In a programming project, it is easy to lose track of which files need to be reprocessed or recompiled after a change is made in some part of the source. Make provides a simple mechanism for maintaining up-to-date versions of programs that result from many operations on a number of files. It is possible to tell Make the sequence of commands that create certain files, and the list of files that require other files to be current before the operations can be done. Whenever a change is made in any part of the program, the Make command will create the proper files simply, correctly, and with a minimum amount of effort.
The basic operation of Make is to find the name of a needed target in the description, ensure that all of the files on which it depends exist and are up to date, and then create the target if it has not been modified since its generators were. The description file really defines the graph of dependencies; Make does a depth-first search of this graph to determine what work is really necessary.
Make also provides a simple macro substitution facility and the ability to encapsulate commands in a single file for convenient administration.
It is common practice to divide large programs into smaller, more manageable pieces. The pieces may require quite different treatments: some may need to be run through a macro processor, some may need to be processed by a sophisticated program generator (e.g., Yacc[1] or Lex[2]). The outputs of these generators may then have to be compiled with special options and with certain definitions and declarations. The code resulting from these transformations may then need to be loaded together with certain libraries under the control of special options. Related maintenance activities involve running complicated test scripts and installing validated modules. Unfortunately, it is very easy for a programmer to forget which files depend on which others, which files have been modified recently, and the exact sequence of operations needed to make or exercise a new version of the program. After a long editing session, one may easily lose track of which files have been changed and which object modules are still valid, since a change to a declaration can obsolete a dozen other files. Forgetting to compile a routine that has been changed or that uses changed declarations will result in a program that will not work, and a bug that can be very hard to track down. On the other hand, recompiling everything in sight just to be safe is very wasteful.
The program described in this report mechanizes many of the activities of program development and maintenance. If the information on inter-file dependences and command sequences is stored in a file, the simple command
Make is most useful for medium-sized programming projects; it does not solve the problems of maintaining multiple source versions or of describing huge programs. Make was designed for use on Unix, but a version runs on GCOS.
The basic operation of make is to update a target file by ensuring that all of the files on which it depends exist and are up to date, then creating the target if it has not been modified since its dependents were. Make does a depth-first search of the graph of dependences. The operation of the command depends on the ability to find the date and time that a file was last modified.
To illustrate, let us consider a simple example: A program named prog is made by compiling and loading three C-language files x.c, y.c, and z.c with the lS library. By convention, the output of the C compilations will be found in files named x.o, y.o, and z.o. Assume that the files x.c and y.c share some declarations in a file named defs, but that z.c does not. That is, x.c and y.c have the line
Make operates using three sources of information: a user-supplied description file (as above), file names and ``last-modified'' times from the file system, and built-in rules to bridge some of the gaps. In our example, the first line says that prog depends on three ``.o'' files. Once these object files are current, the second line describes how to load them to create prog. The third line says that x.o and y.o depend on the file defs. From the file system, make discovers that there are three ``.c'' files corresponding to the needed ``.o'' files, and uses built-in information on how to generate an object from a source file (i.e., issue a ``cc -c'' command).
The following long-winded description file is equivalent to the one above, but takes no advantage of make's innate knowledge:
If none of the source or object files had changed since the last time prog was made, all of the files would be current, and the command
If no target name is given on the make command line, the first target mentioned in the description is created; otherwise the specified targets are made. The command
If the file exists after the commands are executed, its time of last modification is used in further decisions; otherwise the current time is used. It is often quite useful to include rules with mnemonic names and commands that do not actually produce a file with that name. These entries can take advantage of make's ability to generate files and substitute macros. Thus, an entry ``save'' might be included to copy a certain set of files, or an entry ``cleanup'' might be used to throw away unneeded intermediate files. In other cases one may maintain a zero-length file purely to keep track of the time at which certain actions were performed. This technique is useful for maintaining remote archives and listings.
Make has a simple macro mechanism for substituting in dependency lines and command strings. Macros are defined by command arguments or description file lines with embedded equal signs. A macro is invoked by preceding the name by a dollar sign; macro names longer than one character must be parenthesized. The name of the macro is either the single character after the dollar sign or a name inside parentheses. The following are valid macro invocations:
The following sections detail the form of description files and the command line, and discuss options and built-in rules in more detail.
A description file contains three types of information: macro definitions, dependency information, and executable commands. There is also a comment convention: all characters after a sharp (#) are ignored, as is the sharp itself. Blank lines and lines beginning with a sharp are totally ignored. If a non-comment line is too long, it can be continued using a backslash. If the last character of a line is a backslash, the backslash, newline, and following blanks and tabs are replaced by a single blank.
A macro definition is a line containing an equal sign not preceded by a colon or a tab. The name (string of letters and digits) to the left of the equal sign (trailing blanks and tabs are stripped) is assigned the string of characters following the equal sign (leading blanks and tabs are stripped.) The following are valid macro definitions:
Other lines give information about target files. The general form of an entry is:
A dependency line may have either a single or a double colon. A target name may appear on more than one dependency line, but all of those lines must be of the same (single or double colon) type.
