Are string literals const?

Question

Both GCC and Clang do not complain if I assign a string literal to a char*, even when using lots of pedantic options (-Wall -W -pedantic -std=c99):

char *foo = "bar"; 

while they (of course) do complain if I assign a const char* to a char*.

Does this mean that string literals are considered to be of char* type? Shouldn't they be const char*? It's not defined behavior if they get modified!

And (an uncorrelated question) what about command line parameters (ie: argv): is it considered to be an array of string literals?

Answer 1

They are of type char[N] where N is the number of characters including the terminating \0. So yes you can assign them to char*, but you still cannot write to them (the effect will be undefined).

Wrt argv: It points to an array of pointers to strings. Those strings are explicitly modifiable. You can change them and they are required to hold the last stored value.

Answer 2

For completeness sake the C99 draft standard(C89 and C11 have similar wording) in section 6.4.5 String literals paragraph 5 says:

[...]a byte or code of value zero is appended to each multibyte character sequence that results from a string literal or literals. The multibyte character sequence is then used to initialize an array of static storage duration and length just sufficient to contain the sequence. For character string literals, the array elements have type char, and are initialized with the individual bytes of the multibyte character sequence;[...]

So this says a string literal has static storage duration(lasts the lifetime of the program) and it's type is char[](not char *) and its length is the size of the string literal with an appended zero. *Paragraph 6` says:

If the program attempts to modify such an array, the behavior is undefined.

So attempting to modify a string literal is undefined behavior regardless of the fact that they are not const.

With respect to argv in section 5.1.2.2.1 Program startup paragraph 2 says:

If they are declared, the parameters to the main function shall obey the following constraints:

[...]

-The parameters argc and argv and the strings pointed to by the argv array shall be modifiable by the program, and retain their last-stored values between program startup and program termination.

So argv is not considered an array of string literals and it is ok to modify the contents of argv.