Add %g support to mg_snprintf()

docs/README.md

@@ -2469,34 +2469,6 @@ char *buf[sizeof(data)/2];
 unsigned long val = mg_unhex(data, sizeof(data) - 1); // val is now 123
 ```
 
-
-### mg\_asprintf(), mg\_vasprintf()
-
-```c
-int mg_asprintf(char **buf, size_t size, const char *fmt, ...);
-int mg_vasprintf(char **buf, size_t size, const char *fmt, va_list ap);
-```
-
-Print message specified by printf-like format string `fmt` into a buffer
-pointed by `buf` of size `size`. If `size` is large enough to hold the whole
-message, then a message is stored in `*buf`. If it does not fit, then a large
-enough buffer is allocated to hold a message, and `buf` is changed to point to
-that buffer.
-
-Parameters:
-- `buf` - Pointer to pointer to output buffer
-- `size` - Pre-allocated buffer size
-- `fmt` - printf-like format string
-
-Return value: Number of bytes printed
-
-Usage example:
-
-```c
-char buf[1024], *pbuf = &buf;
-mg_asprintf(&pbuf, sizeof(buf), "Hello, %s!", "world"); // buf is now "Hello, world!"
-```
-
 ### mg\_snprintf(), mg\_vsnprintf()
 ```c
 size_t mg_snprintf(char *buf, size_t len, const char *fmt, ...);
@@ -2521,10 +2493,12 @@ Supported format specifiers:
 - `hhd`, `hd`, `d`, `ld`, `lld` - for `char`, `short`, `int`, `long`, `int64_t`
 - `hhu`, `hu`, `u`, `lu`, `llu` - same but for unsigned variants
 - `hhx`, `hx`, `x`, `lx`, `llx` - same, unsigned and hex output
-- `s` - `for char *`
+- `s` - for `char *`
-- `Q` - `for char *`, outputs double-quoted JSON-escaped string (extension)
+- `Q` - for `char *`, outputs double-quoted JSON-escaped string (extension)
-- `c` - `for char`
+- `M` - for `size_t (*)(char *, size_t, va_list *)`, calls another print function (extension)
-- `%` - `the `%` character itself
+- `g`, `f` - for `double`
+- `c` - for `char`
+- `%` - the `%` character itself
 - `p` - for any pointer, prints `0x.....` hex value
 - `%X.Y` - optional width and precision modifiers
 - `%.*` - optional precision modifier specified as `int` argument
@@ -2539,6 +2513,64 @@ mg_snprintf(buf, sizeof(buf), "%.2s", "abcdef");        // ab
 mg_snprintf(buf, sizeof(buf), "%.*s", 2, "abcdef");     // ab
 mg_snprintf(buf, sizeof(buf), "%05x", 123);             // 00123
 mg_snprintf(buf, sizeof(buf), "%%-%3s", "a");           // %-  a
+mg_snprintf(buf, sizeof(buf), "hi, %Q", "a");           // hi, "a"
+mg_snprintf(buf, sizeof(buf), "r: %M, %d", f,1,2,7);    // r: 3, 7
+
+// Printing sub-function for %M specifier. Grabs two int parameters
+size_t f(char *buf, size_t len, va_list *ap) {
+  int a = va_arg(*ap, int);
+  int a = va_arg(*ap, int);
+  return mg_snprintf(buf, len, "%d", a + b);
+}
+```
+
+### mg\_asprintf(), mg\_vasprintf()
+
+```c
+int mg_asprintf(char **buf, size_t size, const char *fmt, ...);
+int mg_vasprintf(char **buf, size_t size, const char *fmt, va_list ap);
+```
+
+Print message specified by printf-like format string `fmt` into a buffer
+pointed by `buf` of size `size`. If `size` is large enough to hold the whole
+message, then a message is stored in `*buf`. If it does not fit, then a large
+enough buffer is allocated to hold a message, and `buf` is changed to point to
+that buffer.
+
+Parameters:
+- `buf` - Pointer to pointer to output buffer
+- `size` - Pre-allocated buffer size
+- `fmt` - printf-like format string
+
+Return value: Number of bytes printed
+
+Usage example:
+
+```c
+char buf[16], *pbuf = buf;
+mg_asprintf(&pbuf, sizeof(buf), "Hello, %s!", "world"); // buf is now "Hello, world!"
