Q23753: fseek() Function Requires a long for the Offset into a File

Article: Q23753
Product(s): See article
Version(s): 3.00 4.00 5.00 5.10 6.00 6.00a | 5.10 6.00 6.00a
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 6-FEB-1991

Question:

I am using fseek() to seek in a file, but it fails when I try to seek
beyond 32K. Is this a limit of fseek()?

Response:

The offset parameter to fseek() takes a value of type long, so this
problem is not likely to be the result of fseek() limits. On the other
hand, this situation does occur if you are using variables of type int
to calculate the offset. Because an int is limited to a maximum value
of 32,767 (32K), values assigned to ints between 32K and 64K are
treated as negative.

An int will be automatically typecast to a long when passed to fseek()
but when this occurs, it is sign-extended. Thus, an int value above
32K that is represented as negative will be sign-extended to a
negative long. This, in turn, specifies a negative offset into the
file, which fails to produce the expected results.

When using the fseek() routine, just be sure to use a long for
specifying the offset into the file.

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