Characters and Strings

Storage

The right column shows (in extreme detail) the allocation of memory for program string-intro.c, based upon the above run. Starting at the top of the program:

• first is allocated space for four characters, beginning in storage location 359157264 (see bottom part of the table). Following the normal approach of initializing arrays, the letters, C, o, l, and d are stored in these locations. The program does not specify what data might be located after this part of memory.
• second is allocated space for six characters, beginning in storage location 359157248. In C, a string contains a sequence of characters, followed by a null character (code zero). Since World contains five characters, the string requires six characters to include the code 0 at the end.
• In organizing memory, the compiler decided not to use the space between second and first for data storage. Although these memory locations are present, the data in those unallocated memory addresses may be left over from the work of previous programs.
• third is allocated space for sixteen characters, beginning in storage location 359157232. As with first, this space is initialized with specified characters. As an array of characters (not a string), no code zero is placed in memory at the end of this array.
• fourth specifies the address of a character (e.g., a pointer to the character). In this case, fourth is given the address that begins the string second defined earlier. Note that fourth refers to a location in memory (359157224), and the address of second (359157248) is stored in the variable fourth.
• fifth specifies the address of a character. The address of a character can be the base address of a character array. A char * may be considered either the location of a single character or the starting point for a string. In this case, information for variable fifth is located at 359157216, and that location contains the starting location 4196464 for the literal string "Hello" — compilers often reserve a separate part of main memory for literal data, such as literal strings.

Output

The first set of printf statements access the first three characters in each character array. Within a printf statement, the %c format prints exactly one data element as a character, so that three characters are printed for each printf statement here. Note that arrays and subscripts work the same whether the variable is declared as an array or as the base address of an array found elsewhere.

The second set of printf statements display where each variable is mapped in main memory. The output shown above maps to the memory schematic on the right.

The third set of printf statements print data as C strings. In C, a string variable identifies a starting or base address, and the string is considered to continue until a code 0 or null character is encountered.

• For variables, second, fourth, and fifth, the character data were stored with a null character at the end, and these character strings are printed without difficulty.
• For the variable third, the initialization placed characters in the array, but no null character was at the end. Rather, from the mapping of memory identified in the table, the string "World" was located immediately after the characters in the third array. When printing third, the printf started with the first character of third (i.e., the C character) and continued character by character until reaching a null. Since no null character was encountered in the processing of the third array, printing continued with the data from the second array.
• For the variable first, the array declaration specified four characters, without a null character at the end. Although this works fine for arrays, work with strings requires processing to continue until a null is found. In this case, first is stored in memory at the end of the program area, and we have no idea what might follow. Thus, processing proceeds with the printing of random material until a null is found.

Schematic Memory Diagram