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

21 Oct 2025

On Sun, Oct 19, 2025 at 6:07 PM Stefano Brivio  wrote:
...
On Fri, 17 Oct 2025 14:28:36 +0800
Yumei Huang  wrote:
...
Signed-off-by: Yumei Huang 
---
 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?  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)."

It's my fault. I should have waited to see if you have more comments
before sending the new version.
...
Note that I didn't review this patch or the rest of this series, yet.
--
Stefano
--
Thanks,

Yumei Huang

    