of the variable s
&p: 0xBF893E88 <-- the address of the variable p
p[0]: 0x8048734 <-- the address of the 'c'
*p: 0x8048734 <-- the address of the 'c'
p[1]: 0x8048680 <-- INVALID - the value stored just after the variable p (the program may segfault here)
*p[1]: 0x55 <-- INVALID - the value stored at location 0x8048680 (again, the program may segfault here)
(*p)[1]: 0x62 <-- the 'b'
*(p[1]): 0x55 <-- INVALID - the same as *p[1]
(*p): 0x8048734 <-- the address of the 'c'
(*p)++: 0x8048734 <-- the address of the 'c' - the pointer is POST incremented
(*p)[0]: 0x62 <-- the 'b'
(*p): 0x8048735 <-- the address of the 'b'
++(*p): 0x8048736 <-- the address of the 'a' - the pointer is PRE incremented
(*p)[0]: 0x61 <-- the 'a'
(*p): 0x8048736 <-- the address of the 'a'
