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Chapter 7: Conditional Parts of Makefiles 79
ifeq (arg1, arg2)
ifeq ’arg1’ ’arg2’
ifeq "arg1" "arg2"
ifeq "arg1" ’arg2’
ifeq ’arg1’ "arg2"
Expand all variable references in arg1 and arg2 and compare them. If they
are identical, the text-if-true is effective; otherwise, the text-if-false, if any, is
effective.
Often you want to test if a variable has a non-empty value. When the value
results from complex expansions of variables and functions, expansions you
would consider empty may actually contain whitespace characters and thus are
not seen as empty. However, you can use the strip function (see Section 8.2
[Text Functions], page 84) to avoid interpreting whitespace as a non-empty
value. For example:
ifeq ($(strip $(foo)),)
text-if-empty
endif
will evaluate text-if-empty even if the expansion of $(foo) contains whitespace
characters.
ifneq (arg1, arg2)
ifneq ’arg1’ ’arg2’
ifneq "arg1" "arg2"
ifneq "arg1" ’arg2’
ifneq ’arg1’ "arg2"
Expand all variable references in arg1 and arg2 and compare them. If they
are different, the text-if-true is effective; otherwise, the text-if-false, if any, is
effective.
ifdef variable-name
The ifdef form takes the name of a variable as its argument, not a reference
to a variable. The value of that variable has a non-empty value, the text-if-true
is effective; otherwise, the text-if-false, if any, is effective. Variables that have
never been defined have an empty value. The text variable-name is expanded,
so it could be a variable or function that expands to the name of a variable.
For example:
bar = true
foo = bar
ifdef $(foo)
frobozz = yes
endif
The variable reference $(foo) is expanded, yielding bar, which is considered
to be the name of a variable. The variable bar is not expanded, but its value
is examined to determine if it is non-empty.
Note that ifdef only tests whether a variable has a value. It does not expand
the variable to see if that value is nonempty. Consequently, tests using ifdef
- GNU Make 1
- Short Contents 5
- Table of Contents 7
- 1 Overview of make 11
- 2 An Introduction to Makefiles 13
- 2.2 A Simple Makefile 14
- 2.6 Another Style of Makefile 18
- 3 Writing Makefiles 21
- 3.1.1 Splitting Long Lines 22
- 3.3 Including Other Makefiles 23
- 3.4 The Variable MAKEFILES 24
- 3.5 How Makefiles Are Remade 24
- 3.7 How make Reads a Makefile 26
- Conditional Directives 27
- Rule Definition 27
- 3.8 Secondary Expansion 28
- 4 Writing Rules 31
- 4.2 Types of Prerequisites 32
- 4.3.1 Wildcard Examples 33
- 4.3.3 The Function wildcard 34
- 4.4.2 The vpath Directive 36
- 4.5 Phony Targets 39
- 4.11 Static Pattern Rules 46
- 4.12 Double-Colon Rules 48
- %.d: %.c 49
- @set -e; rm -f $@; \ 49
- 5 Writing Recipes in Rules 51
- 5.2 Recipe Echoing 53
- 5.3 Recipe Execution 54
- 5.3.2 Choosing the Shell 55
- 5.4 Parallel Execution 57
- 5.5 Errors in Recipes 59
- 5.7 Recursive Use of make 60
- 5.8 Defining Canned Recipes 66
- 5.9 Using Empty Recipes 67
- 6 How to Use Variables 69
- 6.3.1 Substitution References 72
- 6.3.2 Computed Variable Names 73
- 6.5 Setting Variables 75
- 6.7 The override Directive 78
- 6.9 Undefining Variables 79
- 6.13 Suppressing Inheritance 82
- 6.14 Other Special Variables 83
- 7.1 Example of a Conditional 87
- 7.2 Syntax of Conditionals 88
- 8.1 Function Call Syntax 93
- 8.3 Functions for File Names 97
- 8.5 The foreach Function 100
- 8.6 The file Function 101
- 8.7 The call Function 102
- 8.8 The value Function 103
- 8.9 The eval Function 103
- 8.10 The origin Function 104
- 8.11 The flavor Function 105
- 8.14 The guile Function 107
- 8.13 The shell Function 107
- 9 How to Run make 109
- 9.5 Overriding Variables 113
- 9.7 Summary of Options 114
- 10 Using Implicit Rules 121
- 10.4 Chains of Implicit Rules 127
- 10.5.2 Pattern Rule Examples 129
- 10.5.3 Automatic Variables 130
- 10.5.4 How Patterns Match 132
- 10.7 Old-Fashioned Suffix Rules 135
- 12 Extending GNU make 143
- 12.2 Loading Dynamic Objects 145
- 12.2.1 The load Directive 146
- Loaded Object Licensing 147
- Data Structures 148
- Registering Functions 148
- Registered Function Interface 148
- GNU make Facilities 148
- Memory Management 149
- 12.2.4 Example Loaded Object 149
- 13 Features of GNU make 153
- 15 Makefile Conventions 159
- 15.2 Utilities in Makefiles 160
- CFLAGS = -g 161
- ALL_CFLAGS = -I. $(CFLAGS) 161
- Appendix A Quick Reference 175
- Index of Concepts 201
- 192 GNU make 202
- Index of Concepts 193 203
- 194 GNU make 204
- Index of Concepts 195 205
- 196 GNU make 206
- Index of Concepts 197 207
- 198 GNU make 208
- 200 GNU make 210
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