Provide a fallback implementation of [v]asprintf()

Some environments (e.g. Windows + MSVC) do not provide asprintf() or
vasprintf(). This tries to detect their presence, and provides suitable
fallback implementations when not available.

meson.build

@@ -102,6 +102,11 @@ endif
 if cc.has_header_symbol('string.h', 'strndup', prefix: system_ext_define)
     configh_data.set('HAVE_STRNDUP', 1)
 endif
+if cc.has_header_symbol('stdio.h', 'asprintf', prefix: system_ext_define)
+    configh_data.set('HAVE_ASPRINTF', 1)
+elif cc.has_header_symbol('stdio.h', 'vasprintf', prefix: system_ext_define)
+    configh_data.set('HAVE_VASPRINTF', 1)
+endif
 if cc.has_header_symbol('stdlib.h', 'secure_getenv', prefix: system_ext_define)
     configh_data.set('HAVE_SECURE_GETENV', 1)
 elif cc.has_header_symbol('stdlib.h', '__secure_getenv', prefix: system_ext_define)

src/utils.c

@@ -161,3 +161,48 @@ istrncmp(const char *a, const char *b, size_t n)
     }
     return 0;
 }
+
+#if !(defined(HAVE_ASPRINTF) && HAVE_ASPRINTF)
+int
+asprintf(char **strp, const char *fmt, ...)
+{
+    int ret;
+    va_list ap;
+    va_start(ap, fmt);
+    ret = vasprintf(strp, fmt, ap);
+    va_end(ap);
+    return ret;
+}
+
+# if !(defined(HAVE_VASPRINTF) && HAVE_VASPRINTF)
+int
+vasprintf(char **strp, const char *fmt, va_list ap)
+{
+    int ret;
+    char *buf;
+    va_list ap_copy;
+
+    /*
+     * The value of the va_list parameter is undefined after the call to
+     * vsnprintf() returns: pass a copy to make sure "ap" remains valid.
+     */
+    va_copy(ap_copy, ap);
+    ret = vsnprintf(NULL, 0, fmt, ap_copy);
+    va_end(ap_copy);
+
+    if (ret < 0)
+        return ret;
+
+    if (!(buf = malloc(ret + 1)))
+        return -1;
+
+    if ((ret = vsnprintf(buf, ret + 1, fmt, ap)) < 0) {
+        free(buf);
+        return ret;
+    }
+
+    *strp = buf;
+    return ret;
+}
+# endif /* !HAVE_VASPRINTF */
+#endif /* !HAVE_ASPRINTF */

src/utils.h

@@ -259,4 +259,12 @@ unmap_file(char *string, size_t size);
 #define ATTR_PACKED
 #endif
 
+#if !(defined(HAVE_ASPRINTF) && HAVE_ASPRINTF)
+int asprintf(char **strp, const char *fmt, ...) ATTR_PRINTF(2, 3);
+# if !(defined(HAVE_VASPRINTF) && HAVE_VASPRINTF)
+#  include <stdarg.h>
+int vasprintf(char **strp, const char *fmt, va_list ap);
+# endif /* !HAVE_VASPRINTF */
+#endif /* !HAVE_ASPRINTF */
+
 #endif /* UTILS_H */