+if (pbuf != buf) free(pbuf);
+```
+
+### mg\_mprintf(), mg\_vmprintf()
+
+```c
+char *mg_mprintf(const char *fmt, ...);
+char *mg_vmprintf(const char *fmt, va_list ap);
+```
+
+Print message into an allocated memory buffer. Caller must free the result.
+
+Parameters:
+- `fmt` - printf-like format string
+
+Return value: allocated memory buffer
+
+Usage example:
+
+```c
+char *msg = mg_mprintf("Double quoted string: %Q!", "hi");
+free(msg);
 ```
 
 ### mg\_to64()

mongoose.c

@@ -5066,7 +5066,7 @@ size_t mg_snprintf(char *buf, size_t len, const char *fmt, ...) {
   va_list ap;
   size_t n;
   va_start(ap, fmt);
-  n = mg_vsnprintf(buf, len, fmt, ap);
+  n = mg_vsnprintf(buf, len, fmt, &ap);
   va_end(ap);
   return n;
 }
@@ -5107,7 +5107,7 @@ size_t mg_vasprintf(char **buf, size_t size, const char *fmt, va_list ap) {
   size_t len;
 
   va_copy(ap_copy, ap);
-  len = mg_vsnprintf(*buf, size, fmt, ap_copy);
+  len = mg_vsnprintf(*buf, size, fmt, &ap_copy);
   va_end(ap_copy);
 
   if (len >= size) {
@@ -5116,7 +5116,7 @@ size_t mg_vasprintf(char **buf, size_t size, const char *fmt, va_list ap) {
       len = 0;
     } else {
       va_copy(ap_copy, ap);
-      len = mg_vsnprintf(*buf, len + 1, fmt, ap_copy);
+      len = mg_vsnprintf(*buf, len + 1, fmt, &ap_copy);
       va_end(ap_copy);
     }
   }
@@ -5133,6 +5133,21 @@ size_t mg_asprintf(char **buf, size_t size, const char *fmt, ...) {
   return ret;
 }
 
+char *mg_vmprintf(const char *fmt, va_list ap) {
+  char *s = NULL;
+  mg_vasprintf(&s, 0, fmt, ap);
+  return s;
+}
+
+char *mg_mprintf(const char *fmt, ...) {
+  char *s = NULL;
+  va_list ap;
+  va_start(ap, fmt);
+  mg_vasprintf(&s, 0, fmt, ap);
+  va_end(ap);
+  return s;
+}
+
 uint64_t mg_tou64(struct mg_str str) {
   uint64_t result = 0;
   size_t i = 0;
@@ -5218,7 +5233,9 @@ static size_t mg_copyq(char *buf, size_t len, size_t n, char *p, size_t k) {
   return j + extra;
 }
 
-size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list ap) {
+typedef size_t (*mg_spfn_t)(char *, size_t, va_list *);
+
+size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list *ap) {
   size_t i = 0, n = 0;
   while (fmt[i] != '\0') {
     if (fmt[i] == '%') {
@@ -5231,7 +5248,7 @@ size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list ap) {
       if (c == '.') {
         c = fmt[++i];
         if (c == '*') {
-          pr = (size_t) va_arg(ap, int);
+          pr = (size_t) va_arg(*ap, int);
           c = fmt[++i];
         } else {
           pr = 0;
@@ -5244,18 +5261,23 @@ size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list ap) {
         if (c == 'l') is_long++, c = fmt[++i];
       }
       if (c == 'p') x = 1, is_long = 1;
-      if (c == 'd' || c == 'u' || c == 'x' || c == 'X' || c == 'p') {
+      if (c == 'd' || c == 'u' || c == 'x' || c == 'X' || c == 'p' ||
+          c == 'g') {
         bool s = (c == 'd'), h = (c == 'x' || c == 'X' || c == 'p');
-        char tmp[30];
+        char tmp[40];
         size_t xl = x ? 2 : 0;
-        if (is_long == 2) {
+        if (c == 'g' || c == 'f') {
-          int64_t v = va_arg(ap, int64_t);
