#include <malloc.h>
#include <stdio.h>
#include <stdio_ext.h>

int main(int argc, char** argv)
{
    int buf, chars, i;
    fprintf(stderr,"BUFSIZ = %d\n",BUFSIZ);
    fprintf(stderr,"uninitialised stdout buf_size = %d\n",__fbufsize(stdout));
    setvbuf(stdout, (char*)NULL, _IOFBF, 0);
    fprintf(stderr,"default stdout buf_size = %d\n",__fbufsize(stdout));

    if (argc == 3) {
        buf=atoi(argv[1]);   //chars to allocate
        chars=atoi(argv[2]); //chars to write to test buffering

        char* new_buf=malloc(buf); //can't free until fclose(stdout)
        if (setvbuf(stdout, new_buf, _IOFBF, buf)) {
            fprintf(stderr,"setvbuf() failed\n");
        }
        fprintf(stderr,"specified buf_size = %d\n",__fbufsize(stdout));

        if (chars) {
            for (i=1; i<chars; i++) putchar('.'); putchar('$');
            putchar('.');
            sleep(5); //to see buffering of output
        }
    }
}
/*
    Note currently for glibc (2.3.5) the following call does not change the
    the buffer size, and more problematically does not give any indication
    that the new size request was ignored:
        setvbuf(stdout,(char*)NULL,_IOFBF,8192);

    The ISO C99 standard section 7.19.5.6 on the setvbuf function says:

    ... If buf is not a null pointer, the array it points to may be used
    instead of a buffer allocated by the setvbuf function and the argument
    size specifies the size of the array; otherwise, size may determine
    the size of a buffer allocated by the setvbuf function. ...

    Andreas Schwab at least takes the above to mean setvbuf(....,size)
    is only a hint from the application which I don't agree with.

    FreeBSD's libc seems more sensible in this regard. From the man page:

    The size argument may be given as zero to obtain deferred optimal-size
    buffer allocation as usual.  If it is not zero, then except for
    unbuffered files, the buf argument should point to a buffer at least size
    bytes long; this buffer will be used instead of the current buffer.  (If
    the size argument is not zero but buf is NULL, a buffer of the given size
    will be allocated immediately, and released on close.  This is an extension
    to ANSI C; portable code should use a size of 0 with any NULL buffer.)
*/