Re: [PATCH v6 2/4] util: Introduce read_file() and read_file_integer() function

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

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On Tue, Oct 21, 2025 at 11:50:59PM +0200, Stefano Brivio wrote:
> On Tue, 21 Oct 2025 17:32:58 +0800
> Yumei Huang <yuhuang@redhat.com> wrote:
> 
> > On Sun, Oct 19, 2025 at 6:07 PM Stefano Brivio <sbrivio@redhat.com> wrote:
> > >
> > > On Fri, 17 Oct 2025 14:28:36 +0800
> > > Yumei Huang <yuhuang@redhat.com> wrote:
> > >  
> > > > Signed-off-by: Yumei Huang <yuhuang@redhat.com>
> > > > ---
> > > >  util.c | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > > >  util.h |  8 ++++++
> > > >  2 files changed, 92 insertions(+)
> > > >
> > > > diff --git a/util.c b/util.c
> > > > index c492f90..5c8c4bc 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
> > > > +*/
> > > > +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;
> > > > +     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("File %s truncated, buffer too small", path);
> > > > +             return -2;
> > > > +     }
> > > > +
> > > > +     buf[total_read] = '\0';
> > > > +
> > > > +     return total_read;
> > > > +}
> > > > +
> > > > +/**
> > > > + * 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];
> > > > +     ssize_t bytes_read;
> > > > +     intmax_t value;
> > > > +     char *end;
> > > > +
> > > > +     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..3f9f296 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);
> > > > +ssize_t 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);
> > > > @@ -250,6 +252,12 @@ static inline const char *af_name(sa_family_t af)
> > > >
> > > >  #define UINT16_STRLEN                (sizeof("65535"))
> > > >
> > > > +/* Each byte expands to at most 3 decimal digits since 0xff == 255.
> > > > + * Plus 2 extra bytes for the sign and null terminator.
> > > > + * See https://stackoverflow.com/a/10536254.  
> > >
> > > This is not an acceptable form of attribution according to the
> > > CC BY-SA 3.0 terms. See:
> > >
> > >   https://stackoverflow.com/help/licensing
> > >   https://creativecommons.org/licenses/by-sa/3.0/
> > >
> > > and checksum.h in this tree for some examples of how to combine
> > > different licensing terms in a single file, in a way that's
> > > human-readable but still machine-friendly (for license / compliance
> > > scanners such as REUSE).
> > >
> > > As I commented on a previous version, anyway, I don't think we need
> > > this at all. I guess my comment was ignored though.  
> > 
> > I guess you meant the comment of suggesting using BUFSIZ in V2?
> 
> Right, but on v4, and that was just Friday for everybody involved...
> 
> > David replied as quote:
> > 
> >  "We could use BUFSIZ, but it's massive overkill for
> > reading a single integer: 8192 versus ~21 bytes (or ~42 bytes if
> > intmax_t were 128-bit)."
> 
> I wanted to reply to that because sure, BUFSIZ is typically 8192 bytes
> on glibc and 1024 with musl, but adding 10 or 8192 to the stack pointer
> doesn't really make a difference.
> 
> It's not like we allocate that memory anyway, and I don't think any of
> that memory (or unused holes on the stack we create) is prefetched. And
> regardless of all that... we don't use these functions on any data
> path, it's just during configuration. It can be (relatively) slow.

Right.  Sorry, I didn't follow up yet, but after I wrote that, Stefano
convinced me that BUFSIZ is fine.  I originally suggested avoiding the
arbitrary buffer largely because I thought having an INTMAX_STRLEN
constant might be useful for other reasons too, but honestly, not
very.

-- 
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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