12.7 String Constants

A string constant represents a series of characters. It starts with ‘"’ and ends with ‘"’; in between are the contents of the string. Quoting special characters such as ‘"’, ‘\’ and newline in the contents works in string constants as in character constants. In a string constant, ‘'’ does not need to be quoted.

A string constant defines an array of characters which contains the specified characters followed by the null character (code 0). Using the string constant is equivalent to using the name of an array with those contents. In simple cases, where there are no backslash escape sequences, the length in bytes of the string constant is one greater than the number of characters written in it.

As with any array in C, using the string constant in an expression converts the array to a pointer (see Pointers) to the array’s zeroth element (see Accessing Array Elements). This pointer will have type char * because it points to an element of type char. char * is an example of a type designator for a pointer type (see Pointer Type Designators). That type is used for strings generally, not just the strings expressed as constants in a program.

Thus, the string constant "Foo!" is almost equivalent to declaring an array like this

char string_array_1[] = {'F', 'o', 'o', '!', '\0' };

and then using string_array_1 in the program. There are two differences, however:

• The string constant doesn’t define a name for the array.
• The string constant is probably stored in a read-only area of memory.

Newlines are not allowed in the text of a string constant. The motive for this prohibition is to catch the error of omitting the closing ‘"’. To put a newline in a constant string, write it as ‘\n’ in the string constant.

A real null character in the source code inside a string constant causes a warning. To put a null character in the middle of a string constant, write ‘\0’ or ‘\000’.

Consecutive string constants are effectively concatenated. Thus,

"Fo" "o!"   is equivalent to   "Foo!"

This is useful for writing a string containing multiple lines, like this:

"This message is so long that it needs more than\n"
"a single line of text.  C does not allow a newline\n"
"to represent itself in a string constant, so we have to\n"
"write \\n to put it in the string.  For readability of\n"
"the source code, it is advisable to put line breaks in\n"
"the source where they occur in the contents of the\n"
"constant.\n"

The sequence of a backslash and a newline is ignored anywhere in a C program, and that includes inside a string constant. Thus, you can write multi-line string constants this way:

"This is another way to put newlines in a string constant\n\
and break the line after them in the source code."

However, concatenation is the recommended way to do this.

You can also write perverse string constants like this,

"Fo\
o!"

but don’t do that—write it like this instead:

"Foo!"

Be careful to avoid passing a string constant to a function that modifies the string it receives. The memory where the string constant is stored may be read-only, which would cause a fatal SIGSEGV signal that normally terminates the function (see Signals. Even worse, the memory may not be read-only. Then the function might modify the string constant, thus spoiling the contents of other string constants that are supposed to contain the same value and are unified by the compiler.