+          double v = va_arg(*ap, double);
+          if (pr == ~0U) pr = 6;
+          k = mg_dtoa(tmp, sizeof(tmp), v, (int) pr);
+        } else if (is_long == 2) {
+          int64_t v = va_arg(*ap, int64_t);
           k = mg_lld(tmp, v, s, h);
         } else if (is_long == 1) {
-          long v = va_arg(ap, long);
+          long v = va_arg(*ap, long);
           k = mg_lld(tmp, s ? (int64_t) v : (int64_t) (unsigned long) v, s, h);
         } else {
-          int v = va_arg(ap, int);
+          int v = va_arg(*ap, int);
           k = mg_lld(tmp, s ? (int64_t) v : (int64_t) (unsigned) v, s, h);
         }
         for (j = 0; j < xl && w > 0; j++) w--;
@@ -5267,12 +5289,15 @@ size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list ap) {
         n += mg_copys(buf, len, n, tmp, k);
         for (j = 0; pad == ' ' && minus && k < w && j + k < w; j++)
           n += mg_copys(buf, len, n, &pad, 1);
+      } else if (c == 'M') {
+        mg_spfn_t fn = va_arg(*ap, mg_spfn_t);
+        n += fn(buf + n, n < len ? len - n : 0, ap);
       } else if (c == 'c') {
-        int p = va_arg(ap, int);
+        int p = va_arg(*ap, int);
         if (n < len) buf[n] = (char) p;
         n++;
       } else if (c == 's' || c == 'Q') {
-        char *p = va_arg(ap, char *);
+        char *p = va_arg(*ap, char *);
         size_t (*fn)(char *, size_t, size_t, char *, size_t) =
             c == 's' ? mg_copys : mg_copyq;
         if (pr == ~0U) pr = p == NULL ? 0 : strlen(p);
@@ -5346,6 +5371,94 @@ double mg_atod(const char *p, int len, int *numlen) {
   return d;
 }
 
+static int addexp(char *buf, int e, int sign) {
+  int n = 0;
+  buf[n++] = 'e';
+  buf[n++] = (char) sign;
+  if (e > 400) return 0;
+  if (e < 10) buf[n++] = '0';
+  if (e >= 100) buf[n++] = (char) (e / 100 + '0'), e -= 100 * (e / 100);
+  if (e >= 10) buf[n++] = (char) (e / 10 + '0'), e -= 10 * (e / 10);
+  buf[n++] = (char) (e + '0');
+  return n;
+}
+
+static int xisinf(double x) {
+  union {
+    double f;
+    uint64_t u;
+  } ieee754 = {x};
+  return ((unsigned) (ieee754.u >> 32) & 0x7fffffff) == 0x7ff00000 &&
+         ((unsigned) ieee754.u == 0);
+}
+
+static int xisnan(double x) {
+  union {
+    double f;
+    uint64_t u;
+  } ieee754 = {x};
+  return ((unsigned) (ieee754.u >> 32) & 0x7fffffff) +
+             ((unsigned) ieee754.u != 0) >
+         0x7ff00000;
+}
+
+size_t mg_dtoa(char *dst, size_t dstlen, double d, int width) {
+  char buf[40];
+  int i, s = 0, n = 0, e = 0;
+  double t, mul, saved;
+  if (d == 0.0) return mg_snprintf(dst, dstlen, "%s", "0");
+  if (xisinf(d)) return mg_snprintf(dst, dstlen, "%s", d > 0 ? "inf" : "-inf");
+  if (xisnan(d)) return mg_snprintf(dst, dstlen, "%s", "nan");
+  if (d < 0.0) d = -d, buf[s++] = '-';
+
+  // Round
+  saved = d;
+  mul = 1.0;
+  while (d >= 10.0 && d / mul >= 10.0) mul *= 10.0;
+  while (d <= 1.0 && d / mul <= 1.0) mul /= 10.0;
+  for (i = 0, t = mul * 5; i < width; i++) t /= 10.0;
+  d += t;
+  // Calculate exponent, and 'mul' for scientific representation
+  mul = 1.0;
