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
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 (see Variadic Functions)
and non-local exits (see 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. See 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. See Character Handling.
Names that begin with `LC_' followed by an uppercase letter may be
used for additional macros specifying locale attributes.
Names of all existing mathematics functions (see Mathematics)
suffixed with `f' or `l' are reserved for corresponding
functions that operate on float and long double arguments,
Names that begin with `SIG' followed by an uppercase letter are
reserved for additional signal names. See Standard Signals.
Names that begin with `SIG_' followed by an uppercase letter are
reserved for additional signal actions. See Basic Signal Handling.
Names beginning with `str', `mem', or `wcs' followed by a
lowercase letter are reserved for additional string and array functions.
See 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
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_'
The header file sys/stat.h reserves names prefixed with `st_'
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.
Published under the terms of the GNU General Public License