|GNU Make Manual||www.imodulo.com · 2003-04-05|
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After each shell command returns,
make looks at its exit status. If the command completed successfully, the next command line is executed in a new shell; after the last command line is finished, the rule is finished.
If there is an error (the exit status is nonzero),
make gives up on the current rule, and perhaps on all rules.
Sometimes the failure of a certain command does not indicate a problem. For example, you may use the
mkdir command to ensure that a directory exists. If the directory already exists,
mkdir will report an error, but you probably want
make to continue regardless.
To ignore errors in a command line, write a
- at the beginning of the line's text (after the initial tab). The
- is discarded before the command is passed to the shell for execution.
clean: -rm -f *.o
rm to continue even if it is unable to remove a file.
When you run
make with the
-ignore-errors flag, errors are ignored in all commands of all rules. A rule in the makefile for the special target
.IGNORE has the same effect, if there are no prerequisites. These ways of ignoring errors are obsolete because
- is more flexible.
When errors are to be ignored, because of either a
- or the
make treats an error return just like success, except that it prints out a message that tells you the status code the command exited with, and says that the error has been ignored.
When an error happens that
make has not been told to ignore, it implies that the current target cannot be correctly remade, and neither can any other that depends on it either directly or indirectly. No further commands will be executed for these targets, since their preconditions have not been achieved.
make gives up immediately in this circumstance, returning a nonzero status. However, if the
-keep-going flag is specified,
make continues to consider the other prerequisites of the pending targets, remaking them if necessary, before it gives up and returns nonzero status. For example, after an error in compiling one object file,
make -k will continue compiling other object files even though it already knows that linking them will be impossible. Summary of Options.
The usual behavior assumes that your purpose is to get the specified targets up to date; once
make learns that this is impossible, it might as well report the failure immediately. The
-k option says that the real purpose is to test as many of the changes made in the program as possible, perhaps to find several independent problems so that you can correct them all before the next attempt to compile. This is why Emacs'
compile command passes the
-k flag by default. Usually when a command fails, if it has changed the target file at all, the file is corrupted and cannot be used--or at least it is not completely updated. Yet the file's time stamp says that it is now up to date, so the next time
make runs, it will not try to update that file. The situation is just the same as when the command is killed by a signal; Interrupts. So generally the right thing to do is to delete the target file if the command fails after beginning to change the file.
make will do this if
.DELETE_ON_ERROR appears as a target. This is almost always what you want
make to do, but it is not historical practice; so for compatibility, you must explicitly request it.
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