Add configuration to allow stdlib conflicts

Useful to implement or override standard library functions instead of
avoiding the name collisions

printf.c

@@ -36,14 +36,6 @@
 #include "printf.h"
 
 
-// define this globally (e.g. gcc -DPRINTF_INCLUDE_CONFIG_H ...) to include the
-// printf_config.h header file
-// default: undefined
-#ifdef PRINTF_INCLUDE_CONFIG_H
-#include "printf_config.h"
-#endif
-
-
 // 'ntoa' conversion buffer size, this must be big enough to hold one converted
 // numeric number including padded zeros (dynamically created on stack)
 // default: 32 byte

printf.h

@@ -36,6 +36,22 @@
 #include <stddef.h>
 
 
+// define this globally (e.g. gcc -DPRINTF_INCLUDE_CONFIG_H ...) to include the
+// printf_config.h header file
+// default: undefined
+#ifdef PRINTF_INCLUDE_CONFIG_H
+#include "printf_config.h"
+#endif
+
+
+// avoid conflicts with standard library APIs by using underscore-appended
+// function names
+// default: activated
+#ifndef PRINTF_ALLOW_STDLIB_CONFLICTS
+#define PRINTF_AVOID_STDLIB_CONFLICTS
+#endif
+
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -57,7 +73,11 @@ void _putchar(char character);
  * \param format A string that specifies the format of the output
  * \return The number of characters that are written into the array, not counting the terminating null character
  */
-#define printf printf_
+#ifdef PRINTF_AVOID_STDLIB_CONFLICTS
+#define printf  printf_
+#else
+#define printf_ printf
+#endif
 int printf_(const char* format, ...);
 
 
@@ -68,7 +88,11 @@ int printf_(const char* format, ...);
  * \param format A string that specifies the format of the output
  * \return The number of characters that are WRITTEN into the buffer, not counting the terminating null character
  */
-#define sprintf sprintf_
+#ifdef PRINTF_AVOID_STDLIB_CONFLICTS
+#define sprintf  sprintf_
+#else
+#define sprintf_ sprintf
+#endif
 int sprintf_(char* buffer, const char* format, ...);
 
 
@@ -82,8 +106,13 @@ int sprintf_(char* buffer, const char* format, ...);
  *         null character. A value equal or larger than count indicates truncation. Only when the returned value
  *         is non-negative and less than count, the string has been completely written.
  */
-#define snprintf  snprintf_
-#define vsnprintf vsnprintf_
+#ifdef PRINTF_AVOID_STDLIB_CONFLICTS
+#define snprintf   snprintf_
+#define vsnprintf  vsnprintf_
+#else
+#define snprintf_  snprintf
+#define vsnprintf_ vsnprintf
+#endif
 int  snprintf_(char* buffer, size_t count, const char* format, ...);
 int vsnprintf_(char* buffer, size_t count, const char* format, va_list va);
 
@@ -94,7 +123,11 @@ int vsnprintf_(char* buffer, size_t count, const char* format, va_list va);
  * \param va A value identifying a variable arguments list
  * \return The number of characters that are WRITTEN into the buffer, not counting the terminating null character
  */
-#define vprintf vprintf_
+#ifdef PRINTF_AVOID_STDLIB_CONFLICTS
+#define vprintf  vprintf_
+#else
+#define vprintf_ vprintf
+#endif
 int vprintf_(const char* format, va_list va);