Traditional string representation in C

Traditional string representation in C

In C character strings are represented by a sequence of bytes finished by a trailing zero byte. For example, if you got:

char *Name = "lcc-win32";

You will have in memory something like this:

We will have at each of the position of the string array a byte containing a number: the ASCII equivalent of a letter. The array will be followed by a zero byte. Zero is not an ASCII character, and can't appear in character strings, so it means that the string finishes there.

This design is quite ancient, and dates to the beginning of the C language. It has several flaws, as you can imme 727j94h diately see:

• There is no way to know the length of a string besides parsing the whole character array until a zero byte is found.
• Any error where you forget to assign the last terminated byte, or this byte gets overwritten will have catastrophic consequences.
• There is no way to enforce indexing checks.

The most frequently used function of this library are:

• "strlen" that returns an integer containing the length of the string. Example:

int len = strlen("Some character string");

Note that the length of the string is the number of characters without counting the trailing zero. The physical length of the string includes this zero byte however, and this has been (and will be) the source of an infinite number of bugs!

• "strcmp" that compares two strings. If the strings are equal it returns zero. If the first is greater (in the lexicographical sense) than the second it returns a value greater than zero. If the first string is less than the second it returns some value less than zero. The order for the strings is based in the ASCII character set.

• "strcpy" copies one string into another. strcpy(dst,src) copies the src string into the dst string. This means it will start copying characters from the beginning of the src location to the dst location until it finds a zero byte in the src string. No checks are ever done, and it is assumed that the dst string contains sufficient space to hold the src string. If not, the whole program will be destroyed. One of the most common errors in C programming is forgetting these facts.
• "strcat" appends a character string to another. strcat(src,app) will add all the characters of "app" at the end of the "src" string. For instance, if we have the string pointer that has the characters "lcc-win32" as above, and we call the function strcat(str," compiler") we will obtain the following sequence:

The common operations for strings are defined in the header file <string.h>

You will find the details in the online documentation.

Besides these functions in the standard C library, the operating system itself provides quite a few other functions that relate to strings. Besides some relicts of the 16 bit past like lstrcat and others, we find really useful functions, especially for UNICODE handling.