What are the rules about using an underscore in a C identifier

Question

It s common in C   to name member variables with some kind of prefix to denote the fact that they re member variables  rather than local variables or parameters  If you ve come from an MFC background  you ll probably use m foo  I ve also seen myFoo occasionally   C   or possibly just  NET  seems to recommend using just an underscore  as in  foo  Is this allowed by the C   standard

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The rules  which did not change in C  11     Reserved in any scope  including for use as implementation macros    identifiers beginning with an underscore followed immediately by an uppercase letter identifiers containing adjacent underscores  or  double underscore    Reserved in the global namespace    identifiers beginning with an underscore  Also  everything in the std namespace is reserved   You are allowed to add template specializations  though      From the 2003 C   Standard      17 4 3 1 2 Global names  lib global names       Certain sets of names and function signatures are always reserved to the implementation          Each name that contains a double underscore      or begins with an underscore followed by an uppercase letter  2 11  is reserved to the implementation for any use    Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace 165         165  Such names are also reserved in namespace   std  17 4 3 1      Because C   is based on the C standard  1 1 2  C  03  and C99 is a normative reference  1 2 1  C  03  these also apply  from the 1999 C Standard      7 1 3 Reserved identifiers      Each header declares or defines all identifiers listed in its associated subclause  and   optionally declares or defines identifiers listed in its associated future library directions subclause and identifiers which are always reserved either for any use or for use as file scope identifiers          All identifiers that begin with an underscore and either an uppercase letter or another   underscore are always reserved for any use    All identifiers that begin with an underscore are always reserved for use as identifiers   with file scope in both the ordinary and tag name spaces    Each macro name in any of the following subclauses  including the future library   directions  is reserved for use as specified if any of its associated headers is included    unless explicitly stated otherwise  see 7 1 4     All identifiers with external linkage in any of the following subclauses  including the   future library directions  are always reserved for use as identifiers with external   linkage 154   Each identifier with file scope listed in any of the following subclauses  including the   future library directions  is reserved for use as a macro name and as an identifier with   file scope in the same name space if any of its associated headers is included          No other identifiers are reserved  If the program declares or defines an identifier in a   context in which it is reserved  other than as allowed by 7 1 4   or defines a reserved   identifier as a macro name  the behavior is undefined       If the program removes  with  undef  any macro definition of an identifier in the first   group listed above  the behavior is undefined       154  The list of reserved identifiers with external linkage includes errno  math errhandling  setjmp  and va end    Other restrictions might apply  For example  the POSIX standard reserves a lot of identifiers that are likely to show up in normal code    Names beginning with a capital E followed a digit or uppercase letter    may be used for additional error code names   Names that begin with either is or to followed by a lowercase letter   may be used for additional character testing and conversion functions   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 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   Names that begin with SIG  followed by an uppercase letter are reserved   for additional signal actions   Names beginning with str  mem  or wcs followed by a lowercase letter are reserved   for additional string and array functions   Names beginning with PRI or SCN followed by any lowercase letter or X are reserved   for additional format specifier macros  Names that end with  t are reserved   for additional type names     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 that standard     Personally I just don t start identifiers with underscores  New addition to my rule  Don t use double underscores anywhere  which is easy as I rarely use underscore   After doing research on this article I no longer end my identifiers with  t as this is reserved by the POSIX standard   The rule about any identifier ending with  t surprised me a lot  I think that is a POSIX standard  not sure yet  looking for clarification and official chapter and verse  This is from the GNU libtool manual  listing reserved names   CesarB provided the following link to the POSIX 2004 reserved symbols and notes  that many other reserved prefixes and suffixes     can be found there    The POSIX 2008 reserved symbols are defined here   The restrictions are somewhat more nuanced than those above

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From MSDN      Use of two sequential underscore characters        at the beginning of an identifier  or a single leading underscore followed by a capital letter  is reserved for C   implementations in all scopes  You should avoid using one leading underscore followed by a lowercase letter for names with file scope because of possible conflicts with current or future reserved identifiers    This means that you can use a single underscore as a member variable prefix  as long as it s followed by a lower-case letter   This is apparently taken from section 17 4 3 1 2 of the C   standard  but I can t find an original source for the full standard online   See also this question

User · Answer

Yes  underscores may be used anywhere in an identifier  I believe the rules are  any of a-z  A-Z    in the first character and those  0-9 for the following characters   Underscore prefixes are common in C code -- a single underscore means  private   and double underscores are usually reserved for use by the compiler

