std::setvbuf(3)

NAME
std::setvbuf - std::setvbuf

Synopsis
   Defined in header <cstdio>
   int setvbuf( std::FILE* stream, char* buffer, int mode, std::size_t size );
   Changes the buffering mode of the given file stream stream as indicated by the
   argument mode. In addition,
     * If buffer is a null pointer, resizes 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
       std::fclose) before the lifetime of the array pointed to by buffer ends. The
       contents of the array after a successful call to std::setvbuf are indeterminate
       and any attempt to use it is undefined behavior.

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

Return value
   0 on success or nonzero on failure.

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.

Example
   One use case for changing buffer size is when a better size is known. // Run this code
 #include <cstdio>
 #include <cstdlib>
 #include <iostream>
 #include <sys/stat.h>
 int main()
 {
     std::FILE* fp = std::fopen("/tmp/test.txt", "w+");
     if (!fp)
     {
         std::perror("fopen");
         return EXIT_FAILURE;
     }
     struct stat stats;
     if (fstat(fileno(fp), &stats) == -1) // POSIX only
     {
         std::perror("fstat");
         return EXIT_FAILURE;
     }
     std::cout << "BUFSIZ is " << BUFSIZ << ", but optimal block size is "
               << stats.st_blksize << '\n';
     if (std::setvbuf(fp, nullptr, _IOFBF, stats.st_blksize) != 0)
     {
         std::perror("setvbuf failed"); // POSIX version sets errno
         return EXIT_FAILURE;
     }
     // Read entire file: use truss/strace to observe the read(2) syscalls used
     for (int ch; (ch = std::fgetc(fp)) != EOF;)
     {}
     std::fclose(fp);
     return EXIT_SUCCESS;
 }

Possible output:
 BUFSIZ is 8192, but optimal block size is 65536

See also
   setbuf sets the buffer for a file stream
             (function)
   setbuf provides user-supplied buffer or turns this filebuf unbuffered
   [virtual] (virtual protected member function of std::basic_filebuf<CharT,Traits>)
   C documentation for
   setvbuf