Wikipedia:Reference desk/Archives/Computing/2020 December 14

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December 14

difference of files

Is there a Unix command that can compare two sorted files and output only those lines that are in file A and not in file B? (If not, I can easily write a Python program to do it.) —Tamfang (talk) 02:10, 14 December 2020 (UTC)

Do you want to see lines from B that are not in A? RudolfRed (talk) 03:44, 14 December 2020 (UTC)

You can use the comm command: comm -23 A B Graeme Bartlett (talk) 04:17, 14 December 2020 (UTC)

If the lines in file A are not unique, you may still see lines that are in file B but with a lower multiplicity. It is a bit like bag subtraction. If the multiplicity of a line in file A equals m_A and that in file B is m_B, then the number in the output is equal to 0 if m_A ≤ m_B, and is m_A − m_B otherwise (truncated subtraction).  --Lambiam 12:22, 14 December 2020 (UTC)

Good point. If that's an issue and you really want lines in A that are not in B at all, you can strip out the duplicate lines using uniq (or by using sort -u in the first place) before comparing the files. But then you won't know how many times each line that is not in B occurs in A. You may prefer to use fgrep -v -x -f B A (also known as grep -F -v -x -f B A). However, this method does not take advantage of the files being sorted, so it needs to hold the whole of file B in memory while it runs through file A, which may be problematic if B is a large file. --174.95.161.129 (talk) 21:43, 14 December 2020 (UTC)

I am not sure that is a valid concern anymore, with mmap(2). Elizium23 (talk) 21:49, 14 December 2020 (UTC)

"May" be. --174.95.161.129 (talk) 22:20, 14 December 2020 (UTC)

Thank you, comm did the trick – once I made sure the files were sorted the same way! —Tamfang (talk) 03:11, 15 December 2020 (UTC)

how to set make file to compile by c11 standards?

I'm trying to compile my file with the following makefile:

obj-m := message_slot.o KDIR := /lib/modules/$(shell uname -r)/build PWD := $(shell pwd) CFLAGS=-O3 -std=c++0x -pg -D_DEBUG -g -c -Wall

all: $(MAKE) -C $(KDIR) M=$(PWD) modules

clean: $(MAKE) -C $(KDIR) M=$(PWD) clean

But I keep this getting this error:

/home/eran/CLionProjects/tau_os_ex3/message_slot.c:29:5: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement] How can I set the standard to c11 for good? --Exx8 (talk) 22:57, 14 December 2020 (UTC)

@Exx8: According to [1] use -std=c11. RudolfRed (talk) 01:09, 15 December 2020 (UTC)