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The rules to avoid collision of names are both in the C   standard  see Stroustrup book  and mentioned by C   gurus  Sutter  etc     Personal rule  Because I did not want to deal with cases  and wanted a simple rule  I have designed a personal one that is both simple and correct    When naming a symbol  you will avoid collision with compiler OS standard libraries if you    never start a symbol with an underscore never name a symbol with two consecutive underscores inside     Of course  putting your code in an unique namespace helps to avoid collision  too  but won t protect against evil macros   Some examples   I use macros because they are the more code-polluting of C C   symbols  but it could be anything from variable name to class name    define  WRONG  define   WRONG AGAIN  define RIGHT   define WRONG  WRONG  define RIGHT RIGHT  define RIGHT x RIGHT   Extracts from C  0x draft  From the n3242 pdf file  I expect the final standard text to be similar       17 6 3 3 2 Global names  global names       Certain sets of names and function signatures are always reserved to the implementation           Each name that contains a double underscore     or begins with an underscore followed by an uppercase letter  2 12  is reserved to the implementation for any use           Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace    But also      17 6 3 3 5 User-defined literal suffixes  usrlit suffix       Literal suffix identifiers that do not start with an underscore are reserved for future standardization    This last clause is confusing  unless you consider that a name starting with one underscore and followed by a lowercase letter would be Ok if not defined in the global namespace

User · Answer

As for the other part of the question  it s common to put the underscore at the end of the variable name to not clash with anything internal   I do this even inside classes and namespaces because I then only have to remember one rule  compared to  at the end of the name in global scope  and the beginning of the name everywhere else

User · Answer

As for the other part of the question  it s common to put the underscore at the end of the variable name to not clash with anything internal   I do this even inside classes and namespaces because I then only have to remember one rule  compared to  at the end of the name in global scope  and the beginning of the name everywhere else

User · Answer

The rules to avoid collision of names are both in the C   standard  see Stroustrup book  and mentioned by C   gurus  Sutter  etc     Personal rule  Because I did not want to deal with cases  and wanted a simple rule  I have designed a personal one that is both simple and correct    When naming a symbol  you will avoid collision with compiler OS standard libraries if you    never start a symbol with an underscore never name a symbol with two consecutive underscores inside     Of course  putting your code in an unique namespace helps to avoid collision  too  but won t protect against evil macros   Some examples   I use macros because they are the more code-polluting of C C   symbols  but it could be anything from variable name to class name    define  WRONG  define   WRONG AGAIN  define RIGHT   define WRONG  WRONG  define RIGHT RIGHT  define RIGHT x RIGHT   Extracts from C  0x draft  From the n3242 pdf file  I expect the final standard text to be similar       17 6 3 3 2 Global names  global names       Certain sets of names and function signatures are always reserved to the implementation           Each name that contains a double underscore     or begins with an underscore followed by an uppercase letter  2 12  is reserved to the implementation for any use           Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace    But also      17 6 3 3 5 User-defined literal suffixes  usrlit suffix       Literal suffix identifiers that do not start with an underscore are reserved for future standardization    This last clause is confusing  unless you consider that a name starting with one underscore and followed by a lowercase letter would be Ok if not defined in the global namespace

User · Answer

From MSDN      Use of two sequential underscore characters        at the beginning of an identifier  or a single leading underscore followed by a capital letter  is reserved for C   implementations in all scopes  You should avoid using one leading underscore followed by a lowercase letter for names with file scope because of possible conflicts with current or future reserved identifiers    This means that you can use a single underscore as a member variable prefix  as long as it s followed by a lower-case letter   This is apparently taken from section 17 4 3 1 2 of the C   standard  but I can t find an original source for the full standard online   See also this question

User · Answer

From MSDN      Use of two sequential underscore characters        at the beginning of an identifier  or a single leading underscore followed by a capital letter  is reserved for C   implementations in all scopes  You should avoid using one leading underscore followed by a lowercase letter for names with file scope because of possible conflicts with current or future reserved identifiers    This means that you can use a single underscore as a member variable prefix  as long as it s followed by a lower-case letter   This is apparently taken from section 17 4 3 1 2 of the C   standard  but I can t find an original source for the full standard online   See also this question

