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 #38
Topic:
errors and fstat with file off_t Relevant Sections: 5.3.1.4 Classification: No change

Section 5.3.1.4 open() Errors - Assume off_t can take the values between -7 and 7. Assume also that an Ada binding has created a file 8 bytes big. Shall open() work or fail? If fail, with what error code? If work, what does fstat() return? (Paul Wanish IBM)

Interpretation Response
IEEE Std 1003.1-1990 does not specify error conditions for open() or for fstat() for the case where the file size cannot be represented in an object of type off_t. A conforming implementation could define such error conditions.

Unless such an error condition is defined by the implementation, open() must succeed in this example. The value in the st_size field of the structure returned by fstat() on the file descriptor returned by this open() is unspecified for the case where the actual value is larger than can be stored in an object of type off_t.

A call to fstat() must fail if it is impossible to represent the file size in a variable of type off_t. When this occurs, erreno must be set to a non-zero value that is not one of the values specified for fstat() by POSIX.1.

Rationale for Interpretation
An implementation of fstat() might express the failure to set the st_size field by setting the values of the fields other than st_size in the structure pointed to by the buf argument to the proper values. fstat() could return -1 with errno set to a value defined by the implementation, and documented to mean that the file size exceeds the size that can be stored in type off_t. Such an implementation of fstat() could even store the true file size in an implementation-defined member of the stat structure, of some appropriate type.

It is suggested that a future revision of IEEE Std 1003.1 specify the behavior of stat() and fstat() 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.