+  while (d >= 10.0 && d / mul >= 10.0) mul *= 10.0, e++;
+  while (d < 1.0 && d / mul < 1.0) mul /= 10.0, e--;
+  // printf(" --> %g %d %g %g\n", saved, e, t, mul);
+
+  if (e >= width) {
+    n = (int) mg_dtoa(buf, sizeof(buf), saved / mul, width);
+    // printf(" --> %.*g %d [%.*s]\n", 10, d / t, e, n, buf);
+    n += addexp(buf + s + n, e, '+');
+    return mg_snprintf(dst, dstlen, "%.*s", n, buf);
+  } else if (e <= -width) {
+    n = (int) mg_dtoa(buf, sizeof(buf), saved / mul, width);
+    // printf(" --> %.*g %d [%.*s]\n", 10, d / mul, e, n, buf);
+    n += addexp(buf + s + n, -e, '-');
+    return mg_snprintf(dst, dstlen, "%.*s", n, buf);
+  } else {
+    for (i = 0, t = mul; t >= 1.0 && s + n < (int) sizeof(buf); i++) {
+      int ch = (int) (d / t);
+      if (n > 0 || ch > 0) buf[s + n++] = (char) (ch + '0');
+      d -= ch * t;
+      t /= 10.0;
+    }
+    // printf(" --> [%g] -> %g %g (%d) [%.*s]\n", saved, d, t, n, s + n, buf);
+    if (n == 0) buf[s++] = '0';
+    while (t >= 1.0 && n + s < (int) sizeof(buf)) buf[n++] = '0', t /= 10.0;
+    if (s + n < (int) sizeof(buf)) buf[n + s++] = '.';
+    // printf(" 1--> [%g] -> [%.*s]\n", saved, s + n, buf);
+    for (i = 0, t = 0.1; s + n < (int) sizeof(buf) && n < width; i++) {
+      int ch = (int) (d / t);
+      buf[s + n++] = (char) (ch + '0');
+      d -= ch * t;
+      t /= 10.0;
+    }
+  }
+  while (n > 0 && buf[s + n - 1] == '0') n--;  // Trim trailing zeros
+  if (n > 0 && buf[s + n - 1] == '.') n--;     // Trim trailing dot
+  buf[s + n] = '\0';
+  return mg_snprintf(dst, dstlen, "%s", buf);
+}
+
 #ifdef MG_ENABLE_LINES
 #line 1 "src/timer.c"
 #endif

mongoose.h

@@ -715,18 +715,21 @@ bool mg_match(struct mg_str str, struct mg_str pattern, struct mg_str *caps);
 bool mg_globmatch(const char *pattern, size_t plen, const char *s, size_t n);
 bool mg_commalist(struct mg_str *s, struct mg_str *k, struct mg_str *v);
 bool mg_split(struct mg_str *s, struct mg_str *k, struct mg_str *v, char delim);
-size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list ap);
+size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list *ap);
 size_t mg_snprintf(char *, size_t, const char *fmt, ...);
 char *mg_hex(const void *buf, size_t len, char *dst);
 void mg_unhex(const char *buf, size_t len, unsigned char *to);
 unsigned long mg_unhexn(const char *s, size_t len);
 size_t mg_asprintf(char **, size_t, const char *fmt, ...);
 size_t mg_vasprintf(char **buf, size_t size, const char *fmt, va_list ap);
+char *mg_mprintf(const char *fmt, ...);
+char *mg_vmprintf(const char *fmt, va_list ap);
 int mg_check_ip_acl(struct mg_str acl, uint32_t remote_ip);
 int64_t mg_to64(struct mg_str str);
 uint64_t mg_tou64(struct mg_str str);
 size_t mg_lld(char *buf, int64_t val, bool is_signed, bool is_hex);
 double mg_atod(const char *buf, int len, int *numlen);
+size_t mg_dtoa(char *buf, size_t len, double d, int width);
 
 
 

src/str.c

@@ -164,7 +164,7 @@ size_t mg_snprintf(char *buf, size_t len, const char *fmt, ...) {
   va_list ap;
   size_t n;
   va_start(ap, fmt);
-  n = mg_vsnprintf(buf, len, fmt, ap);
+  n = mg_vsnprintf(buf, len, fmt, &ap);
