add OSMO_STRBUF_PRINTF()
We are using macros like this or different workarounds in libmsc. In the course
of implementing inter-MSC handover, I am encountering yet another such
situation of appending multiple strings to a limited char buffer. Standardize.
Add a unit test to utils_test.c.
Change-Id: I2497514e26c5e7a5d88985fc7e58343be1a027b2
diff --git a/include/osmocom/core/utils.h b/include/osmocom/core/utils.h
index fe360b3..16159d3 100644
--- a/include/osmocom/core/utils.h
+++ b/include/osmocom/core/utils.h
@@ -145,4 +145,89 @@
const char osmo_luhn(const char* in, int in_len);
+/*! State for OSMO_STRBUF_APPEND() and OSMO_STRBUF_PRINTF(). See there for examples. */
+struct osmo_strbuf {
+ /*! Point to the start of a string buffer. */
+ char *buf;
+ /*! Total sizeof() the buffer buf points at. */
+ size_t len;
+ /*! Current writing position in buf (end of the string written so far). */
+ char *pos;
+ /*! After all OSMO_STRBUF_APPEND operations, reflects the total number of characters that would be written had
+ * buf been large enough. Like snprintf()'s return value, this does not include the terminating nul character.
+ * Hence, to allocate an adequately sized buffer, add 1 to this number. */
+ size_t chars_needed;
+};
+
+/*! Append a string to a buffer, as printed by an snprintf()-like function and with similar bounds checking.
+ * Make sure to never write past the end of the buffer, and collect the total size that would be needed.
+ *
+ * // an example function implementation to append: write N spaces.
+ * int print_spaces(char *dst, size_t dst_len, int n)
+ * {
+ * int i;
+ * if (n < 0)
+ * return -EINVAL;
+ * for (i = 0; i < n && i < dst_len; i++)
+ * dst[i] = ' ';
+ * if (dst_len)
+ * dst[OSMO_MIN(dst_len - 1, n)] = '\0';
+ * // return the n that we would have liked to write if space were available:
+ * return n;
+ * }
+ *
+ * // append above spaces as well as an snprintf()
+ * void strbuf_example()
+ * {
+ * char buf[23];
+ * struct osmo_strbuf sb = { .buf = buf, .len = sizeof(buf) };
+ *
+ * OSMO_STRBUF_APPEND(sb, print_spaces, 5);
+ * OSMO_STRBUF_APPEND(sb, snprintf, "The answer is %d but what is the question?", 42);
+ * OSMO_STRBUF_APPEND(sb, print_spaces, 423423);
+ *
+ * printf("%s\n", buf);
+ * printf("would have needed %zu bytes\n", sb.chars_needed);
+ * }
+ *
+ * \param[inout] STRBUF A struct osmo_strbuf instance.
+ * \param[in] func A function with a signature of int func(char *dst, size_t dst_len [, args]) with semantics like
+ * snprintf().
+ * \param[in] args Arguments passed to func, if any.
+ */
+#define OSMO_STRBUF_APPEND(STRBUF, func, args...) do { \
+ if (!(STRBUF).pos) \
+ (STRBUF).pos = (STRBUF).buf; \
+ size_t remain = (STRBUF).buf ? (STRBUF).len - ((STRBUF).pos - (STRBUF).buf) : 0; \
+ int l = func((STRBUF).pos, remain, ##args); \
+ if (l < 0 || l > remain) \
+ (STRBUF).pos = (STRBUF).buf + (STRBUF).len; \
+ else \
+ (STRBUF).pos += l; \
+ if (l > 0) \
+ (STRBUF).chars_needed += l; \
+ } while(0)
+
+/*! Shortcut for OSMO_STRBUF_APPEND() invocation using snprintf().
+ *
+ * int strbuf_example2(char *buf, size_t buflen)
+ * {
+ * int i;
+ * struct osmo_strbuf sb = { .buf = buf, .len = buflen };
+ *
+ * OSMO_STRBUF_PRINTF(sb, "T minus");
+ * for (i = 10; i; i--)
+ * OSMO_STRBUF_PRINTF(sb, " %d", i);
+ * OSMO_STRBUF_PRINTF(sb, " ... Lift off!");
+ *
+ * return sb.chars_needed;
+ * }
+ *
+ * \param[inout] STRBUF A struct osmo_strbuf instance.
+ * \param[in] fmt Format string passed to snprintf.
+ * \param[in] args Additional arguments passed to snprintf, if any.
+ */
+#define OSMO_STRBUF_PRINTF(STRBUF, fmt, args...) \
+ OSMO_STRBUF_APPEND(STRBUF, snprintf, fmt, ##args)
+
/*! @} */