CODEWITHSUNDEEP
cpointers
Sivakumar K R
Sivakumar K R

Reputation: 45

Accessing a Value from pointer which has manually assigned address

I have assigned some random address to a pointer of a particular data type. Then I stored a value in that particular address. When I run the program, it terminates abruptly.

char *c=2000;
*c='A';
printf("%u",c);
printf("%d",*c);

I could be able to print the value of c in first printf statement. But I couldn't fetch the value stored in that address through the second one. I have executed in Cygwin GCC compiler and also in online ideone.com compiler. In ideone.com compiler it shows runtime error. What's the reason behind this?

Upvotes: 0

Views: 94

Answers (2)

Sadique
Sadique

Reputation: 22823

char *c=2000;

Assignment (and initialization) of integer values to pointers is implementation defined behavior.

Implementation-defined behavior is defined by the ISO C Standard in section 3.4.1 as:

unspecified behavior where each implementation documents how the choice is made

EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right.

Any code that relies on implementation defined behaviour is only guaranteed to work under a specific platform and/or compiler. Portable programs should try to avoid such behaviour.

Upvotes: 1

Jonathan Leffler
Jonathan Leffler

Reputation: 753615

When you assign the address 2000 to the pointer c, you are assuming that will be a valid address. Generally, though, it is not a valid address. You can't choose addresses at random and expect the compiler (and operating system) to have allocated that memory for you to use. In particular, the first page of memory (often 4 KiB, usually at least 1 KiB) is completely off limits; all attempts to read or write there are more usually indicative of bugs than intentional behaviour, and the MMU (memory management unit) is configured to reject attempts to access that memory.

If you're using an embedded microprocessor, the rules might well be different, but on a general purpose o/s like Windows with Cygwin, addresses under 0x1000 (4 KiB) are usually verboten.

You can print the address (you did it unreliably, but presumably your compiler didn't warn you; mine would have warned me about using a format for a 4-byte integer quantity to print an 8-byte address). But you can't reliably read or write the data at the address. There could be machines (usually mainframes) where simply reading an invalid address (even without accessing the memory it points at) generates a memory fault.

So, as Acme said in their answer,you've invoked undefined behaviour. You've taken the responsibility from the compiler for assigning a valid address to your pointer, but you chose an invalid value. A crash is the consequence of your bad decision.

Upvotes: 2

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