[PATCH v5 2/4] util: Introduce read_file() and read_file_integer() function

[Yumei Huang <yuhuang@redhat.com>]

Signed-off-by: Yumei Huang <yuhuang@redhat.com>
---
 util.c | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 util.h | 10 +++++++
 2 files changed, 94 insertions(+)

diff --git a/util.c b/util.c
index c492f90..ff21bf6 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_perror("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..acfbd08 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 +252,14 @@ static inline const char *af_name(sa_family_t af)
 
 #define UINT16_STRLEN		(sizeof("65535"))
 
+/* Each byte holds roughly 2.4 decimal digits:
+ *    1 bit = log10(2) ~= 0.30103 decimal digits
+ *    1 byte = 8 bits = 8 x 0.30103 ~= 2.408 decimal digits
+ * Using sizeof(intmax_t) * 3 provides a safe upper bound,
+ * plus 2 extra bytes for the sign and null terminator.
+ */
+#define INTMAX_STRLEN		(sizeof(intmax_t) * 3 + 2)
+
 /* inet address (- '\0') + port (u16) (- '\0') + ':' + '\0' */
 #define SOCKADDR_INET_STRLEN					\
 	(INET_ADDRSTRLEN-1 + UINT16_STRLEN-1 + sizeof(":"))
-- 
2.47.0