C/Pointers
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| ... || ... || ... || ... || colspan="2" | ... | | ... || ... || ... || ... || colspan="2" | ... | ||
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| − | | 0xBF893E93 || style="color:red" | 0x08 || || | + | | 0xBF893E93 || style="color:red" | 0x08 || || ? (MSB) || colspan="2" | |
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| − | | 0xBF893E92 || style="color:red" | 0x04 || || | + | | 0xBF893E92 || style="color:red" | 0x04 || || ? || colspan="2" | |
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| − | | 0xBF893E91 || style="color:red" | 0x86 || || | + | | 0xBF893E91 || style="color:red" | 0x86 || || ? || colspan="2" | |
|- | |- | ||
| − | | 0xBF893E90 || style="color:red" | 0x80 || || | + | | 0xBF893E90 || style="color:red" | 0x80 || || ? (LSB) || This is p[1] || 0xBF893E8C + (sizeof(char*) * 1) → 0xBF893E8C + (4 * 1) → 0xBF893E8C + 4 → 0xBF893E90 |
|- | |- | ||
| 0xBF893E8F || style="color:#4B2" | 0x08 || || s (MSB) || colspan="2" | | | 0xBF893E8F || style="color:#4B2" | 0x08 || || s (MSB) || colspan="2" | | ||
Revision as of 00:00, 9 March 2012
A helper for people starting out with pointers:
test.c
#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'Memory Map
These values and addresses will probably be different each time you run the program
| Address | Value | ASCII | Variable | Notes | |
|---|---|---|---|---|---|
| ... | ... | ... | ... | ... | |
| 0xBF893E93 | 0x08 | ? (MSB) | |||
| 0xBF893E92 | 0x04 | ? | |||
| 0xBF893E91 | 0x86 | ? | |||
| 0xBF893E90 | 0x80 | ? (LSB) | This is p[1] | 0xBF893E8C + (sizeof(char*) * 1) → 0xBF893E8C + (4 * 1) → 0xBF893E8C + 4 → 0xBF893E90 | |
| 0xBF893E8F | 0x08 | s (MSB) | |||
| 0xBF893E8E | 0x04 | s | |||
| 0xBF893E8D | 0x87 | s | |||
| 0xBF893E8C | 0x34 | s (LSB) | This is p[0] | 0xBF893E8C + (sizeof(char*) * 0) → 0xBF893E8C + (4 * 0) → 0xBF893E8C + 0 → 0xBF893E8C | |
| 0xBF893E8B | 0xBF | p (MSB) | |||
| 0xBF893E8A | 0x89 | p | |||
| 0xBF893E89 | 0x3E | p | |||
| 0xBF893E88 | 0x8C | p (LSB) | |||
| ... | ... | ... | ... | ... | |
| 0x08048737 | 0x00 | '\0' | This is s[3] | 0x08048734 + (sizeof(char) * 3) → 0x08048734 + (1 * 3) → 0x08048734 + 3 → 0x08048737 | |
| 0x08048736 | 0x63 | 'a' | This is s[2] | 0x08048734 + (sizeof(char) * 2) → 0x08048734 + (1 * 2) → 0x08048734 + 2 → 0x08048736 | |
| 0x08048735 | 0x62 | 'b' | This is s[1] | 0x08048734 + (sizeof(char) * 1) → 0x08048734 + (1 * 1) → 0x08048734 + 1 → 0x08048735 | |
| 0x08048734 | 0x63 | 'c' | This is s[0] | 0x08048734 + (sizeof(char) * 0) → 0x08048734 + (1 * 0) → 0x08048734 + 0 → 0x08048734 | |
| ... | ... | ... | ... | ... | |
| 0x08048680 | 0x55 | ||||
| ... | ... | ... | ... | ... | |
