Re: [PATCH v9 2/5] util: Introduce read_file() and read_file_integer() function

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Nov 25, 2025 at 03:26:35PM +0800, Yumei Huang wrote:
> Signed-off-by: Yumei Huang <yuhuang@redhat.com>
> ---
>  util.c | 90 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  util.h |  1 +
>  2 files changed, 91 insertions(+)
> 
> diff --git a/util.c b/util.c
> index ab23463..c56f920 100644
> --- a/util.c
> +++ b/util.c
> @@ -589,6 +589,96 @@ int write_file(const char *path, const char *buf)
>  	return len == 0 ? 0 : -1;
>  }
>  
> +/**
> + * read_file() - Read contents of file into a NULL-terminated buffer
> + * @path:	Path to file to read
> + * @buf:	Buffer to store file contents
> + * @buf_size:	Size of buffer
> + *
> + * Return: number of bytes read on success, negative error code on failure
> + */
> +static ssize_t read_file(const char *path, char *buf, size_t buf_size)
> +{
> +	int fd = open(path, O_RDONLY | O_CLOEXEC);
> +	size_t total_read = 0;
> +
> +	if (fd < 0)
> +		return -errno;
> +
> +	if (!buf_size) {
> +		close(fd);
> +		return -EINVAL;
> +	}

Nit: usually it's preferable to put the error checks with the least
"impact" first.  So in this case, it would be slightly better to check
buf_size first, before even opening the file.  What you have is still
correct though, so this isn't worth a respin.

> +
> +	while (total_read < buf_size) {
> +		ssize_t rc = read(fd, buf + total_read, buf_size - total_read);
> +
> +		if (rc < 0) {
> +			int errno_save = errno;
> +			close(fd);
> +			return -errno_save;
> +		}
> +
> +		if (rc == 0)
> +			break;
> +
> +		total_read += rc;
> +	}
> +
> +	close(fd);
> +
> +	if (total_read == buf_size) {
> +		buf[buf_size - 1] = '\0';
> +		return -ENOBUFS;
> +	}
> +
> +	buf[total_read] = '\0';
> +
> +	return total_read;
> +}
> +
> +/**
> + * read_file_integer() - Read an integer value from a file
> + * @path:	Path to file to read
> + * @fallback:	Default value if file can't be read
> + *
> + * Return: integer value, @fallback on failure
> + */
> +intmax_t read_file_integer(const char *path, intmax_t fallback)
> +{
> +	ssize_t bytes_read;
> +	char buf[BUFSIZ];
> +	intmax_t value;
> +	char *end;
> +
> +	bytes_read = read_file(path, buf, sizeof(buf));
> +
> +	if (bytes_read < 0)
> +		goto error;
> +
> +	if (bytes_read == 0) {
> +		debug("Empty file %s", path);
> +		goto error;
> +	}
> +
> +	errno = 0;
> +	value = strtoimax(buf, &end, 10);
> +	if (*end && *end != '\n') {
> +		debug("Non-numeric content in %s", path);
> +		goto error;
> +	}
> +	if (errno) {
> +		debug("Out of range value in %s: %s", path, buf);
> +		goto error;
> +	}
> +
> +	return value;
> +
> +error:
> +	debug("Using %"PRIdMAX" as default value", fallback);

I think this does need to say what it's using this default for.  So
something like:
	"Couldn't read %s, using %"PRIdMAX"d as default value"

> +	return fallback;
> +}
> +
>  #ifdef __ia64__
>  /* Needed by do_clone() below: glibc doesn't export the prototype of __clone2(),
>   * use the description from clone(2).
> diff --git a/util.h b/util.h
> index a0b2ada..6dec14b 100644
> --- a/util.h
> +++ b/util.h
> @@ -229,6 +229,7 @@ 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);
> +intmax_t read_file_integer(const char *path, intmax_t fallback);
>  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);
>  int read_all_buf(int fd, void *buf, size_t len);
> -- 
> 2.51.1
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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