Hi,
This bit from setvbuf(3):
The size parameter may be given as zero to obtain
deferred optimal-size buffer allocation as usual.
doesn't describe what's happening in setvbuf.c. When size is 0,
fp->_fb.base is allocated immediately:
/*
* Find optimal I/O size for seek optimization. This also returns
* a `tty flag' to suggest that we check isatty(fd), but we do not
* care since our caller told us how to buffer.
*/
flags |= __swhatbuf(fp, &iosize, &ttyflag);
if (size == 0) {
buf = NULL; /* force local allocation */
size = iosize;
}
/* Allocate buffer if needed. */
if (buf == NULL) {
if ((buf = malloc(size)) == NULL) {
/*
* Unable to honor user's request. We will return
* failure, but try again with file system size.
*/
ret = EOF;
if (size != iosize) {
size = iosize;
buf = malloc(size);
}
}
if (buf == NULL) {
/* No luck; switch to unbuffered I/O. */
Allocation is not "deferred," unless I've misunderstood something.
Not sure whether it's important to mention that the allocation
occurs immediately as opposed to whenever I/O is first attempted,
so I've just pared the remark down.
--
Scott Cheloha
Index: lib/libc/stdio/setvbuf.3
===================================================================
RCS file: /cvs/src/lib/libc/stdio/setvbuf.3,v
retrieving revision 1.4
diff -u -p -r1.4 setvbuf.3
--- lib/libc/stdio/setvbuf.3 26 Nov 2014 18:16:32 -0000 1.4
+++ lib/libc/stdio/setvbuf.3 29 Jul 2017 02:21:49 -0000
@@ -90,7 +90,7 @@ fully buffered
The
.Fa size
parameter may be given as zero
-to obtain deferred optimal-size buffer allocation as usual.
+to obtain an optimal-size buffer allocation.
If it is not zero, then except for unbuffered files, the
.Fa buf
argument should point to a buffer at least
