std::basic_filebuf::seekpos

 

C++

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Technical Specifications

 

Input/output library

I/O manipulators

C-style I/O

Buffers

basic_streambuf

basic_filebuf

basic_stringbuf

strstreambuf (deprecated in C++98)

basic_syncbuf (C++20)

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ios_base

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ostrstream (deprecated in C++98)

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basic_osyncstream (C++20)

Types

streamoff

streamsize

fpos

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iostream_category (C++11)

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std::basic_filebuf

Public member functions

basic_filebuf::basic_filebuf

basic_filebuf::~basic_filebuf

basic_filebuf::operator= (C++11)

basic_filebuf::swap (C++11)

basic_filebuf::is_open

basic_filebuf::open

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Protected member functions

basic_filebuf::showmanyc

basic_filebuf::underflow

basic_filebuf::uflow

basic_filebuf::pbackfail

basic_filebuf::overflow

basic_filebuf::setbuf

basic_filebuf::seekoff

basic_filebuf::seekpos

basic_filebuf::sync

basic_filebuf::imbue

Non-member functions

swap(std::basic_filebuf) (C++11)

 

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.

If the file is open for writing, first writes the put area and any unshift sequences required by the currently imbued locale, using overflow()

Then repositions the file pointer, as if by calling std::fsetpos()

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
which	-	defines which of the input and/or output sequences to affect. It can be one or a combination of the following constants: 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

See also

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