Re: [PATCH v3 2/4] util: Introduce read_file() and read_file_long() function

On Tue, Oct 14, 2025 at 03:38:34PM +0800, Yumei Huang wrote:

Signed-off-by: Yumei Huang --- util.c | 92 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ util.h | 2 ++ 2 files changed, 94 insertions(+)

diff --git a/util.c b/util.c index c492f90..d331f08 100644 --- a/util.c +++ b/util.c @@ -579,6 +579,98 @@ 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 +*/

Looks ok, but I think there's a simpler way.

+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; + bool truncated = false; + + if (fd < 0) { + warn_perror("Could not open %s", path); + return -1; + } + + while (total_read < buf_size - 1) { + rc = read(fd, buf + total_read, buf_size - 1 - total_read);

The '- 1' is to leave space for the \0, but if you instead attempt to read the entire buffer...

+ + if (rc < 0 ) {

(nit: extra space before ')')

+ warn_perror("Couldn't read from %s", path); + close(fd); + return -1; + } + + if (rc == 0) { + break; + } + + total_read += rc; + + if (total_read == buf_size - 1) { + char test_byte; + rc = read(fd, &test_byte, 1); + if (rc >0) { + truncated = true; + warn_perror("File %s truncated, buffer too small", path); + } + }

...then you can tell if you have to truncate by finishing the loop then checking if (total_read < buf_size). If it is, there's space for the \0, otherwise there isn't and you report truncation. No need for test_byte.

+ } + + close(fd); + + if (total_read < buf_size){ + buf[total_read] = '\0';

And if you test for truncation and exit early, you can make this unconditional.

+ } + + return truncated ? -2 : (int)total_read; +} + +/** + * read_file_long() - Read a long integer value from a file

When I first read this name I thought it was for reading a long file, rather than reading a long (int) from a file. Not immediately sure how to clarify that. read_file_long_int() is clear, but awkward. A better choice might be to change this to use strtoimax() and call it read_file_integer().

+ * @path: Path to the sysctl file + * @fallback: Default value if file can't be read + * + * Return: Parameter value, fallback on failure +*/ +long read_file_long(const char *path, long fallback) +{ + char buf[32];

Rather than just using a semi-arbitrary 32 here, I'd suggest defining a new constant similar to UINT16_STRLEN. Except that's trickier for a type that doesn't have a known fixed width. Pity the C library doesn't have constants for these AFAICT.

+ char *end; + long value; + int bytes_read; + + bytes_read = read_file(path, buf, sizeof(buf)); + if (bytes_read < 0) { + debug("Unable to read %s", path);

If there's a an error on open() or read(), this will produce two very similar error messages in a row, which isn't ideal.

+ return fallback; + } + + if (bytes_read == 0) { + debug("Empty file %s", path); + return fallback; + }

Might be worth checking strtol()'s behaviour on an empty string to see if this special case would already be handled below.

+ + errno = 0; + value = strtol(buf, &end, 10); + if (*end && *end != '\n') { + debug("Invalid format in %s", path); + return fallback; + } + if (errno || value < 0 || value > LONG_MAX) {

No need to exclude negative values here. (value > LONG_MAX) can never be true since value is a long.

+ debug("Invalid value in %s: %ld", path, value);

If errno != 0, value might be uninitialised here, and certainly won't have something useful. Better to print the contents as a string.

+ 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..e509bec 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); +long read_file_long(const char *path, long 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); -- 2.47.0

-- 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