From 074b6d45b0aab99753b64dd5423531fb31e46748 Mon Sep 17 00:00:00 2001 From: Michal Privoznik Date: Thu, 17 Jan 2013 08:35:53 +0100 Subject: [PATCH] safe{read,write}: Document usage with nonblocking FD Currently, whenever somebody calls saferead() on nonblocking FD (safewrite() is totally interchangeable for purpose of this message) he might get wrong return value. For instance, in the first iteration some data is read. The number of bytes read is stored into local variable 'nread'. However, in next iterations we can get -1 from read() with errno == EAGAIN, in which case the -1 is returned despite fact some data has already been read. So the caller gets confused. Bare read() should be used for nonblocking FD. --- src/util/virutil.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/src/util/virutil.c b/src/util/virutil.c index b36e9d39dc..24ba954949 100644 --- a/src/util/virutil.c +++ b/src/util/virutil.c @@ -98,7 +98,11 @@ verify(sizeof(gid_t) <= sizeof(unsigned int) && #define VIR_FROM_THIS VIR_FROM_NONE -/* Like read(), but restarts after EINTR */ +/* Like read(), but restarts after EINTR. Doesn't play + * nicely with nonblocking FD and EAGAIN, in which case + * you want to use bare read(). Or even use virSocket() + * if the FD is related to a socket rather than a plain + * file or pipe. */ ssize_t saferead(int fd, void *buf, size_t count) { @@ -118,7 +122,11 @@ saferead(int fd, void *buf, size_t count) return nread; } -/* Like write(), but restarts after EINTR */ +/* Like write(), but restarts after EINTR. Doesn't play + * nicely with nonblocking FD and EAGAIN, in which case + * you want to use bare write(). Or even use virSocket() + * if the FD is related to a socket rather than a plain + * file or pipe. */ ssize_t safewrite(int fd, const void *buf, size_t count) {