Re: [PATCH v9 2/5] util: Introduce read_file() and read_file_integer() function

On Wed, 26 Nov 2025 15:07:25 +1100 David Gibson wrote:

On Tue, Nov 25, 2025 at 03:26:35PM +0800, Yumei Huang wrote:

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Signed-off-by: Yumei Huang --- util.c | 90 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ util.h | 1 + 2 files changed, 91 insertions(+)

diff --git a/util.c b/util.c index ab23463..c56f920 100644 --- a/util.c +++ b/util.c @@ -589,6 +589,96 @@ int write_file(const char *path, const char *buf) return len == 0 ? 0 : -1; }

+/** + * read_file() - Read contents of file into a NULL-terminated buffer + * @path: Path to file to read + * @buf: Buffer to store file contents + * @buf_size: Size of buffer + * + * Return: number of bytes read on success, negative error code on failure + */ +static 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; + + if (fd < 0) + return -errno; + + if (!buf_size) { + close(fd); + return -EINVAL; + }

Nit: usually it's preferable to put the error checks with the least "impact" first. So in this case, it would be slightly better to check buf_size first, before even opening the file. What you have is still correct though, so this isn't worth a respin.

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+ + while (total_read < buf_size) { + ssize_t rc = read(fd, buf + total_read, buf_size - total_read); + + if (rc < 0) { + int errno_save = errno; + close(fd); + return -errno_save; + } + + if (rc == 0) + break; + + total_read += rc; + } + + close(fd); + + if (total_read == buf_size) { + buf[buf_size - 1] = '\0'; + return -ENOBUFS; + } + + buf[total_read] = '\0'; + + return total_read; +} + +/** + * read_file_integer() - Read an integer value from a file + * @path: Path to 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) +{ + ssize_t bytes_read; + char buf[BUFSIZ]; + intmax_t value; + char *end; + + bytes_read = read_file(path, buf, sizeof(buf)); + + if (bytes_read < 0) + goto error; + + if (bytes_read == 0) { + debug("Empty file %s", path); + goto error; + } + + errno = 0; + value = strtoimax(buf, &end, 10); + if (*end && *end != '\n') { + debug("Non-numeric content in %s", path); + goto error; + } + if (errno) { + debug("Out of range value in %s: %s", path, buf); + goto error; + } + + return value; + +error: + debug("Using %"PRIdMAX" as default value", fallback);

I think this does need to say what it's using this default for. So something like: "Couldn't read %s, using %"PRIdMAX"d as default value"

Right, in some cases it's printed in the message just before (as long as we don't have multi-threading, at least), but in the most common case (kernel too old): --- $ ./pasta -d [...] 0.0163: Using 4 as default value 0.0163: Using 120000 as default value 0.0164: Using TCP RTO parameters, syn_retries: 6, syn_linear_timeouts: 4, rto_max: 120 --- The rest of the comments from myself and David aren't worth a re-spin, but I think this one is, because it might actually confuse users.

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+ return fallback; +} + #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 a0b2ada..6dec14b 100644 --- a/util.h +++ b/util.h @@ -229,6 +229,7 @@ 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); +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); -- 2.51.1

-- Stefano