User · Answer

The rules to avoid collision of names are both in the C   standard  see Stroustrup book  and mentioned by C   gurus  Sutter  etc     Personal rule  Because I did not want to deal with cases  and wanted a simple rule  I have designed a personal one that is both simple and correct    When naming a symbol  you will avoid collision with compiler OS standard libraries if you    never start a symbol with an underscore never name a symbol with two consecutive underscores inside     Of course  putting your code in an unique namespace helps to avoid collision  too  but won t protect against evil macros   Some examples   I use macros because they are the more code-polluting of C C   symbols  but it could be anything from variable name to class name    define  WRONG  define   WRONG AGAIN  define RIGHT   define WRONG  WRONG  define RIGHT RIGHT  define RIGHT x RIGHT   Extracts from C  0x draft  From the n3242 pdf file  I expect the final standard text to be similar       17 6 3 3 2 Global names  global names       Certain sets of names and function signatures are always reserved to the implementation           Each name that contains a double underscore     or begins with an underscore followed by an uppercase letter  2 12  is reserved to the implementation for any use           Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace    But also      17 6 3 3 5 User-defined literal suffixes  usrlit suffix       Literal suffix identifiers that do not start with an underscore are reserved for future standardization    This last clause is confusing  unless you consider that a name starting with one underscore and followed by a lowercase letter would be Ok if not defined in the global namespace

User · Answer

Yes  underscores may be used anywhere in an identifier  I believe the rules are  any of a-z  A-Z    in the first character and those  0-9 for the following characters   Underscore prefixes are common in C code -- a single underscore means  private   and double underscores are usually reserved for use by the compiler

User · Answer

From MSDN      Use of two sequential underscore characters        at the beginning of an identifier  or a single leading underscore followed by a capital letter  is reserved for C   implementations in all scopes  You should avoid using one leading underscore followed by a lowercase letter for names with file scope because of possible conflicts with current or future reserved identifiers    This means that you can use a single underscore as a member variable prefix  as long as it s followed by a lower-case letter   This is apparently taken from section 17 4 3 1 2 of the C   standard  but I can t find an original source for the full standard online   See also this question

User · Answer

Yes  underscores may be used anywhere in an identifier  I believe the rules are  any of a-z  A-Z    in the first character and those  0-9 for the following characters   Underscore prefixes are common in C code -- a single underscore means  private   and double underscores are usually reserved for use by the compiler

User · Answer

As for the other part of the question  it s common to put the underscore at the end of the variable name to not clash with anything internal   I do this even inside classes and namespaces because I then only have to remember one rule  compared to  at the end of the name in global scope  and the beginning of the name everywhere else

User · Answer

As for the other part of the question  it s common to put the underscore at the end of the variable name to not clash with anything internal   I do this even inside classes and namespaces because I then only have to remember one rule  compared to  at the end of the name in global scope  and the beginning of the name everywhere else

User · Answer

The rules to avoid collision of names are both in the C   standard  see Stroustrup book  and mentioned by C   gurus  Sutter  etc     Personal rule  Because I did not want to deal with cases  and wanted a simple rule  I have designed a personal one that is both simple and correct    When naming a symbol  you will avoid collision with compiler OS standard libraries if you    never start a symbol with an underscore never name a symbol with two consecutive underscores inside     Of course  putting your code in an unique namespace helps to avoid collision  too  but won t protect against evil macros   Some examples   I use macros because they are the more code-polluting of C C   symbols  but it could be anything from variable name to class name    define  WRONG  define   WRONG AGAIN  define RIGHT   define WRONG  WRONG  define RIGHT RIGHT  define RIGHT x RIGHT   Extracts from C  0x draft  From the n3242 pdf file  I expect the final standard text to be similar       17 6 3 3 2 Global names  global names       Certain sets of names and function signatures are always reserved to the implementation           Each name that contains a double underscore     or begins with an underscore followed by an uppercase letter  2 12  is reserved to the implementation for any use           Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace    But also      17 6 3 3 5 User-defined literal suffixes  usrlit suffix       Literal suffix identifiers that do not start with an underscore are reserved for future standardization    This last clause is confusing  unless you consider that a name starting with one underscore and followed by a lowercase letter would be Ok if not defined in the global namespace

User · Answer

Yes  underscores may be used anywhere in an identifier  I believe the rules are  any of a-z  A-Z    in the first character and those  0-9 for the following characters   Underscore prefixes are common in C code -- a single underscore means  private   and double underscores are usually reserved for use by the compiler

[c++] What are the rules about using an underscore in a C++ identifier?

17.4.3.1.2 Global names [lib.global.names]

7.1.3 Reserved identifiers

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