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

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 26 Nov 2025 15:07:25 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> 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"

Right, in some cases it's printed in the message just before (as long
as we don't have multi-threading, at least), but in the most common
case (kernel too old):

---
$ ./pasta -d
[...]
0.0163: Using 4 as default value
0.0163: Using 120000 as default value
0.0164: Using TCP RTO parameters, syn_retries: 6, syn_linear_timeouts: 4, rto_max: 120
---

The rest of the comments from myself and David aren't worth a re-spin,
but I think this one is, because it might actually confuse users.

> 
> > +	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

-- 
Stefano