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SubjectRe: [PATCH v2 4/5] tools api: add a lightweight buffered reading api
Hello,

On Fri, Apr 3, 2020 at 12:44 AM Ian Rogers <irogers@google.com> wrote:
>
> The synthesize benchmark shows the majority of execution time going to
> fgets and sscanf, necessary to parse /proc/pid/maps. Add a new buffered
> reading library that will be used to replace these calls in a follow-up
> CL.
>
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
> tools/lib/api/io.h | 107 +++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 107 insertions(+)
> create mode 100644 tools/lib/api/io.h
>
> diff --git a/tools/lib/api/io.h b/tools/lib/api/io.h
> new file mode 100644
> index 000000000000..5aa5b0e26a7a
> --- /dev/null
> +++ b/tools/lib/api/io.h
> @@ -0,0 +1,107 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Lightweight buffered reading library.
> + *
> + * Copyright 2019 Google LLC.
> + */
> +#ifndef __API_IO__
> +#define __API_IO__
> +
> +struct io {
> + /* File descriptor being read/ */
> + int fd;
> + /* Size of the read buffer. */
> + unsigned int buf_len;
> + /* Pointer to storage for buffering read. */
> + char *buf;
> + /* End of the storage. */
> + char *end;
> + /* Currently accessed data pointer. */
> + char *data;
> + /* Set true on when the end of file on read error. */
> + bool eof;
> +};
> +
> +static inline void io__init(struct io *io, int fd,
> + char *buf, unsigned int buf_len)
> +{
> + io->fd = fd;
> + io->buf_len = buf_len;
> + io->buf = buf;
> + io->end = buf;
> + io->data = buf;
> + io->eof = false;
> +}
> +
> +/* Reads one character from the "io" file with similar semantics to fgetc. */
> +static inline int io__get_char(struct io *io)
> +{
> + char *ptr = io->data;
> +
> + if (ptr == io->end) {
> + ssize_t n = read(io->fd, io->buf, io->buf_len);
> +
> + if (n <= 0) {
> + io->eof = true;
> + return -1;
> + }
> + ptr = &io->buf[0];
> + io->end = &io->buf[n];
> + }
> + io->data = ptr + 1;
> + return *ptr;
> +}
> +
> +/* Read a hexadecimal value with no 0x prefix into the out argument hex.
> + * Returns -1 on error or if nothing is read, otherwise returns the character
> + * after the hexadecimal value.
> + */
> +static inline int io__get_hex(struct io *io, __u64 *hex)
> +{
> + bool first_read = true;
> +
> + *hex = 0;
> + while (true) {
> + char ch = io__get_char(io);
> +

Maybe you can add this

if (io->eof)
return 0;

Please see below


> + if (ch < 0)
> + return ch;
> + if (ch >= '0' && ch <= '9')
> + *hex = (*hex << 4) | (ch - '0');
> + else if (ch >= 'a' && ch <= 'f')
> + *hex = (*hex << 4) | (ch - 'a' + 10);
> + else if (ch >= 'A' && ch <= 'F')
> + *hex = (*hex << 4) | (ch - 'A' + 10);
> + else if (first_read)
> + return -1;
> + else
> + return ch;
> + first_read = false;
> + }
> +}

What if a file contains hex digits at the end (without trailing spaces)?
I guess it'd see EOF and return -1, right?

And it'd better to be clear when it sees a big hex numbers -
it could have a comment that it'd simply discard upper bits
or return an error.

> +
> +/* Read a decimal value into the out argument dec.
> + * Returns -1 on error or if nothing is read, otherwise returns the character
> + * after the decimal value.
> + */
> +static inline int io__get_dec(struct io *io, __u64 *dec)
> +{
> + bool first_read = true;
> +
> + *dec = 0;
> + while (true) {
> + char ch = io__get_char(io);
> +
> + if (ch < 0)
> + return ch;
> + if (ch >= '0' && ch <= '9')
> + *dec = (*dec * 10) + ch - '0';
> + else if (first_read)
> + return -1;
> + else
> + return ch;
> + first_read = false;
> + }
> +}

Ditto.

Thanks
Namhyung


> +
> +#endif /* __API_IO__ */
> --
> 2.26.0.rc2.310.g2932bb562d-goog
>

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