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54 GNU make
5.7.3 Communicating Options to a Sub-make
Flags such as ‘-s’ and ‘-k’ are passed automatically to the sub-make through the variable
MAKEFLAGS. This variable is set up automatically by make to contain the flag letters that
make received. Thus, if you do ‘make -ks’ then MAKEFLAGS gets the value ‘ks’.
As a consequence, every sub-make gets a value for MAKEFLAGS in its environment. In
response, it takes the flags from that value and processes them as if they had been given as
arguments. See Section 9.7 [Summary of Options], page 104.
Likewise variables defined on the command line are passed to the sub-make through
MAKEFLAGS. Words in the value of MAKEFLAGS that contain ‘=’, make treats as variable
definitions just as if they appeared on the command line. See Section 9.5 [Overriding
Variables], page 103.
The options ‘-C’, ‘-f’, ‘-o’, and ‘-W’ are not put into MAKEFLAGS; these options are not
passed down.
The ‘-j’ option is a special case (see Section 5.4 [Parallel Execution], page 47). If you set
it to some numeric value ‘N’ and your operating system supports it (most any UNIX system
will; others typically won’t), the parent make and all the sub-makes will communicate to
ensure that there are only ‘N’ jobs running at the same time between them all. Note that
any job that is marked recursive (see Section 9.3 [Instead of Executing Recipes], page 101)
doesn’t count against the total jobs (otherwise we could get ‘N’ sub-makes running and have
no slots left over for any real work!)
If your operating system doesn’t support the above communication, then ‘-j 1’ is always
put into MAKEFLAGS instead of the value you specified. This is because if the ‘-j’ option
were passed down to sub-makes, you would get many more jobs running in parallel than
you asked for. If you give ‘-j’ with no numeric argument, meaning to run as many jobs as
possible in parallel, this is passed down, since multiple infinities are no more than one.
If you do not want to pass the other flags down, you must change the value of MAKEFLAGS,
like this:
subsystem:
cd subdir && $(MAKE) MAKEFLAGS=
The command line variable definitions really appear in the variable MAKEOVERRIDES,
and MAKEFLAGS contains a reference to this variable. If you do want to pass flags down
normally, but don’t want to pass down the command line variable definitions, you can reset
MAKEOVERRIDES to empty, like this:
MAKEOVERRIDES =
This is not usually useful to do. However, some systems have a small fixed limit on the
size of the environment, and putting so much information into the value of MAKEFLAGS can
exceed it. If you see the error message ‘Arg list too long’, this may be the problem. (For
strict compliance with POSIX.2, changing MAKEOVERRIDES does not affect MAKEFLAGS if the
special target ‘.POSIX’ appears in the makefile. You probably do not care about this.)
A similar variable MFLAGS exists also, for historical compatibility. It has the same
value as MAKEFLAGS except that it does not contain the command line variable defini-
tions, and it always begins with a hyphen unless it is empty (MAKEFLAGS begins with
a hyphen only when it begins with an option that has no single-letter version, such as
- 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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