Interpretations

Answering questions that may arise related to the meaning of portions of an IEEE standard concerning specific applications.

IEEE Standards Interpretation for IEEE Std 1003.1™-1990 IEEE Standard for Information Technology--Portable Operating System Interfaces (POSIX®)

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Interpretation Request #30
Topic: Max # of directory levels Relevant Sections: 2.3.6

I wish for an interpretation of ISO/IEC 9945-1 (IEEE Std 1003.1) regarding the maximum number of directory levels that a conforming implementation must support. The value {PATH_MAX} defines the maximum number of bytes in a pathname. The error [ENAMETOOLONG] is returned when the length of the path or file arguments exceeds {PATH_MAX}. All other references to {PATH_MAX} are to arguments or the result of the pathconf() function.

All the arguments that are checked against {PATH_MAX} go thru pathname resolution (2.3.6). However, the current working directory mechanism can be used to effectively by-pass any limits imposed by {PATH_MAX}. If the pathname is too long, change the current working directory to some directory on the pathname, delete up to and including that directory, and try again. {PATH_MAX} only controls how many directories can be described at any one call.

If this is the correct interpretation, it would prevent a conforming implementation on file systems that associates each file with an absolute pathname (with filename) stored in a limited size array. Such a file system is used on A Series from Unisys, and I suspect on many other operating systems. I hope that instead the correct interpretation is that the maximum number of directory levels that a conforming implementation must support is limited by {PATH_MAX} when expressed as an absolute pathname. Note that this would not restrict what an implementation may support, only what it must support.

Interpretation Response
No, {PATH_MAX} was not intended to constrain actual absolute pathnames. {PATH_MAX} specifies the length of the string that can be passed to any of a number of POSIX.1 interfaces as an argument that represents a pathname. IEEE Std 1003.1-1990 does not state an upper limit or a lower limit for the number of levels in directory hierarchies that must be supported.

Since {PATH_MAX} is not the maximum length of an absolute path, the absolute path for a file can be much longer than {PATH_MAX} as long as the pathname used to refer to the file is not longer than {PATH_MAX}.

Rationale for Interpretation
None.