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Last update: 1997-05-20
9945-1-90 #37
Classification: No change.
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Topic: off_t error messages
Relevant Sections: not specified
Defect Report:
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3. off_t has a finite upper bound. No error conditions are specifically
identified for functions that attempt to exceed the inherent limit of off_t.
Take for example lseek(fildes, 2**31-2,SEEK_SET);write(filedes,'abcd',4).
Do any characters get written? Page 119, lines 204-205 imply 2 bytes would get
written and subsequent call would get EFBIG. When more than one binding
is supported, is EFBIG set to a size that all binds on the implementation
can cope with? What is returned from lseek() and fcntl() when the resulting
offset exceeds the size of off_t? I assume, EINVAL.
What must be documented in the Conformance Document when underlying file
file systems and other bindings permit different limits than the 'C' binds?
(Paul Wanish IBM)
WG15 response for ISO/IEC 9945-1:1990 (IEEE Std 1003.1-1990)
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There is an error for lseek() that applies to this
situation:
[EINVAL] The whence argument is not a proper value,
or the resulting file offset would be invalid.
The result of attempting a write() that would cause the file offset
to exceed the maximum value that can be stored in an object of type
off_t is unspecified. Although ISO/IEC 9945-1:1990 states (page
119, lines 195-196) that:
Before successful return from write(), the file offset shall
be incremented by the number of bytes actually written.
there is no error specified for the case where advancing the file
pointer would produce an offset with no well-defined value. Since
write() does not return this offset and need not examine it in
this case, there is no requirement that an error condition be
detected. This also applies to the interfaces from the
C Standard that can extend the size of a file (fwrite(), fprintf(),
etc.).
The description of the [EINVAL] error condition for fcntl() with
the F_GETLK, F_SETLK, or F_SETLKW flag refers only to invalid
data in the structure passed to fcntl(). The only way to set a
lock on the portion of a file beyond the size that can be
represented in type off_t is to set l_len to 0 to lock to the
end of the file, and that is the only way that information that
refers to that portion of the file can be returned by fcntl().
Issues related to harmonizing semantics with standards other than
the C Standard are beyond the scope of ISO/IEC 9945-1:1990
There is no requirement in ISO/IEC 9945-1 that a mismatch in
the ability to handle file sizes between POSIX.1 and the C
Standard be documented. Note that the C Standard provides
interfaces to be used in manipulating the file offsets for
very large files (fgetpos(), fsetpos()).
Rationale for Interpretation:
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POSIX.1 does not specify a specific relationship among the maximum
file size, {SSIZE_MAX}, the maximum value that can be stored in an
object of type off_t, and the storage capacity of a particular
medium or filesystem. Page 119, lines 204-205 refer to the case
where there is no more room for data, which is not necessarily the
same as the case where a write would cause the offset of the file
pointer to exceed the maximum value that can be stored in an object
of type off_t.
An application that needs to use file offsets that are larger
than can be represented in type off_t should, if possible,
use the fgetpos() and fsetpos() interfaces from the C Standard
rather than using lseek().
It is suggested that a future revision of ISO/IEC 9945-1
specify the behavior of fcntl() when used on files whose sizes
cannot be represented in variables of type off_t. This
condition can arise when file systems are mounted from a remote
POSIX.1 system on which off_t is a larger type than on the local
system.
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