   va_end(ap);
   return n;
 }
@@ -205,7 +205,7 @@ size_t mg_vasprintf(char **buf, size_t size, const char *fmt, va_list ap) {
   size_t len;
 
   va_copy(ap_copy, ap);
-  len = mg_vsnprintf(*buf, size, fmt, ap_copy);
+  len = mg_vsnprintf(*buf, size, fmt, &ap_copy);
   va_end(ap_copy);
 
   if (len >= size) {
@@ -214,7 +214,7 @@ size_t mg_vasprintf(char **buf, size_t size, const char *fmt, va_list ap) {
       len = 0;
     } else {
       va_copy(ap_copy, ap);
-      len = mg_vsnprintf(*buf, len + 1, fmt, ap_copy);
+      len = mg_vsnprintf(*buf, len + 1, fmt, &ap_copy);
       va_end(ap_copy);
     }
   }
@@ -231,6 +231,21 @@ size_t mg_asprintf(char **buf, size_t size, const char *fmt, ...) {
   return ret;
 }
 
+char *mg_vmprintf(const char *fmt, va_list ap) {
+  char *s = NULL;
+  mg_vasprintf(&s, 0, fmt, ap);
+  return s;
+}
+
+char *mg_mprintf(const char *fmt, ...) {
+  char *s = NULL;
+  va_list ap;
+  va_start(ap, fmt);
+  mg_vasprintf(&s, 0, fmt, ap);
+  va_end(ap);
+  return s;
+}
+
 uint64_t mg_tou64(struct mg_str str) {
   uint64_t result = 0;
   size_t i = 0;
@@ -316,7 +331,9 @@ static size_t mg_copyq(char *buf, size_t len, size_t n, char *p, size_t k) {
   return j + extra;
 }
 
-size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list ap) {
+typedef size_t (*mg_spfn_t)(char *, size_t, va_list *);
+
+size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list *ap) {
   size_t i = 0, n = 0;
   while (fmt[i] != '\0') {
     if (fmt[i] == '%') {
@@ -329,7 +346,7 @@ size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list ap) {
       if (c == '.') {
         c = fmt[++i];
         if (c == '*') {
-          pr = (size_t) va_arg(ap, int);
+          pr = (size_t) va_arg(*ap, int);
           c = fmt[++i];
         } else {
           pr = 0;
@@ -342,18 +359,23 @@ size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list ap) {
         if (c == 'l') is_long++, c = fmt[++i];
       }
       if (c == 'p') x = 1, is_long = 1;
-      if (c == 'd' || c == 'u' || c == 'x' || c == 'X' || c == 'p') {
+      if (c == 'd' || c == 'u' || c == 'x' || c == 'X' || c == 'p' ||
+          c == 'g') {
         bool s = (c == 'd'), h = (c == 'x' || c == 'X' || c == 'p');
-        char tmp[30];
+        char tmp[40];
         size_t xl = x ? 2 : 0;
-        if (is_long == 2) {
+        if (c == 'g' || c == 'f') {
-          int64_t v = va_arg(ap, int64_t);
+          double v = va_arg(*ap, double);
+          if (pr == ~0U) pr = 6;
+          k = mg_dtoa(tmp, sizeof(tmp), v, (int) pr);
+        } else if (is_long == 2) {
+          int64_t v = va_arg(*ap, int64_t);
           k = mg_lld(tmp, v, s, h);
         } else if (is_long == 1) {
-          long v = va_arg(ap, long);
+          long v = va_arg(*ap, long);
           k = mg_lld(tmp, s ? (int64_t) v : (int64_t) (unsigned long) v, s, h);
         } else {
-          int v = va_arg(ap, int);
+          int v = va_arg(*ap, int);
           k = mg_lld(tmp, s ? (int64_t) v : (int64_t) (unsigned) v, s, h);
         }
         for (j = 0; j < xl && w > 0; j++) w--;
