Ka0s

Well... you are saying that it's not possible to change a string literal declared has a pointer but i've compiled this code and it worked:


When i run the program it shows "hello" followed by "aello".
Can you tell me why can i modify the string pointed by "char *a"?

lectricpharaoh

My understanding (though you might want to wait until grumpy or another person more knowledgeable than I weighs in on the subject) is that your program is exhibiting 'undefined behavior'. The reason for the immutable nature of string literals is mainly to reduce redundancy, making the executable smaller. Take the following code:A compiler can see that both are being initialized with identical string literals, and can thus store a single copy of the contents, and point both pointers at it. Allowing modification of the contents could thus have unintended consequences.

Some systems will actually store the string in memory flagged as read-only, and a write access will raise a hardware exception, causing your program to crash. Others will not detect the access, but just because an illegal operation is not trapped, that does not make it okay. Remember that 'undefined' means your program may take any action, including acting in a manner you think is 'correct'.

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Sane

And in support of lectricpharaoh's post: your code crashed, after printing the first line, when I attempted to run it.

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grumpy

The short answer, as electricpharoah said, is undefined behaviour.

For discussion another day, there are a few historic reasons the code will compile, although the runtime behaviour is not defined. This is due to very old (well before the first C standard) anomalies in the C language related to the relationship between string literals, arrays of char, and pointers to char, from days when the language did not support the const keyword.