In a symlink pointing to '127.0.1.1:+xxxxx', what is the plus character for?

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On my Linux filesystem, a symlinks points to 127.0.1.1:+xxxxx.



Why the plus sign? Could there also be a minus? Why not just 127.0.1.1:xxxxx?






linux networking symbolic-link ip-address







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  • Is this symbolic link under /proc?
    – Joshua
    5 hours ago










  • @Joshua: More likely under ~/.mozilla/ or similar. Firefox/Thunderbird, and iirc Steam, use such symlinks for locking.
    – grawity
    4 mins ago














up vote
13
down vote

favorite
3












On my Linux filesystem, a symlinks points to 127.0.1.1:+xxxxx.



Why the plus sign? Could there also be a minus? Why not just 127.0.1.1:xxxxx?






linux networking symbolic-link ip-address







share|improve this question









New contributor




myMethod is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.



















  • Is this symbolic link under /proc?
    – Joshua
    5 hours ago










  • @Joshua: More likely under ~/.mozilla/ or similar. Firefox/Thunderbird, and iirc Steam, use such symlinks for locking.
    – grawity
    4 mins ago












up vote
13
down vote

favorite
3









up vote
13
down vote

favorite
3






3





On my Linux filesystem, a symlinks points to 127.0.1.1:+xxxxx.



Why the plus sign? Could there also be a minus? Why not just 127.0.1.1:xxxxx?






linux networking symbolic-link ip-address







share|improve this question









New contributor




myMethod is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











On my Linux filesystem, a symlinks points to 127.0.1.1:+xxxxx.



Why the plus sign? Could there also be a minus? Why not just 127.0.1.1:xxxxx?






linux networking symbolic-link ip-address




linux networking symbolic-link ip-address






share|improve this question









New contributor




myMethod is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











share|improve this question









New contributor




myMethod is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









share|improve this question




share|improve this question








edited 18 mins ago









muru

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asked 9 hours ago









myMethod

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myMethod is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.





New contributor





myMethod is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






myMethod is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











  • Is this symbolic link under /proc?
    – Joshua
    5 hours ago










  • @Joshua: More likely under ~/.mozilla/ or similar. Firefox/Thunderbird, and iirc Steam, use such symlinks for locking.
    – grawity
    4 mins ago
















  • Is this symbolic link under /proc?
    – Joshua
    5 hours ago










  • @Joshua: More likely under ~/.mozilla/ or similar. Firefox/Thunderbird, and iirc Steam, use such symlinks for locking.
    – grawity
    4 mins ago















Is this symbolic link under /proc?
– Joshua
5 hours ago




Is this symbolic link under /proc?
– Joshua
5 hours ago












@Joshua: More likely under ~/.mozilla/ or similar. Firefox/Thunderbird, and iirc Steam, use such symlinks for locking.
– grawity
4 mins ago




@Joshua: More likely under ~/.mozilla/ or similar. Firefox/Thunderbird, and iirc Steam, use such symlinks for locking.
– grawity
4 mins ago










2 Answers
2






active

oldest

votes

















up vote
17
down vote



accepted










Symbolic links which don't point to a file have no generic meaning at all. In this case it might be the process ID, or a port with some special protocol spoken over it, or another identifier. It all depends on what program made it.



Software which creates these links simply takes advantage of the facts that 1) a symlink's target may be non-existent or even total nonsense; 2) creating a symlink is a single-syscall completely atomic operation (as is reading its target), unlike creating a regular file which takes at least 3 separate system calls.