@@ -365,12 +387,15 @@ size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list ap) {
         n += mg_copys(buf, len, n, tmp, k);
         for (j = 0; pad == ' ' && minus && k < w && j + k < w; j++)
           n += mg_copys(buf, len, n, &pad, 1);
+      } else if (c == 'M') {
+        mg_spfn_t fn = va_arg(*ap, mg_spfn_t);
+        n += fn(buf + n, n < len ? len - n : 0, ap);
       } else if (c == 'c') {
-        int p = va_arg(ap, int);
+        int p = va_arg(*ap, int);
         if (n < len) buf[n] = (char) p;
         n++;
       } else if (c == 's' || c == 'Q') {
-        char *p = va_arg(ap, char *);
+        char *p = va_arg(*ap, char *);
         size_t (*fn)(char *, size_t, size_t, char *, size_t) =
             c == 's' ? mg_copys : mg_copyq;
         if (pr == ~0U) pr = p == NULL ? 0 : strlen(p);
@@ -443,3 +468,91 @@ double mg_atod(const char *p, int len, int *numlen) {
   if (numlen != NULL) *numlen = i;
   return d;
 }
+
+static int addexp(char *buf, int e, int sign) {
+  int n = 0;
+  buf[n++] = 'e';
+  buf[n++] = (char) sign;
+  if (e > 400) return 0;
+  if (e < 10) buf[n++] = '0';
+  if (e >= 100) buf[n++] = (char) (e / 100 + '0'), e -= 100 * (e / 100);
+  if (e >= 10) buf[n++] = (char) (e / 10 + '0'), e -= 10 * (e / 10);
+  buf[n++] = (char) (e + '0');
+  return n;
+}
+
+static int xisinf(double x) {
+  union {
+    double f;
+    uint64_t u;
+  } ieee754 = {x};
+  return ((unsigned) (ieee754.u >> 32) & 0x7fffffff) == 0x7ff00000 &&
+         ((unsigned) ieee754.u == 0);
+}
+
+static int xisnan(double x) {
+  union {
+    double f;
+    uint64_t u;
+  } ieee754 = {x};
+  return ((unsigned) (ieee754.u >> 32) & 0x7fffffff) +
+             ((unsigned) ieee754.u != 0) >
+         0x7ff00000;
+}
+
+size_t mg_dtoa(char *dst, size_t dstlen, double d, int width) {
+  char buf[40];
+  int i, s = 0, n = 0, e = 0;
+  double t, mul, saved;
+  if (d == 0.0) return mg_snprintf(dst, dstlen, "%s", "0");
+  if (xisinf(d)) return mg_snprintf(dst, dstlen, "%s", d > 0 ? "inf" : "-inf");
+  if (xisnan(d)) return mg_snprintf(dst, dstlen, "%s", "nan");
+  if (d < 0.0) d = -d, buf[s++] = '-';
+
+  // Round
+  saved = d;
+  mul = 1.0;
+  while (d >= 10.0 && d / mul >= 10.0) mul *= 10.0;
+  while (d <= 1.0 && d / mul <= 1.0) mul /= 10.0;
+  for (i = 0, t = mul * 5; i < width; i++) t /= 10.0;
+  d += t;
+  // Calculate exponent, and 'mul' for scientific representation
+  mul = 1.0;
+  while (d >= 10.0 && d / mul >= 10.0) mul *= 10.0, e++;
+  while (d < 1.0 && d / mul < 1.0) mul /= 10.0, e--;
+  // printf(" --> %g %d %g %g\n", saved, e, t, mul);
+
+  if (e >= width) {
+    n = (int) mg_dtoa(buf, sizeof(buf), saved / mul, width);
+    // printf(" --> %.*g %d [%.*s]\n", 10, d / t, e, n, buf);
+    n += addexp(buf + s + n, e, '+');
+    return mg_snprintf(dst, dstlen, "%.*s", n, buf);
+  } else if (e <= -width) {
+    n = (int) mg_dtoa(buf, sizeof(buf), saved / mul, width);
+    // printf(" --> %.*g %d [%.*s]\n", 10, d / mul, e, n, buf);
+    n += addexp(buf + s + n, -e, '-');
+    return mg_snprintf(dst, dstlen, "%.*s", n, buf);
+  } else {
