I have a hex string for example \xF5\x17\x30\x91\x00\xA1\xC9\x00\xDF\xFF, when trying to use strlen() function to get the length of that hex string it returns 4!

const char string_[] = { "\xF5\x17\x30\x91\x00\xA1\xC9\x00\xDF\xFF" };
unsigned int string_length = strlen(string_);
printf("%d", string_length); // the result: 4

Is the strlen() function dealing with that hex as a string, or is something unclear to me?

CodePudding user response：

For string functions in the C standard library, a character with value zero, also called a null character, marks the end of a string. Your string contains \x00, which designates a null character, so the string ends there. There are four non-null characters before it, so strlen returns four.

C 2018 7.1.1 1 says:

A string is a contiguous sequence of characters terminated by and including the first null character… The length of a string is the number of bytes preceding the null character…

C 2018 7.24.6.3 2 says:

The strlen function computes the length of the string pointed to by s [its first argument].

You could compute the size of your array as sizeof string_ (because it is an array of char) or sizeof string_ / sizeof *string_ (to compute the number of elements regardless of type), but this will include a terminating null character because defining an array with [] and letting the length be computed from a string literal initializer includes the terminating null character of the string literal. You may need to hard-code the length of the array, possibly using #define to define a preprocessor macro, and use that length in the array definition and in other places where the length is needed.

CodePudding user response：

strlen() iterate to \0 (similar to \x0). Then it count only "\xF5\x17\x30\x91", it's four character.

CodePudding user response：

It is because you have zero at index [4]

string_[0] == 0xF5
string_[1] == 0x17
string_[2] == 0x30
string_[3] == 0x91
string_[4] == 0
...

"\xf5" puts char having integer value 0xf5 at position [0]

To see it as a string you need to escape the \ character

const char string_[] = "\\xF5\\x17\\x30\\x91\\x00\\xA1\\xC9\\x00\\xDF\\xFF";

CodePudding user response：

At compile time, your "string" appears as consecutive hex values expressed in C syntax inside a pair of quotation marks.

strlen() is a run time function that scans through a series of bytes, looking for the first instance of a zero-value byte.

It's good to understand the difference between "compile time" and "run time".