# std::basic_filebuf<CharT,Traits>::setbuf

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< cpp‎ | io‎ | basic filebuf

protected: virtual std::basic_streambuf<CharT, Traits>* setbuf( char_type* s, std::streamsize n ) |  |

If s is a null pointer and n is zero, the filebuf becomes unbuffered for output, meaning `pbase()` and `pptr()` are null and any output is immediately sent to file.

Otherwise, a call to `setbuf()` replaces the internal buffer (the controlled character sequence) with the user-supplied character array whose first element is pointed to by s and allows this std::basic_filebuf object to use up to n bytes in that array for buffering.

This function is protected virtual, it may only be called through `pubsetbuf()` or from member functions of a user-defined class derived from `std::basic_filebuf`.

### Parameters

s | - | pointer to the first `CharT` in the user-provided buffer or null
n | - | the number of `CharT` elements in the user-provided buffer or zero

### Return value

this

### Notes

The conditions when this function may be used and the way in which the provided buffer is used is implementation-defined.

- GCC 4.6 libstdc++

  `setbuf()` may only be called when the std::basic_filebuf is not associated with a file (has no effect otherwise). With a user-provided buffer, reading from file reads `n-1` bytes at a time.

- Clang++3.0 libc++

  `setbuf()` may be called after opening the file, but before any I/O (may crash otherwise). With a user-provided buffer, reading from file reads largest multiples of 4096 that fit in the buffer.

- Visual Studio 2010

  `setbuf()` may be called at any time, even after some I/O took place. Current contents of the buffer, if any, are lost.

The standard does not define any behavior for this function except that setbuf(0, 0) called before any I/O has taken place is required to set unbuffered output.

### Example

Provides a 10k buffer for reading. On linux, the strace utility may be used to observe the actual number of bytes read.

Run this code

    #include <fstream>
    #include <iostream>
    #include <string>

    int main()
    {
        int cnt = 0;
        std::ifstream file;
        char buf[10241];

        file.rdbuf()->pubsetbuf(buf, sizeof buf);
        file.open("/usr/share/dict/words");

        for (std::string line; getline(file, line);)
            ++cnt;
        std::cout << cnt << '\n';
    }

### 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 173 | C++98 | the type of n was misspecified as int | corrected to std::streamsize

### See also

pubsetbuf | invokes setbuf() (public member function of `std::basic_streambuf<CharT,Traits>`)
setvbuf | sets the buffer and its size for a file stream (function)

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