Changes between Version 5 and Version 6 of const

Timestamp:

May 13, 2010, 5:21:45 PM (15 years ago)

Author:

andersk@mit.edu

Comment:

Add a note about C library functions accepting pointers to strings.

const

@@ -37 +37 @@
 == `const` and strings ==
 
-For historical reasons, literal strings have type `char[]` by default.  However, literal strings are still put into a read-only section of the program image, and it is invalid to modify or free one.  Therefore literal strings should always be stored in a `const char *`, never in a `char *`.
+Prehistoric C compilers did not support `const`, so to maintain compatibility with old code, literal strings have type `char[]` by default.  However, literal strings are still put into a read-only section of the program image, and it is invalid to modify or free one.  Therefore literal strings should always be stored in a `const char *`, never in a `char *`.
 
 If you compile with `gcc -Wwrite-strings`, literal strings will be typed `const char[]` like they should be, to prevent you from making the mistake of storing one in a `char *`.
+
+String functions in the standard C library accept `const char *` and return `char *`, again for compatibility; this does not cause problems because of the implicit conversion from `char *` to `const char *`.  (Note that abuses such as `const char *p; char *q = strstr(p, "");` are still unsafe even though they fail to be disallowed by the signature of `strstr`.)  However, a few functions like `execv` that deal with ''pointers'' to strings (or arrays of strings) were forced to choose one or the other, and they take `char **` or `char *const *` rather than `const char *const *`.  A cast is required to use these functions with `const char *` strings.  glibc’s `getopt` is a [http://www.kernel.org/doc/man-pages/online/pages/man3/getopt.3.html#CONFORMING_TO particularly special case].
 
 == Using `const` effectively ==