+    for (i = 0, t = mul; t >= 1.0 && s + n < (int) sizeof(buf); i++) {
+      int ch = (int) (d / t);
+      if (n > 0 || ch > 0) buf[s + n++] = (char) (ch + '0');
+      d -= ch * t;
+      t /= 10.0;
+    }
+    // printf(" --> [%g] -> %g %g (%d) [%.*s]\n", saved, d, t, n, s + n, buf);
+    if (n == 0) buf[s++] = '0';
+    while (t >= 1.0 && n + s < (int) sizeof(buf)) buf[n++] = '0', t /= 10.0;
+    if (s + n < (int) sizeof(buf)) buf[n + s++] = '.';
+    // printf(" 1--> [%g] -> [%.*s]\n", saved, s + n, buf);
+    for (i = 0, t = 0.1; s + n < (int) sizeof(buf) && n < width; i++) {
+      int ch = (int) (d / t);
+      buf[s + n++] = (char) (ch + '0');
+      d -= ch * t;
+      t /= 10.0;
+    }
+  }
+  while (n > 0 && buf[s + n - 1] == '0') n--;  // Trim trailing zeros
+  if (n > 0 && buf[s + n - 1] == '.') n--;     // Trim trailing dot
+  buf[s + n] = '\0';
+  return mg_snprintf(dst, dstlen, "%s", buf);
+}

src/str.h

@@ -31,15 +31,18 @@ bool mg_match(struct mg_str str, struct mg_str pattern, struct mg_str *caps);
 bool mg_globmatch(const char *pattern, size_t plen, const char *s, size_t n);
 bool mg_commalist(struct mg_str *s, struct mg_str *k, struct mg_str *v);
 bool mg_split(struct mg_str *s, struct mg_str *k, struct mg_str *v, char delim);
-size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list ap);
+size_t mg_vsnprintf(char *buf, size_t len, const char *fmt, va_list *ap);
 size_t mg_snprintf(char *, size_t, const char *fmt, ...);
 char *mg_hex(const void *buf, size_t len, char *dst);
 void mg_unhex(const char *buf, size_t len, unsigned char *to);
 unsigned long mg_unhexn(const char *s, size_t len);
 size_t mg_asprintf(char **, size_t, const char *fmt, ...);
 size_t mg_vasprintf(char **buf, size_t size, const char *fmt, va_list ap);
+char *mg_mprintf(const char *fmt, ...);
+char *mg_vmprintf(const char *fmt, va_list ap);
 int mg_check_ip_acl(struct mg_str acl, uint32_t remote_ip);
 int64_t mg_to64(struct mg_str str);
 uint64_t mg_tou64(struct mg_str str);
 size_t mg_lld(char *buf, int64_t val, bool is_signed, bool is_hex);
 double mg_atod(const char *buf, int len, int *numlen);
+size_t mg_dtoa(char *buf, size_t len, double d, int width);

test/unit_test.c

@@ -1,3 +1,5 @@
+#include "float.h"  // For DBL_EPSILON and HUGE_VAL
+#include "math.h"
 #include "mongoose.h"
 
 static int s_num_tests = 0;
@@ -1354,10 +1356,10 @@ static bool sn(const char *fmt, ...) {
   n = (size_t) vsnprintf(buf2, sizeof(buf2), fmt, ap);
   va_end(ap);
   va_start(ap, fmt);
-  n1 = mg_vsnprintf(buf, sizeof(buf), fmt, ap);
+  n1 = mg_vsnprintf(buf, sizeof(buf), fmt, &ap);
   va_end(ap);
   va_start(ap, fmt);
-  n2 = mg_vsnprintf(tmp, 0, fmt, ap);
+  n2 = mg_vsnprintf(tmp, 0, fmt, &ap);
   va_end(ap);
   result = n1 == n2 && n1 == n && strcmp(buf, buf2) == 0;
   if (!result)
@@ -1372,6 +1374,12 @@ static bool sccmp(const char *s1, const char *s2, int expected) {
   return n1 == expected;
 }
 
+static size_t pf1(char *buf, size_t len, va_list *ap) {
+  int a = va_arg(*ap, int);
+  int b = va_arg(*ap, int);
+  return mg_snprintf(buf, len, "%d", a + b);
+}
+
 static void test_str(void) {
