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readv.2: Update details on glibc readv()/writev() wrapper behavior 

And add a historical detail about Linux 2.0.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2015-01-20 17:37:40 +01:00
parent bc9ed112b7
commit 8a930bf121
1 changed files with 31 additions and 12 deletions
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@ -226,7 +226,6 @@ first appeared in Linux 2.6.30; library support was added in glibc 2.10.
.BR pwritev ():
nonstandard, but present also on the modern BSDs.
.SH NOTES
.SS C library/kernel ABI differences
POSIX.1-2001 allows an implementation to place a limit on
the number of items that can be passed in
.IR iov .
@ -236,28 +235,48 @@ in
.I <limits.h>
or at run time via the return value from
.IR sysconf(_SC_IOV_MAX) .
On Linux, the limit advertised by these mechanisms is 1024,
which is the true kernel limit.
However, the glibc wrapper functions do some extra work if
they detect that the underlying kernel system call failed because this
limit was exceeded.
On modern Linux systems, the limit is 1024.
Back in Linux 2.0 days, this limit was 16.
\"
.SS Historical C library/kernel ABI differences
To deal with the fact that
.B IOV_MAX
was so low on early versions of Linux,
the glibc wrapper functions for
.BR readv ()
and
.BR writev ()
did some extra work if they detected that the underlying kernel
system call failed because this limit was exceeded.
In the case of
.BR readv (),
the wrapper function allocates a temporary buffer large enough
the wrapper function allocated a temporary buffer large enough
for all of the items specified by
.IR iov ,
passes that buffer in a call to
passed that buffer in a call to
.BR read (2),
copies data from the buffer to the locations specified by the
copied data from the buffer to the locations specified by the
.I iov_base
fields of the elements of
.IR iov ,
and then frees the buffer.
and then freed the buffer.
The wrapper function for
.BR writev ()
performs the analogous task using a temporary buffer and a call to
performed the analogous task using a temporary buffer and a call to
.BR write (2).
The need for this extra effort in the glibc wrapper functions
went away with Linux 2.2 and later.
However, glibc continued to provide this behavior until version 2.10.
Starting with glibc version 2.9,
the wrapper functions provide this behavior only if the library detects
that the system is running a Linux kernel older than version 2.6.18
(an arbitrarily selected kernel version).
And since glibc 2.20
(which requires a minimum Linux kernel version of 2.6.32),
the glibc wrapper functions always just directly invoke the system calls.
\"
.SS C library/kernel ABI differences
The raw
.BR preadv ()
and