Thus symlink creation can be abused as a way of locking (ensuring single instance of a program) even when other mechanisms may be unreliable. The program doesn't need the symlink to actually resolve to a real file: it only cares about whether creating the link succeeds, or whether it fails due to it already existing.






share|improve this answer
















  • 1




    Thank you for the detailed answer.
    – myMethod
    8 hours ago

















up vote
9
down vote













As far as I know the "+" means that the number after the IP (the "xxxxx") refers to a "process ID" (not a port which usually uses the notation [IP-address]:[portnumber]).






share|improve this answer




















  • I just checked it. In my case it really does.
    – myMethod
    8 hours ago






  • 1




    @myMethod glad to help, feel free to "accept" my answer, if it answered you're question.
    – Albin
    8 hours ago











  • Is there documentation of this format somewhere that you could link to?
    – David Z
    7 hours ago










  • Not sure, I just remember it from hands on experience.
    – Albin
    7 hours ago










  • It sounds like you're talking about a symlink in a special directory, like /proc or /dev. Can you explain what this symlink is used for?
    – Barmar
    2 hours ago










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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes








up vote
17
down vote



accepted










Symbolic links which don't point to a file have no generic meaning at all. In this case it might be the process ID, or a port with some special protocol spoken over it, or another identifier. It all depends on what program made it.



Software which creates these links simply takes advantage of the facts that 1) a symlink's target may be non-existent or even total nonsense; 2) creating a symlink is a single-syscall completely atomic operation (as is reading its target), unlike creating a regular file which takes at least 3 separate system calls.



Thus symlink creation can be abused as a way of locking (ensuring single instance of a program) even when other mechanisms may be unreliable. The program doesn't need the symlink to actually resolve to a real file: it only cares about whether creating the link succeeds, or whether it fails due to it already existing.






share|improve this answer
















  • 1




    Thank you for the detailed answer.
    – myMethod
    8 hours ago














up vote
17
down vote



accepted










Symbolic links which don't point to a file have no generic meaning at all. In this case it might be the process ID, or a port with some special protocol spoken over it, or another identifier. It all depends on what program made it.



Software which creates these links simply takes advantage of the facts that 1) a symlink's target may be non-existent or even total nonsense; 2) creating a symlink is a single-syscall completely atomic operation (as is reading its target), unlike creating a regular file which takes at least 3 separate system calls.



Thus symlink creation can be abused as a way of locking (ensuring single instance of a program) even when other mechanisms may be unreliable. The program doesn't need the symlink to actually resolve to a real file: it only cares about whether creating the link succeeds, or whether it fails due to it already existing.






share|improve this answer
















  • 1




    Thank you for the detailed answer.
    – myMethod
    8 hours ago












up vote
17
down vote



accepted







up vote
17
down vote



accepted






Symbolic links which don't point to a file have no generic meaning at all. In this case it might be the process ID, or a port with some special protocol spoken over it, or another identifier. It all depends on what program made it.



Software which creates these links simply takes advantage of the facts that 1) a symlink's target may be non-existent or even total nonsense; 2) creating a symlink is a single-syscall completely atomic operation (as is reading its target), unlike creating a regular file which takes at least 3 separate system calls.



Thus symlink creation can be abused as a way of locking (ensuring single instance of a program) even when other mechanisms may be unreliable. The program doesn't need the symlink to actually resolve to a real file: it only cares about whether creating the link succeeds, or whether it fails due to it already existing.






share|improve this answer












Symbolic links which don't point to a file have no generic meaning at all. In this case it might be the process ID, or a port with some special protocol spoken over it, or another identifier. It all depends on what program made it.



Software which creates these links simply takes advantage of the facts that 1) a symlink's target may be non-existent or even total nonsense; 2) creating a symlink is a single-syscall completely atomic operation (as is reading its target), unlike creating a regular file which takes at least 3 separate system calls.



Thus symlink creation can be abused as a way of locking (ensuring single instance of a program) even when other mechanisms may be unreliable. The program doesn't need the symlink to actually resolve to a real file: it only cares about whether creating the link succeeds, or whether it fails due to it already existing.







share|improve this answer












share|improve this answer



share|improve this answer










answered 8 hours ago









grawity

220k32449514




220k32449514







  • 1




    Thank you for the detailed answer.
    – myMethod
    8 hours ago












  • 1




    Thank you for the detailed answer.
    – myMethod
    8 hours ago







1




1




Thank you for the detailed answer.
– myMethod
8 hours ago




Thank you for the detailed answer.
– myMethod
8 hours ago












up vote
9
down vote













As far as I know the "+" means that the number after the IP (the "xxxxx") refers to a "process ID" (not a port which usually uses the notation [IP-address]:[portnumber]).






