Are the strings in argv modifiable?

Question

I just wrote a small program that reads command line arguments in C, nothing too difficult. I was also modifying them, for example changing the first character of the parameter to uppercase.

I know that you shouldn't modify string literals as it can cause undefined behavior, so was just wondering if the strings in the *argv[] are literals that you shouldn't change.

int main(int argc, char *argv[])

Answer 1

From the C11 standard draft N1570, §5.1.2.2.1/2:

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.

They are modifiable. That means they are not string literals.

But be careful: the upper citation only refers to pointers to strings, excluding the obligatory null pointer at argv[argc]¹.
From the C11 standard draft N1570, §5.1.2.2.1/2 (same as above)¹:

argv[argc] shall be a null pointer

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Notes:

• Something regarding this sentence:

  I know that you shouldn't modify string literals as it can cause undefined behavior [...]

  "can"? It does always. Undefined behavior includes expected, as if well-defined, and unexpected behavior.

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_{¹ Thanks to @black!}