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Representation of Strings

This section is a quick summary of string concepts for beginning C programmers. It describes how character strings are represented in C and some common pitfalls. If you are already familiar with this material, you can skip this section.

A string is an array of char objects. But string-valued variables are usually declared to be pointers of type char *. Such variables do not include space for the text of a string; that has to be stored somewhere else--in an array variable, a string constant, or dynamically allocated memory (see section Memory Allocation). It's up to you to store the address of the chosen memory space into the pointer variable. Alternatively you can store a null pointer in the pointer variable. The null pointer does not point anywhere, so attempting to reference the string it points to gets an error.

By convention, a null character, '\0', marks the end of a string. For example, in testing to see whether the char * variable p points to a null character marking the end of a string, you can write !*p or *p == '\0'.

A null character is quite different conceptually from a null pointer, although both are represented by the integer 0.

String literals appear in C program source as strings of characters between double-quote characters (`"'). In ISO C, string literals can also be formed by string concatenation: "a" "b" is the same as "ab". Modification of string literals is not allowed by the GNU C compiler, because literals are placed in read-only storage.

Character arrays that are declared const cannot be modified either. It's generally good style to declare non-modifiable string pointers to be of type const char *, since this often allows the C compiler to detect accidental modifications as well as providing some amount of documentation about what your program intends to do with the string.

The amount of memory allocated for the character array may extend past the null character that normally marks the end of the string. In this document, the term allocation size is always used to refer to the total amount of memory allocated for the string, while the term length refers to the number of characters up to (but not including) the terminating null character.

A notorious source of program bugs is trying to put more characters in a string than fit in its allocated size. When writing code that extends strings or moves characters into a pre-allocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array. Many of the library functions do not do this for you! Remember also that you need to allocate an extra byte to hold the null character that marks the end of the string.

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