GNU Info

(libc.info)Reserved Names

---

Next: Feature Test Macros Prev: Macro Definitions Up: Using the Library

---

Enter node , (file) or (file)node

Reserved Names
--------------

   The names of all library types, macros, variables and functions that
come from the ISO C standard are reserved unconditionally; your program
*may not* redefine these names.  All other library names are reserved
if your program explicitly includes the header file that defines or
declares them.  There are several reasons for these restrictions:

   * Other people reading your code could get very confused if you were
     using a function named `exit' to do something completely different
     from what the standard `exit' function does, for example.
     Preventing this situation helps to make your programs easier to
     understand and contributes to modularity and maintainability.

   * It avoids the possibility of a user accidentally redefining a
     library function that is called by other library functions.  If
     redefinition were allowed, those other functions would not work
     properly.

   * It allows the compiler to do whatever special optimizations it
     pleases on calls to these functions, without the possibility that
     they may have been redefined by the user.  Some library
     facilities, such as those for dealing with variadic arguments
     (Note: Variadic Functions) and non-local exits (Note: Non-Local
     Exits), actually require a considerable amount of cooperation on
     the part of the C compiler, and with respect to the
     implementation, it might be easier for the compiler to treat these
     as built-in parts of the language.

   In addition to the names documented in this manual, reserved names
include all external identifiers (global functions and variables) that
begin with an underscore (`_') and all identifiers regardless of use
that begin with either two underscores or an underscore followed by a
capital letter are reserved names.  This is so that the library and
header files can define functions, variables, and macros for internal
purposes without risk of conflict with names in user programs.

   Some additional classes of identifier names are reserved for future
extensions to the C language or the POSIX.1 environment.  While using
these names for your own purposes right now might not cause a problem,
they do raise the possibility of conflict with future versions of the C
or POSIX standards, so you should avoid these names.

   * Names beginning with a capital `E' followed a digit or uppercase
     letter may be used for additional error code names.  Note: Error
     Reporting.

   * Names that begin with either `is' or `to' followed by a lowercase
     letter may be used for additional character testing and conversion
     functions.  Note: Character Handling.

   * Names that begin with `LC_' followed by an uppercase letter may be
     used for additional macros specifying locale attributes.  Note:
     Locales.

   * Names of all existing mathematics functions (Note: Mathematics)
     suffixed with `f' or `l' are reserved for corresponding functions
     that operate on `float' and `long double' arguments, respectively.

   * Names that begin with `SIG' followed by an uppercase letter are
     reserved for additional signal names.  Note: Standard Signals.

   * Names that begin with `SIG_' followed by an uppercase letter are
     reserved for additional signal actions.  Note: Basic Signal
     Handling.

   * Names beginning with `str', `mem', or `wcs' followed by a
     lowercase letter are reserved for additional string and array
     functions.  Note: String and Array Utilities.

   * Names that end with `_t' are reserved for additional type names.

   In addition, some individual header files reserve names beyond those
that they actually define.  You only need to worry about these
restrictions if your program includes that particular header file.

   * The header file `dirent.h' reserves names prefixed with `d_'.

   * The header file `fcntl.h' reserves names prefixed with `l_', `F_',
     `O_', and `S_'.

   * The header file `grp.h' reserves names prefixed with `gr_'.

   * The header file `limits.h' reserves names suffixed with `_MAX'.

   * The header file `pwd.h' reserves names prefixed with `pw_'.

   * The header file `signal.h' reserves names prefixed with `sa_' and
     `SA_'.

   * The header file `sys/stat.h' reserves names prefixed with `st_'
     and `S_'.

   * The header file `sys/times.h' reserves names prefixed with `tms_'.

   * The header file `termios.h' reserves names prefixed with `c_',
     `V', `I', `O', and `TC'; and names prefixed with `B' followed by a
     digit.