[Qemu-devel] [PATCH] Add qemu_read_full

[Qemu-devel] [PATCH] Add qemu_read_full

[M. Mohan Kumar]

Add qemu_read_full function

Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
---
 osdep.c       |   29 +++++++++++++++++++++++++++++
 qemu-common.h |    2 ++
 2 files changed, 31 insertions(+), 0 deletions(-)

diff --git a/osdep.c b/osdep.c
index 327583b..7046b32 100644
--- a/osdep.c
+++ b/osdep.c
@@ -127,6 +127,35 @@ ssize_t qemu_write_full(int fd, const void *buf, size_t count)
 }
 
 /*
+ * A variant of read(2) which handles interrupted read.
+ * Simlar to qemu_write_full function
+ *
+ * Return the number of bytes read.
+ *
+ */
+ssize_t qemu_read_full(int fd, void *buf, size_t count)
+{
+    ssize_t ret = 0;
+    ssize_t total = 0;
+
+    while (count) {
+        ret = read(fd, buf, count);
+        if (ret < 0) {
+            if (errno == EINTR) {
+                continue;
+            }
+            break;
+        }
+
+        count -= ret;
+        buf += ret;
+        total += ret;
+    }
+
+    return total;
+}
+
+/*
  * Opens a socket with FD_CLOEXEC set
  */
 int qemu_socket(int domain, int type, int protocol)
diff --git a/qemu-common.h b/qemu-common.h
index b3957f1..0f60e08 100644
--- a/qemu-common.h
+++ b/qemu-common.h
@@ -190,6 +190,8 @@ void qemu_mutex_unlock_iothread(void);
 int qemu_open(const char *name, int flags, ...);
 ssize_t qemu_write_full(int fd, const void *buf, size_t count)
     QEMU_WARN_UNUSED_RESULT;
+ssize_t qemu_read_full(int fd, void *buf, size_t count)
+    QEMU_WARN_UNUSED_RESULT;
 void qemu_set_cloexec(int fd);
 
 #ifndef _WIN32
-- 
1.7.0.4

Re: [Qemu-devel] [PATCH] Add qemu_read_full

[Stefan Hajnoczi]

On Wed, Nov 24, 2010 at 5:18 PM, M. Mohan Kumar <mohan@in.ibm.com> wrote:
> Add qemu_read_full function
>
> Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
> ---
>  osdep.c       |   29 +++++++++++++++++++++++++++++
>  qemu-common.h |    2 ++
>  2 files changed, 31 insertions(+), 0 deletions(-)
>
> diff --git a/osdep.c b/osdep.c
> index 327583b..7046b32 100644
> --- a/osdep.c
> +++ b/osdep.c
> @@ -127,6 +127,35 @@ ssize_t qemu_write_full(int fd, const void *buf, size_t count)
>  }
>
>  /*
> + * A variant of read(2) which handles interrupted read.
> + * Simlar to qemu_write_full function
> + *
> + * Return the number of bytes read.
> + *
> + */

Two points worth noting in the comment (stolen from qemu_write_full):
1. "This function don't work with non-blocking fd's."
2. "Set errno if fewer than `count' bytes are read."  Normal QEMU
style would be to return -errno, so it's important to point out this
exception to the rule.

> +ssize_t qemu_read_full(int fd, void *buf, size_t count)
> +{
> +    ssize_t ret = 0;
> +    ssize_t total = 0;
> +
> +    while (count) {
> +        ret = read(fd, buf, count);
> +        if (ret < 0) {
> +            if (errno == EINTR) {
> +                continue;
> +            }
> +            break;
> +        }
> +
> +        count -= ret;
> +        buf += ret;
> +        total += ret;
> +    }
> +
> +    return total;
> +}

On EOF read(2) will return 0.  Right now we don't handle this case and
go into an infinite loop.

Stefan