Re: [PATCH v4 2/4] util: Introduce read_file() and read_file_integer() function

[Yumei Huang <yuhuang@redhat.com>]

On Thu, Oct 16, 2025 at 2:30 PM David Gibson
<david@gibson.dropbear.id.au> wrote:
>
> On Thu, Oct 16, 2025 at 10:34:21AM +0800, Yumei Huang wrote:
> > Signed-off-by: Yumei Huang <yuhuang@redhat.com>
> > ---
> >  util.c | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  util.h |  3 +++
> >  2 files changed, 87 insertions(+)
> >
> > diff --git a/util.c b/util.c
> > index c492f90..197677e 100644
> > --- a/util.c
> > +++ b/util.c
> > @@ -579,6 +579,90 @@ int write_file(const char *path, const char *buf)
> >       return len == 0 ? 0 : -1;
> >  }
> >
> > +/**
> > + * read_file() - Read contents of file into a buffer
> > + * @path:    File to read
> > + * @buf:     Buffer to store file contents
> > + * @buf_size:        Size of buffer
> > + *
> > + * Return: number of bytes read on success, -1 on any error, -2 on truncation
> > +*/
> > +int read_file(const char *path, char *buf, size_t buf_size)
> > +{
> > +     int fd = open(path, O_RDONLY | O_CLOEXEC);
> > +     size_t total_read = 0;
> > +     ssize_t rc;
> > +
> > +     if (fd < 0) {
> > +             warn_perror("Could not open %s", path);
> > +             return -1;
> > +     }
> > +
> > +     while (total_read < buf_size) {
> > +             rc = read(fd, buf + total_read, buf_size - total_read);
> > +
> > +             if (rc < 0) {
> > +                     warn_perror("Couldn't read from %s", path);
> > +                     close(fd);
> > +                     return -1;
> > +             }
> > +
> > +             if (rc == 0)
> > +                     break;
> > +
> > +             total_read += rc;
> > +     }
> > +
> > +     close(fd);
> > +
> > +     if (total_read == buf_size) {
> > +             warn_perror("File %s truncated, buffer too small", path);
> > +             return -2;
> > +     }
> > +
> > +     buf[total_read] = '\0';
> > +
> > +     return (int)total_read;
>
> Probably makes more sense for total_read and the return type to be ssize_t.

Just tried to be consistent with write_file().  I can change it to
ssize_t if needed.
>
> > +}
> > +
> > +/**
> > + * read_file_integer() - Read an integer value from a file
> > + * @path: 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)
> > +{
> > +     char buf[INTMAX_STRLEN];
> > +     char *end;
>
> passt coding style is to list (where possible) local variables in
> reverse order of line length, so this should go after bytes_read.

Oh, I didn't notice that. Will update later.
>
> > +     intmax_t value;
> > +     int bytes_read;
> > +
> > +     bytes_read = read_file(path, buf, sizeof(buf));
> > +
> > +     if (bytes_read < 0)
> > +             return fallback;
> > +
> > +     if (bytes_read == 0) {
> > +             debug("Empty file %s", path);
> > +             return fallback;
> > +     }
> > +
> > +     errno = 0;
> > +     value = strtoimax(buf, &end, 10);
> > +     if (*end && *end != '\n') {
> > +             debug("Invalid format in %s", path);
> > +             return fallback;
> > +     }
> > +     if (errno) {
> > +             debug("Invalid value in %s: %s", path, buf);
> > +             return fallback;
> > +     }
> > +
> > +     return value;
> > +}
> > +
> >  #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 22eaac5..887d795 100644
> > --- a/util.h
> > +++ b/util.h
> > @@ -222,6 +222,8 @@ 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 read_file(const char *path, char *buf, size_t buf_size);
> > +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);
> > @@ -249,6 +251,7 @@ static inline const char *af_name(sa_family_t af)
> >  }
> >
> >  #define UINT16_STRLEN                (sizeof("65535"))
> > +#define INTMAX_STRLEN                (sizeof("-9223372036854775808"))
>
> It's correct for now, and probably for any systems we're likely to run
> on, but I dislike hard-assuming the size of intmax_t here.  I feel
> like there must be a better way to derive the correct string length,
> but I haven't figured out what it is yet :(.

How about this:

     #define INTMAX_STRLEN (sizeof(intmax_t) * 3 + 2)

Each byte can represent about 2.4 decimal digits as below,
sizeof(intmax_t) * 3 gives us a safe upper bound, +2 for sign and null
terminator.

  1 bit = log₁₀(2) ≈ 0.30103 decimal digits
  1 byte = 8 bits = 8 × 0.30103 ≈ 2.408 decimal digits

>
> >
> >  /* inet address (- '\0') + port (u16) (- '\0') + ':' + '\0' */
> >  #define SOCKADDR_INET_STRLEN                                 \
> > --
> > 2.47.0
> >
>
> --
> 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



-- 
Thanks,

Yumei Huang