Gun make (3) Rules for makefile

1. The type of dependency

GUN make There are two types of dependencies in ：

Routine dependence ： Any one of the dependent files is newer than the target file , It is considered that the goal of the rule is expired and needs to be rebuilt .

order-only rely on ： When order-only When a file in a dependency is updated , If the target does not exist , Generate target ; If the target exists , Do not update the target .

 

routine 、order-only Rely on writing rules ：

TARGETS:NORMAL-PREREQUISITES | ORDER-ONLY-PREREQUISITES

Use “ | ” Separate general dependencies from order-only rely on , If a file appears on both sides at the same time , It is considered as a regular dependency .

 

2. File name wildcard

2.1 Wildcard type

makefile Wildcard characters can be used for file names in , Yes * ,  ？  ,   [...]

Wildcards can be used in the following situations ：

• Used in the target of the rule , Dependency , make In the reading makefile It is automatically matched when （ Wildcard expansion ）.
• Can appear in the command of the rule , The general configuration processing is performed in shell Completed when executing this command .
• In other contexts besides the above two cases , Wildcards cannot be used directly .

 

2.2 wildcard function

In the rules , Wildcards are automatically expanded , But in variable definitions and function references , Wildcards will be invalidated .

If the wildcard is required to take effect , Use wildcard function ：

$(wildcard PATTERN)

 stay makefile in , It is expanded to an existing , Separated by spaces , List of all files matching the pattern .

 

such as :

$(wildcard *.c) To get all the... In the working directory .c File list .

 

3. Directory search

3.1 General search （VPATH）

Through the variable VPATH You can specify the search path for dependent files , Use Space Or in the The colon ： Separate multiple directory files to be searched , The search order is in the order of the directory .

When When the dependent file of the rule does not exist in the current directory ,make These dependent files will be found in the directory specified by this variable .

 

such as ：

VPATH = src:../headers   

It specifies src and ../headers Two search directories , about foo:foo.c  , If foo.c Exist in src Under the table of contents , This rule is equivalent to foo:src:/foo.c

 

3.2 Selective search （vpath）

Selective search can specify different search directories for different types of files .

vpath Medium PATTERN You need to include pattern characters %, It means matching one or more characters . Represents a class of files with the same characteristics .

DIRECTORIES Specifies the directory to search for such files .

 

Three ways to use ：

• vpath PATTERN DIRECTORIES

For all conforming patterns PATTERN The file specifies the search directory DIRECTORIES, Multiple directories use spaces perhaps The colon ： Separate .

• vpath PATTERN

Before clearing, it is in compliance mode PATTERN The search path for the file settings of .

• vpath Clear all set file search paths .

 

such as ：
vpath %.h ../headers 

Express makefile What happened in .h file , If it cannot be found in the current directory , Then go to the catalog ../headers Look for .

Be careful , The specified path here is limited to makefile Appears in the contents of the file .h file , You cannot specify the path where the header file contained in the source file is located . stay .c The header file path contained in the source file needs to use gcc Of -I To specify the .

 

3.3 The mechanism of directory search

make In parsing Makefile The algorithm for saving or discarding file paths when executing rules for files is as follows ：

1. If the rules are Target file stay makefile The directory where the file is located does not exist , Then perform a directory search .
2. If the directory search is successful , This target's... Exists in the specified directory The rules , Then the full pathname searched will be saved as a temporary target file .
3. For all of the rules Dependency file Use the same method to deal with .
4. After the third step of dependency processing ,make The program can then determine whether the goal of the rule needs to be rebuilt , There are two situations ：

• The goal of the rule does not need to be rebuilt , The full pathnames of all files in the rule are valid , The directory of the existing target file will not be changed .
• The goal of the rule needs to be rebuilt , The target file of the rule will be rebuilt in the working directory , Instead of the directory obtained during directory search .
• If you use GPATH instead of VPATH Specify search directory , Then the target is rebuilt , It will be rebuilt in the directory obtained during directory search .

 

3.4 Library files and search directories

makefile have access to -INAME Yes Static library .a Dynamic library .so Do a directory search .