If a target must be created, the sequence of commands is executed. Normally, each command line is printed and then passed to a separate invocation of the Shell after substituting for macros. (The printing is suppressed in silent mode or if the command line begins with an @ sign). Make normally stops if any command signals an error by returning a non-zero error code. (Errors are ignored if the ``-i'' flags has been specified on the make command line, if the fake target name ``.IGNORE'' appears in the description file, or if the command string in the description file begins with a hyphen. Some UNIXcommands return meaningless status). Because each command line is passed to a separate invocation of the Shell, care must be taken with certain commands (e.g., cd and Shell control commands) that have meaning only within a single Shell process; the results are forgotten before the next line is executed.
Before issuing any command, certain macros are set. $@ is set to the name of the file to be ``made''. $? is set to the string of names that were found to be younger than the target. If the command was generated by an implicit rule (see below), $< is the name of the related file that caused the action, and $* is the prefix shared by the current and the dependent file names.
If a file must be made but there are no explicit commands or relevant built-in rules, the commands associated with the name ``.DEFAULT'' are used. If there is no such name, make prints a message and stops.
The make command takes four kinds of arguments: macro definitions, flags, description file names, and target file names.
First, all macro definition arguments (arguments with embedded equal signs) are analyzed and the assignments made. Command-line macros override corresponding definitions found in the description files.
Next, the flag arguments are examined. The permissible flags are
Finally, the remaining arguments are assumed to be the names of targets to be made; they are done in left to right order. If there are no such arguments, the first name in the description files that does not begin with a period is ``made''.
The make program uses a table of interesting suffixes and a set of transformation rules to supply default dependency information and implied commands. (The Appendix describes these tables and means of overriding them.) The default suffix list is:
.o Object file
.c C source file
.e Efl source file
.r Ratfor source file
.f Fortran source file
.s Assembler source file
.y Yacc-C source grammar
.yr Yacc-Ratfor source grammar
.ye Yacc-Efl source grammar
.l Lex source grammar
The following diagram summarizes the default transformation paths. If there are two paths connecting a pair of suffixes, the longer one is used only if the intermediate file exists or is named in the description.
.o
.c .r .e .f .s .y .yr .ye .l .d
.y .l .yr .ye
If the file x.o were needed and there were an x.c in the description or directory, it would be compiled. If there were also an x.l, that grammar would be run through Lex before compiling the result. However, if there were no x.c but there were an x.l, make would discard the intermediate C-language file and use the direct link in the graph above.
It is possible to change the names of some of the compilers used in the default, or the flag arguments with which they are invoked by knowing the macro names used. The compiler names are the macros AS, CC, RC, EC, YACC, YACCR, YACCE, and LEX. The command
As an example of the use of make, we will present the description file used to maintain the make command itself. The code for make is spread over a number of C source files and a Yacc grammar. The description file contains:
The last few entries in the description file are useful maintenance sequences. The ``print'' entry prints only the files that have been changed since the last ``make print'' command. A zero-length file print is maintained to keep track of the time of the printing; the $? macro in the command line then picks up only the names of the files changed since print was touched. The printed output can be sent to a different printer or to a file by changing the definition of the P macro:
The most common difficulties arise from make's specific meaning of dependency. If file x.c has a ``#include "defs"'' line, then the object file x.o depends on defs; the source file x.c does not. (If defs is changed, it is not necessary to do anything to the file x.c, while it is necessary to recreate x.o.)
To discover what make would do, the ``-n'' option is very useful. The command
The debugging flag (``-d'') causes make to print out a very detailed description of what it is doing, including the file times. The output is verbose, and recommended only as a last resort.
I would like to thank S. C. Johnson for suggesting this approach to program maintenance control. I would like to thank S. C. Johnson and H. Gajewska for being the prime guinea pigs during development of make.
The make program itself does not know what file name suffixes are interesting or how to transform a file with one suffix into a file with another suffix. This information is stored in an internal table that has the form of a description file. If the ``-r'' flag is used, this table is not used.
The list of suffixes is actually the dependency list for the name ``.SUFFIXES''; make looks for a file with any of the suffixes on the list. If such a file exists, and if there is a transformation rule for that combination, make acts as described earlier. The transformation rule names are the concatenation of the two suffixes. The name of the rule to transform a ``.r'' file to a ``.o'' file is thus ``.r.o''. If the rule is present and no explicit command sequence has been given in the user's description files, the command sequence for the rule ``.r.o'' is used. If a command is generated by using one of these suffixing rules, the macro $* is given the value of the stem (everything but the suffix) of the name of the file to be made, and the macro $< is the name of the dependent that caused the action.
The order of the suffix list is significant, since it is scanned from left to right, and the first name that is formed that has both a file and a rule associated with it is used. If new names are to be appended, the user can just add an entry for ``.SUFFIXES'' in his own description file; the dependents will be added to the usual list. A ``.SUFFIXES'' line without any dependents deletes the current list. (It is necessary to clear the current list if the order of names is to be changed).
The following is an excerpt from the default rules file: