Re: [PATCH v2 1/2] util: Add helper to write() all of a buffer

[Markus Armbruster <armbru@redhat.com>]

David Gibson <david@gibson.dropbear.id.au> writes:

> write(2) might not write all the data it is given.  Add a write_all_buf()
> helper to keep calling it until all the given data is written, or we get an
> error.
>
> Currently we use write_remainder() to do this operation in pcap_frame().
> That's a little awkward since it requires constructing an iovec, and future
> changes we want to make to write_remainder() will be easier in terms of
> this single buffer helper.
>
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  pcap.c |  3 +--
>  util.c | 25 +++++++++++++++++++++++++
>  util.h |  1 +
>  3 files changed, 27 insertions(+), 2 deletions(-)
>
> diff --git a/pcap.c b/pcap.c
> index 46cc4b0d..e6b5ced4 100644
> --- a/pcap.c
> +++ b/pcap.c
> @@ -86,9 +86,8 @@ static void pcap_frame(const struct iovec *iov, size_t iovcnt,
>  		.caplen = l2len,
>  		.len = l2len
>  	};
> -	struct iovec hiov = { &h, sizeof(h) };
>  
> -	if (write_remainder(pcap_fd, &hiov, 1, 0) < 0 ||
> +	if (write_all_buf(pcap_fd, &h, sizeof(h)) < 0 ||
>  	    write_remainder(pcap_fd, iov, iovcnt, offset) < 0)
>  		debug_perror("Cannot log packet, length %zu", l2len);
>  }
> diff --git a/util.c b/util.c
> index eede4e58..7db7c2e7 100644
> --- a/util.c
> +++ b/util.c
> @@ -582,6 +582,31 @@ int do_clone(int (*fn)(void *), char *stack_area, size_t stack_size, int flags,
>  #endif
>  }
>  
> +/* write_all_buf() - write all of a buffer to an fd
> + * @fd:		File descriptor
> + * @buf:	Pointer to base of buffer
> + * @len:	Length of buffer
> + *
> + * Return: 0 on success, -1 on error (with errno set)
> + *
> + * #syscalls write
> + */
> +int write_all_buf(int fd, const void *buf, size_t len)

I'd call it write_all(), but that's strictly a matter of taste.

> +{
> +	const char *p = buf;
> +	size_t left = len;
> +
> +	while (left) {
> +		ssize_t rc = write(fd, p, left);
> +
> +		if (rc < 0)
> +			return -1;

Loop when errno == EINTR?

> +		p += rc;
> +		left -= rc;
> +	}
> +	return 0;
> +}
> +
>  /* write_remainder() - write the tail of an IO vector to an fd
>   * @fd:		File descriptor
>   * @iov:	IO vector
> diff --git a/util.h b/util.h
> index c7a59d5d..5e67f1fa 100644
> --- a/util.h
> +++ b/util.h
> @@ -200,6 +200,7 @@ void pidfile_write(int fd, pid_t pid);
>  int __daemon(int pidfile_fd, int devnull_fd);
>  int fls(unsigned long x);
>  int write_file(const char *path, const char *buf);
> +int write_all_buf(int fd, const void *buf, size_t len);
>  int write_remainder(int fd, const struct iovec *iov, size_t iovcnt, size_t skip);
>  void close_open_files(int argc, char **argv);