 

The file name of the dependent library searched is determined by .LIBPATTERNS Appoint , It is usually multiple characters that contain patterns % Word , Multiple words are separated by spaces .

according to .LIBPATTERNS The order in which words appear , use NAME Pattern characters that replace words one by one , Get library file name , Search under the search directory according to the library file name .

 

By default , .LIBPATTERNS The value is lib%.so  lib%.a , So usually search first libNAME.so library , Re search libNAME.a library .

 

4. Makefile Target in

4.1 Fake target

Pseudo targets use ：

.PHONY:clean               // Use .PHONY Declare false targets clean
clean:
    rm *.o temp

Pseudo target advantages ：

• When the working directory does not exist clean This file , Output make clean when , The corresponding rule must be executed . But if the current working directory exists clean When this document , When the input make clean, Because this rule does not depend on files , So the target is considered to be up-to-date without executing the commands defined by the rules .
• When a target is declared as a pseudo target ,make When this rule is executed, no attempt is made to find an implicit rule to create it .

 

4.2 Force the target

The target of the mandatory target is a nonexistent file name , And there are no commands or dependencies .

When executing this mandatory goal , Goals are always considered up to date . therefore , If you force the target as a dependency , Cause the rule to be executed .

 

such as ：

clean:FORCE                    // Force the target as a dependency 
    rm $(objects)
FORCE:                         // Force the target （ No dependencies or commands ）

 

4.3 Multiple goals

The multi-target rule means that all targets have the same dependent files .

Multi objective usage ：

aaa bbb:text.g
    generate text.g -$(subst output,[email protected]) > [email protected]

 Equivalent to ：
aaa:text.g
    generate text.g -$(subst output,[email protected]) > aaa 

bbb:text.g
    generate text.g -$(subst output,[email protected]) > bbb

 

4.4 Multi rule target

For a multi rule target , The command to rebuild this target can only appear in one rule .

If multiple rules give the command to rebuild this target at the same time ,make The command defined in the last rule will be used .

 

4.5 Static mode

Static mode rules ：

Rules have multiple goals , And different targets can automatically construct dependent files according to the name of the target file . And the dependent files must be similar, not identical .

4.5.1 grammar ：

TARGETS...:TARGET-PATTERN:PREREQ-PATTERN...

    COMMANDS
    ...

TARGETS Lists a series of target files for this rule .

TARGET-PATTERN and PREREQ-PATTERNS Describes how to generate dependency files for each target file . From target mode TARGET-PATTERN Extract a part of the string from the target name of （ be called stem ）, Use stems instead of dependency patterns PREREQ-PATTERNS To generate the dependent file corresponding to the target .

 

such as ：

objects=foo.o bar.o
all:$(objects)
$(objects):%o:%c
    $(CC) -c $(CFLAGS) $< -o [email protected]

In the rules %o:%c Describes all .o The dependent file of the file is the corresponding .c file .

To the goal foo.o Take its stem foo Instead of the corresponding dependency pattern %.c Mode characters in % Then you can get the dependent files of the target foo.c.

 

The above rule describes the following two specific rules ：

foo.o:foo.c
    $(CC) -c $(CFLAGS) foo.c -o foo.o
bar.o:bar.c
    $(CC) -c $(CFLAGS) bar.c -o bar.o

 

4.5.2 The difference between static patterns and implicit rules

• Implicit rules can be used in whatever On the target that matches it . When there are multiple implicit rules matching the target pattern , Execute only one of the rules , Which one to execute depends on the order in which the rules are defined .
• Static mode rules can only be used in rules clear The reconstruction process of those files pointed out . If there are two rules for a target , be make The execution will prompt an error .

 

4.6 Automatically generate dependencies

gcc adopt -M Option to automatically find the header file contained in the source file , And generate file dependencies .

When you do not need to consider standard library header files in dependencies , Use -MM Parameters .

 

such as ,main.c Only header files are included in def.h, be ：

gcc -M main.c
 Its output is ：
main.o:main.c def.h