// c:\ti\ccsv5\tools\compiler\arm_5.1.1\include\stdio.h
extern _CODE_ACCESS int     fputc(int _c, register FILE *_fp);
extern _CODE_ACCESS int     fputs(const char *_ptr, register FILE *_fp);

// user_interface.cpp
int std::fputc(int _c, register FILE *_fp) {
           ...
           return 1;
}

int std::fputs(const char *_ptr, register FILE *_fp) {
            ...
            return 1;
}

// c:\ARM\GNU Tools ARM Embedded\4.9 2014q4\arm-none-eabi\include\stdio.h
int    _EXFUN(fputc, (int, FILE *));
int    _EXFUN(fputs, (const char *__restrict, FILE *__restrict));

// user_interface.cpp
int fputc(int _c, register FILE *_fp) {
    ...
    return 1;
}

int fputs(const char *_ptr, register FILE *_fp) {
    ...
    return 1;
}

// user_interface.cpp
#ifdef __cplusplus
extern "C" {
#endif

ssize_t _write (int fd, const char* buf, size_t nbyte);

ssize_t _write (int fd __attribute__((unused)), const char* buf __attribute__((unused)), size_t nbyte __attribute__((unused))) {
    ...
    return 1;
}

#ifdef __cplusplus
}
#endif