##### cppreference.com

##### Search

Create account

##### Namespaces

- Page
- Discussion

##### Variants

##### Views

- View
- Edit
- History

##### Actions

# std::setvbuf

From cppreference.com
< cpp‎ | io‎ | c

Defined in header `<cstdio>` |  |
int setvbuf( std::FILE* stream, char* buffer, int mode, std::size_t size ); |  |

Changes the the buffering mode of the given file stream `stream` as indicated by the argument `mode`. In addition,

- If `buffer` is a null pointer, resizes of the internal buffer to `size`.
- If `buffer` is not a null pointer, instructs the stream to use the user-provided buffer of size `size` beginning at `buffer`. The stream must be closed (with fclose) before the lifetime of the array pointed to by `buffer` ends. The contents of the array after a successful call to `setvbuf` are indeterminate and any attempt to use it is undefined behavior.

### [edit] Parameters

stream | - | the file stream to set the buffer to or null pointer to change size and mode only
buffer | - | pointer to a buffer for the stream to use
mode | - | buffering mode to use. It can be one of the following values:
`_IOFBF` | full buffering
`_IOLBF` | line buffering
`_IONBF` | no buffering
size | - | size of the buffer

### [edit] Return value

​0​ on success or nonzero on failure.

### [edit] Notes

This function may only be used after `stream` has been associated with an open file, but before any other operation (other than a failed call to std::setbuf/`std::setvbuf`).

Not all `size` bytes will necessarily be used for buffering: the actual buffer size is usually rounded down to a multiple of 2, a multiple of page size, etc.

On many implementations, line buffering is only available for terminal input streams.

A common error is setting the buffer of stdin or stdout to an array whose lifetime ends before the program terminates:

    int main() {
        char buf[BUFSIZ];
        std::setbuf(stdin, buf);
    } // lifetime of buf ends, undefined behavior

The default buffer size BUFSIZ is expected to be the most efficient buffer size for file I/O on the implementation, but POSIX fstat often provides a better estimate.

### [edit] Example

one use case for changing buffer size is when a better size is known

Run this code

    #include <iostream>
    #include <cstdio>
    #include <stdlib.h>
    #include <sys/stat.h>

    int main()
    {
        std::FILE* fp = std::fopen("test.txt", "r");
        if(!fp) {
           std::perror("fopen"); return 1;
        }

        struct stat stats;
        if(fstat(fileno(fp), &stats) == -1) { // POSIX only
            std::perror("fstat"); return 1;
        }

        std::cout << "BUFSIZ is " << BUFSIZ << ", but optimal block size is "
                  << stats.st_blksize << '\n';
        if(std::setvbuf(fp, NULL, _IOFBF, stats.st_blksize) != 0) {
           perror("setvbuf failed"); // POSIX version sets errno
           return 1;
        }

        int ch;
        while((ch=std::fgetc(fp)) != EOF); // read entire file: use truss/strace to
                                           // observe the read(2) syscalls used
        std::fclose(fp);
    }

Possible output:

    BUFSIZ is 8192, but optimal block size is 65536

### [edit] See also

setbuf | sets the buffer for a file stream (function) [edit]
setbuf [virtual] | provides user-supplied buffer or turns this filebuf unbuffered (virtual protected member function of `std::basic_filebuf`) [edit]
C documentation for setvbuf

Retrieved from "http://en.cppreference.com/mwiki/index.php?title=cpp/io/c/setvbuf&oldid=87575"