NAME¶

std::basic_filebuf::seekpos - std::basic_filebuf::seekpos

Synopsis¶

protected:

virtual pos_type seekpos( pos_type sp,

std::ios_base::openmode which = std::ios_base::in |
std::ios_base::out );

Repositions the file pointer, if possible, to the position indicated by sp. If the
associated file is not open (is_open() == false), fails immediately.

Reposition performs as follows:

1) If the file is open for writing, writes the put area and any unshift sequences
required by the currently imbued locale, using overflow().
2) Repositions the file pointer, as if by calling std::fsetpos().
3) If the file is open for reading, updates the get area if necessary.

If sp was not obtained by calling seekoff() or seekpos() on the same file, the
behavior is undefined.

Parameters¶

sp - file position obtained by seekoff() or seekpos() called earlier on the same
file
defines which of the input and/or output sequences to affect. It can be one
or a combination of the following constants:
which -
Constant Explanation
in affect the input sequence
out affect the output sequence

Return value¶

sp on success or pos_type(off_type(-1)) on failure.

Notes¶

seekpos() is called by std::basic_streambuf::pubseekpos(), which is called by the
single-argument versions of std::basic_istream::seekg() and
std::basic_ostream::seekp().

Many implementations do not update the get area in seekpos(), delegating to
underflow() that is called by the next sgetc().

Example¶

On some implementations, the get area is emptied by seekpos() and the second
underflow() is necessary to observe the effects.

// Run this code

#include <fstream>
#include <iostream>

struct mybuf : std::filebuf
{
pos_type seekpos(pos_type sp, std::ios_base::openmode which)
{
std::cout << "Before seekpos(" << sp << "), size of the get area is "
<< egptr() - eback() << " with "
<< egptr() - gptr() << " read positions available.\n";

pos_type rc = std::filebuf::seekpos(sp, which);

std::cout << "seekpos() returns " << rc << ".\nAfter the call, "
<< "size of the get area is "
<< egptr() - eback() << " with "
<< egptr() - gptr() << " read positions available.\n";
// uncomment if get area is emptied by seekpos()
// std::filebuf::underflow();
// std::cout << "after forced underflow(), size of the get area is "
// << egptr() - eback() << " with "
// << egptr() - gptr() << " read positions available.\n";

return rc;
}
};

int main()
{
mybuf buf;
buf.open("test.txt", std::ios_base::in);
std::istream stream(&buf);
stream.get(); // read one char to force underflow()
stream.seekg(2);
}

Possible output:¶

Before seekpos(2), size of the get area is 110 with 109 read positions available.
seekpos() returns 2.
After the call, size of the get area is 110 with 108 read positions available.

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 55 C++98 seekpos returned an undefined pos_type(off_type(-1))
invalid stream position on failure is returned on failure
LWG 171 C++98 the sequence of the operations of made clear
reposition was not clear

See also¶

pubseekpos invokes seekpos()
(public member function of std::basic_streambuf<CharT,Traits>)
seekoff repositions the file position, using relative addressing
[virtual] (virtual protected member function)
fseek moves the file position indicator to a specific location in a file
(function)