On GNU systems, the file position is truly a character count. You
can specify any character count value as an argument to fseek or
fseeko and get reliable results for any random access file.
However, some ISO C systems do not represent file positions in this
way.
On some systems where text streams truly differ from binary streams, it is impossible to represent the file position of a text stream as a count of characters from the beginning of the file. For example, the file position on some systems must encode both a record offset within the file, and a character offset within the record.
As a consequence, if you want your programs to be portable to these systems, you must observe certain rules:
ftell on a text stream has no predictable
relationship to the number of characters you have read so far. The only
thing you can rely on is that you can use it subsequently as the
offset argument to fseek or fseeko to move back to
the same file position.
fseek or fseeko on a text stream, either the
offset must be zero, or whence must be SEEK_SET and
the offset must be the result of an earlier call to ftell
on the same stream.
ungetc
that haven't been read or discarded. See Unreading.
But even if you observe these rules, you may still have trouble for long
files, because ftell and fseek use a long int value
to represent the file position. This type may not have room to encode
all the file positions in a large file. Using the ftello and
fseeko functions might help here since the off_t type is
expected to be able to hold all file position values but this still does
not help to handle additional information which must be associated with
a file position.
So if you do want to support systems with peculiar encodings for the
file positions, it is better to use the functions fgetpos and
fsetpos instead. These functions represent the file position
using the data type fpos_t, whose internal representation varies
from system to system.
These symbols are declared in the header file stdio.h.
This is the type of an object that can encode information about the file position of a stream, for use by the functions
fgetposandfsetpos.In the GNU C Library,
fpos_tis an opaque data structure that contains internal data to represent file offset and conversion state information. In other systems, it might have a different internal representation.When compiling with
_FILE_OFFSET_BITS == 64on a 32 bit machine this type is in fact equivalent tofpos64_tsince the LFS interface transparently replaces the old interface.
This is the type of an object that can encode information about the file position of a stream, for use by the functions
fgetpos64andfsetpos64.In the GNU C Library,
fpos64_tis an opaque data structure that contains internal data to represent file offset and conversion state information. In other systems, it might have a different internal representation.
This function stores the value of the file position indicator for the stream stream in the
fpos_tobject pointed to by position. If successful,fgetposreturns zero; otherwise it returns a nonzero value and stores an implementation-defined positive value inerrno.When the sources are compiled with
_FILE_OFFSET_BITS == 64on a 32 bit system the function is in factfgetpos64. I.e., the LFS interface transparently replaces the old interface.
This function is similar to
fgetposbut the file position is returned in a variable of typefpos64_tto which position points.If the sources are compiled with
_FILE_OFFSET_BITS == 64on a 32 bits machine this function is available under the namefgetposand so transparently replaces the old interface.
This function sets the file position indicator for the stream stream to the position position, which must have been set by a previous call to
fgetposon the same stream. If successful,fsetposclears the end-of-file indicator on the stream, discards any characters that were “pushed back” by the use ofungetc, and returns a value of zero. Otherwise,fsetposreturns a nonzero value and stores an implementation-defined positive value inerrno.When the sources are compiled with
_FILE_OFFSET_BITS == 64on a 32 bit system the function is in factfsetpos64. I.e., the LFS interface transparently replaces the old interface.
This function is similar to
fsetposbut the file position used for positioning is provided in a variable of typefpos64_tto which position points.If the sources are compiled with
_FILE_OFFSET_BITS == 64on a 32 bits machine this function is available under the namefsetposand so transparently replaces the old interface.