Roland Ver. 4.5 Informações Técnicas descarregar pdf (Página 57)

Chapter 5: Writing Recipes in Rules 47

The eﬀect of the above DOS-speciﬁc processing is that a Makeﬁle that contains ‘SHELL

= /bin/sh’ (as many Unix makeﬁles do), will work on MS-DOS unaltered if you have e.g.

sh.exe installed in some directory along your PATH.

5.4 Parallel Execution

GNU make knows how to execute several recipes at once. Normally, make will execute

only one recipe at a time, waiting for it to ﬁnish before executing the next. However, the

‘-j’ or ‘--jobs’ option tells make to execute many recipes simultaneously. You can inhibit

parallelism in a particular makeﬁle with the .NOTPARALLEL pseudo-target (see Section 4.8

[Special Targets], page 32).

On MS-DOS, the ‘-j’ option has no eﬀect, since that system doesn’t support multi-

processing.

If the ‘-j’ option is followed by an integer, this is the number of recipes to execute at

once; this is called the number of job slots. If there is nothing looking like an integer after

the ‘-j’ option, there is no limit on the number of job slots. The default number of job

slots is one, which means serial execution (one thing at a time).

Handling recursive make invocations raises issues for parallel execution. For more infor-

mation on this, see Section 5.7.3 [Communicating Options to a Sub-make], page 54.

If a recipe fails (is killed by a signal or exits with a nonzero status), and errors are

not ignored for that recipe (see Section 5.5 [Errors in Recipes], page 49), the remaining

recipe lines to remake the same target will not be run. If a recipe fails and the ‘-k’ or

‘--keep-going’ option was not given (see Section 9.7 [Summary of Options], page 104),

make aborts execution. If make terminates for any reason (including a signal) with child

processes running, it waits for them to ﬁnish before actually exiting.

When the system is heavily loaded, you will probably want to run fewer jobs than when

it is lightly loaded. You can use the ‘-l’ option to tell make to limit the number of jobs to

run at once, based on the load average. The ‘-l’ or ‘--max-load’ option is followed by a

ﬂoating-point number. For example,

-l 2.5

will not let make start more than one job if the load average is above 2.5. The ‘-l’ option

with no following number removes the load limit, if one was given with a previous ‘-l’

option.

More precisely, when make goes to start up a job, and it already has at least one job

running, it checks the current load average; if it is not lower than the limit given with ‘-l’,

make waits until the load average goes below that limit, or until all the other jobs ﬁnish.

By default, there is no load limit.

5.4.1 Output During Parallel Execution

When running several recipes in parallel the output from each recipe appears as soon as

it is generated, with the result that messages from diﬀerent recipes may be interspersed,

sometimes even appearing on the same line. This can make reading the output very diﬃcult.

To avoid this you can use the ‘--output-sync’ (‘-O’) option. This option instructs make

to save the output from the commands it invokes and print it all once the commands are

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