share|improve this answer




















  • I just checked it. In my case it really does.
    – myMethod
    8 hours ago






  • 1




    @myMethod glad to help, feel free to "accept" my answer, if it answered you're question.
    – Albin
    8 hours ago











  • Is there documentation of this format somewhere that you could link to?
    – David Z
    7 hours ago










  • Not sure, I just remember it from hands on experience.
    – Albin
    7 hours ago










  • It sounds like you're talking about a symlink in a special directory, like /proc or /dev. Can you explain what this symlink is used for?
    – Barmar
    2 hours ago














up vote
9
down vote













As far as I know the "+" means that the number after the IP (the "xxxxx") refers to a "process ID" (not a port which usually uses the notation [IP-address]:[portnumber]).






share|improve this answer




















  • I just checked it. In my case it really does.
    – myMethod
    8 hours ago






  • 1




    @myMethod glad to help, feel free to "accept" my answer, if it answered you're question.
    – Albin
    8 hours ago











  • Is there documentation of this format somewhere that you could link to?
    – David Z
    7 hours ago










  • Not sure, I just remember it from hands on experience.
    – Albin
    7 hours ago










  • It sounds like you're talking about a symlink in a special directory, like /proc or /dev. Can you explain what this symlink is used for?
    – Barmar
    2 hours ago












up vote
9
down vote










up vote
9
down vote









As far as I know the "+" means that the number after the IP (the "xxxxx") refers to a "process ID" (not a port which usually uses the notation [IP-address]:[portnumber]).






share|improve this answer












As far as I know the "+" means that the number after the IP (the "xxxxx") refers to a "process ID" (not a port which usually uses the notation [IP-address]:[portnumber]).







share|improve this answer












share|improve this answer



share|improve this answer










answered 8 hours ago









Albin

1,250621




1,250621











  • I just checked it. In my case it really does.
    – myMethod
    8 hours ago






  • 1




    @myMethod glad to help, feel free to "accept" my answer, if it answered you're question.
    – Albin
    8 hours ago











  • Is there documentation of this format somewhere that you could link to?
    – David Z
    7 hours ago










  • Not sure, I just remember it from hands on experience.
    – Albin
    7 hours ago










  • It sounds like you're talking about a symlink in a special directory, like /proc or /dev. Can you explain what this symlink is used for?
    – Barmar
    2 hours ago
















  • I just checked it. In my case it really does.
    – myMethod
    8 hours ago






  • 1




    @myMethod glad to help, feel free to "accept" my answer, if it answered you're question.
    – Albin
    8 hours ago











  • Is there documentation of this format somewhere that you could link to?
    – David Z
    7 hours ago










  • Not sure, I just remember it from hands on experience.
    – Albin
    7 hours ago










  • It sounds like you're talking about a symlink in a special directory, like /proc or /dev. Can you explain what this symlink is used for?
    – Barmar
    2 hours ago















I just checked it. In my case it really does.
– myMethod
8 hours ago




I just checked it. In my case it really does.
– myMethod
8 hours ago




1




1




@myMethod glad to help, feel free to "accept" my answer, if it answered you're question.
– Albin
8 hours ago





@myMethod glad to help, feel free to "accept" my answer, if it answered you're question.
– Albin
8 hours ago













Is there documentation of this format somewhere that you could link to?
– David Z
7 hours ago




Is there documentation of this format somewhere that you could link to?
– David Z
7 hours ago












Not sure, I just remember it from hands on experience.
– Albin
7 hours ago




Not sure, I just remember it from hands on experience.
– Albin
7 hours ago












It sounds like you're talking about a symlink in a special directory, like /proc or /dev. Can you explain what this symlink is used for?
– Barmar
2 hours ago




It sounds like you're talking about a symlink in a special directory, like /proc or /dev. Can you explain what this symlink is used for?
– Barmar
2 hours ago










myMethod is a new contributor. Be nice, and check out our Code of Conduct.









 

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