Does C have a string type?

Question

I have recently started programming in C, coming from Java and Python. Now, in my book I have noticed that to make a "Hello World" program, the syntax is something like this:

char message[10]
strcpy(message, "Hello, world!")
printf("%s\n", message);

Now, this example is using a char array and I wondered - what happened to strings? Why can't I simply use one of those? Maybe there is a different way to do this?

Solution 1

C does not and never has had a native string type. By convention, the language uses arrays of char terminated with a null char, i.e., with '\0'. Functions and macros in the language's standard libraries provide support for the null-terminated character arrays, e.g., strlen iterates over an array of char until it encounters a '\0' character and strcpy copies from the source string until it encounters a '\0'.

The use of null-terminated strings in C reflects the fact that C was intended to be only a little more high-level than assembly language. Zero-terminated strings were already directly supported at that time in assembly language for the PDP-10 and PDP-11.

It is worth noting that this property of C strings leads to quite a few nasty buffer overrun bugs, including serious security flaws. For example, if you forget to null-terminate a character string passed as the source argument to strcpy, the function will keep copying sequential bytes from whatever happens to be in memory past the end of the source string until it happens to encounter a 0, potentially overwriting whatever valuable information follows the destination string's location in memory.

In your code example, the string literal "Hello, world!" will be compiled into a 14-byte long array of char. The first 13 bytes will hold the letters, comma, space, and exclamation mark and the final byte will hold the null-terminator character '\0', automatically added for you by the compiler. If you were to access the array's last element, you would find it equal to 0. E.g.:

const char foo[] = "Hello, world!";
assert(foo[12] == '!');
assert(foo[13] == '\0');

However, in your example, message is only 10 bytes long. strcpy is going to write all 14 bytes, including the null-terminator, into memory starting at the address of message. The first 10 bytes will be written into the memory allocated on the stack for message and the remaining four bytes will simply be written on to the end of the stack. The consequence of writing those four extra bytes onto the stack is hard to predict in this case (in this simple example, it might not hurt a thing), but in real-world code it usually leads to corrupted data or memory access violation errors.

Solution 2

There is no string type in C. You have to use char arrays.

By the way your code will not work ,because the size of the array should allow for the whole array to fit in plus one additional zero terminating character.

Solution 3

To note it in the languages you mentioned:

Java:

String str = new String("Hello");

Python:

str = "Hello"

Both Java and Python have the concept of a "string", C does not have the concept of a "string". C has character arrays which can come in "read only" or manipulatable.

C:

char * str = "Hello";  // the string "Hello\0" is pointed to by the character pointer
                       // str. This "string" can not be modified (read only)

or

char str[] = "Hello";  // the characters: 'H''e''l''l''o''\0' have been copied to the 
                       // array str. You can change them via: str[x] = 't'

A character array is a sequence of contiguous characters with a unique sentinel character at the end (normally a NULL terminator '\0'). Note that the sentinel character is auto-magically appended for you in the cases above.

Solution 4

In C, a string simply is an array of characters, ending with a null byte. So a char* is often pronounced "string", when you're reading C code.

Solution 5

C does not support a first class string type.

C++ has std::string

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Comments

[12431234123412341234123]

An array of char that does not have a '\0'-byte in it is not a string.

[shadow0359]

does writing 4 extra bytes result in corruption of initial 10 bytes or corrupt some other memory location(4 bytes) on the stack? In what scenario segmentation fault will be thrown instead of memory corruption?