that completely matches the [op]%[os] pattern on the left hand side

that completely matches, in a case-sensitive manner, the [op]%[os] pattern on the left hand side

If the original value of the variable named by string1 is an empty string, the final result shall be an empty string. Matching shall be case-sensitive.

In all forms of macro expansion, if the value of the macro named by string1 is an empty string, the final result shall be an empty string. If the macro named by string1 does not exist, it is unspecified whether the final result is an empty string or make treats it as an error. The application should ensure that all macros used by make exist (and are explicitly set to an empty string if expected to expand to an empty string).

If the macro named by string1 does not exist, this form shall be equivalent to ...

If the macro named by string1 does not exist, make shall treat this as an error if and only if an attempt to expand the macro named by string1 would be treated as an error; if it is not treated as an error, this form shall be equivalent to ...

that completely matches the [op]%[os] pattern on the left hand side

that completely matches, in a case-sensitive manner, the [op]%[os] pattern on the left hand side

If the original value of the variable named by string1 is an empty string, the final result shall be an empty string. Matching shall be case-sensitive.

In all forms of macro expansion, if the value of the macro named by string1 is an empty string, or if the macro named by string1 does not exist, the final result shall be an empty string.

<small>Note:It is not safe to assume that a macro which has not intentionally been set to a specific value will not exist. See APPLICATION USAGE for more information.</small>

Although make expands macros that do not exist to an empty string, makefile authors should be aware that it is not safe to assume that a macro which has not intentionally been set to a specific value will expand to an empty string for everyone who uses the makefile. There are two reasons for this:

1. When another user executes make, they might happen to have an environment variable of the same name (which they have set for some unrelated purpose) with a non-empty value.

2. A different implementation of make (or a later version of the same implementation) might have a non-empty value for the macro in its default set.

This is one aspect of a more general problem, which is that any macro that is not one for which this standard requires a default value, and is not explicitly set either in the makefile or on the make command line, can have an unexpected value (or unexpectedly not exist) when the makefile is used by a different user or with a different make implementation.

For this reason, it is recommended that makefile authors do not design makefile schemes in which values for non-standard macros are obtained from the user's environment variables. Safer methods of allowing users to configure macro values include:

* Setting the macros to default values in a make include file where the user can edit the values.

* Executing make from one or more wrapper scripts which set macro values on the command line (and which do not obtain those values from environment variables).

Makefile authors who follow this recommendation may wish to check for any macros they have overlooked by using a make implementation that provides, as an extension, a command-line option that causes make to report attempts to expand (or append to) macros that do not exist. Users of makefiles written by others can also benefit from use of such an option to detect the opposite problem (where the author had a macro being set from an environment variable but the user does not have the variable set). This can avoid misbehaviors that would otherwise be hard to debug, such as a file-processing utility reading from standard input because it was not given any pathnames to process.

Makefile authors who choose not to follow the recommendation can minimize the risk of misbehavior by ensuring all non-standard macros have names that begin with a suitable project-specific prefix.

Use of the -e option is strongly discouraged, as it makes the problem discussed above even more likely by introducing the possibility of unexpected values occurring even for macros set in the makefile. If a specific macro needs a value from the environment to override a value set in the makefile, it is safer to set just that macro on the command line, using for example <tt>make MYPROJ_FOO="$MYPROJ_FOO"</tt>. Alternatively, the makefile can be modified to use the <tt>?=</tt> assignment operator for that macro.

Implementations are permitted to include any macro of their choosing in the default set. However, they are encouraged to keep such additions to a minimum in order to reduce the risk of name clashes with user macros.

Implementations are encouraged to provide, as an extension, a command-line option that causes make to report attempts to expand (or append to) macros that do not exist. See APPLICATION USAGE for the intended use cases of such an option.

A future version of this standard may add a command-line option that causes make to report attempts to expand (or append to) macros that do not exist.