   struct mg_str s = mg_strdup(mg_str("a"));
   ASSERT(mg_strcmp(s, mg_str("a")) == 0);
@@ -1447,7 +1455,7 @@ static void test_str(void) {
 
   // Non-standard formatting
   {
-    char buf[100];
+    char buf[100], *p;
     const char *expected;
 
     expected = "\"\"";
@@ -1465,6 +1473,71 @@ static void test_str(void) {
     expected = "\"abc\"";
     mg_snprintf(buf, sizeof(buf), "%.*Q", 3, "abcdef");
     ASSERT(strcmp(buf, expected) == 0);
+
+    p = mg_mprintf("[%s,%M,%s]", "null", pf1, 2, 3, "hi");
+    printf("-> %s\n", p);
+    ASSERT(strcmp(p, "[null,5,hi]") == 0);
+    free(p);
+  }
+
+  {
+    char tmp[40];
+#define DBLWIDTH(a, b) a, b
+#define TESTDOUBLE(fmt_, num_, res_)                                   \
+  do {                                                                 \
+    const char *N = #num_;                                             \
+    size_t n = mg_snprintf(tmp, sizeof(tmp), fmt_, num_);              \
+    printf("[%s] [%s] -> [%s] [%.*s]\n", fmt_, N, res_, (int) n, tmp); \
+    ASSERT(n == strlen(res_));                                         \
+    ASSERT(strcmp(tmp, res_) == 0);                                    \
+  } while (0)
+
+    TESTDOUBLE("%g", 0.0, "0");
+    TESTDOUBLE("%g", 0.123, "0.123");
+    TESTDOUBLE("%g", 0.00123, "0.00123");
+    TESTDOUBLE("%g", 0.123456333, "0.123456");
+    TESTDOUBLE("%g", 123.0, "123");
+    TESTDOUBLE("%g", 11.5454, "11.5454");
+    TESTDOUBLE("%g", 11.0001, "11.0001");
+    TESTDOUBLE("%g", 0.999, "0.999");
+    TESTDOUBLE("%g", 0.999999, "0.999999");
+    TESTDOUBLE("%g", 0.9999999, "1");
+    TESTDOUBLE("%g", 10.9, "10.9");
+    TESTDOUBLE("%g", 10.01, "10.01");
+    TESTDOUBLE("%g", 1.0, "1");
+    TESTDOUBLE("%g", 10.0, "10");
+    TESTDOUBLE("%g", 100.0, "100");
+    TESTDOUBLE("%g", 1000.0, "1000");
+    TESTDOUBLE("%g", 10000.0, "10000");
+    TESTDOUBLE("%g", 100000.0, "100000");
+    TESTDOUBLE("%g", 1000000.0, "1e+06");
+    TESTDOUBLE("%g", 10000000.0, "1e+07");
+    TESTDOUBLE("%g", 100000001.0, "1e+08");
+    TESTDOUBLE("%g", 10.5454, "10.5454");
+    TESTDOUBLE("%g", 999999.0, "999999");
+    TESTDOUBLE("%g", 9999999.0, "1e+07");
+    TESTDOUBLE("%g", 44556677.0, "4.45567e+07");
+    TESTDOUBLE("%g", 1234567.2, "1.23457e+06");
+    TESTDOUBLE("%g", -987.65432, "-987.654");
+    TESTDOUBLE("%g", 0.0000000001, "1e-10");
+    TESTDOUBLE("%g", 2.34567e-57, "2.34567e-57");
+    TESTDOUBLE("%.*g", DBLWIDTH(7, 9999999.0), "9999999");
+    TESTDOUBLE("%.*g", DBLWIDTH(10, 0.123456333), "0.123456333");
+    TESTDOUBLE("%g", 123.456222, "123.456");
+    TESTDOUBLE("%.*g", DBLWIDTH(10, 123.456222), "123.456222");
+    TESTDOUBLE("%g", 600.1234, "600.123");
+    TESTDOUBLE("%g", -600.1234, "-600.123");
+    TESTDOUBLE("%g", 599.1234, "599.123");
+    TESTDOUBLE("%g", -599.1234, "-599.123");
+
+#ifndef _WIN32
+    TESTDOUBLE("%g", (double) INFINITY, "inf");
+    TESTDOUBLE("%g", (double) -INFINITY, "-inf");
+    TESTDOUBLE("%g", (double) NAN, "nan");
+#else
+    TESTDOUBLE("%g", HUGE_VAL, "inf");
+    TESTDOUBLE("%g", -HUGE_VAL, "-inf");
+#endif
   }
 }