GCC+Keil printf cann't retarget to uart

HI,I Want use gun in keil.now,Can't output debug log to uart.I have implement fputc like this:
int fputc(int ch, FILE *f)
{ if (s_debugInit) { UART_SendData(DEBUG_UART, ch); } return ch;
} but when I call printf,don't call fputc function.what's worry?Thanks for your help!

I have add syscalls.c in my keil project.

syscalls.c

#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>

/***************************************************************************/

int _read_r (struct _reent *r, int file, char * ptr, int len)
{ r = r; file = file; ptr = ptr; len = len;

errno = EINVAL; return -1;
}

/***************************************************************************/

int _lseek_r (struct _reent *r, int file, int ptr, int dir)
{ r = r; file = file; ptr = ptr; dir = dir;

return 0;
}

/***************************************************************************/

int _write_r (struct _reent *r, int file, char * ptr, int len)
{ r = r; file = file; ptr = ptr;

#if 0 int index;

/* For example, output string by UART */ for(index=0; index<len; index++) { if (ptr[index] == '\n') { uart_putc('\r'); }

uart_putc(ptr[index]); }
#endif

return len;
}

/***************************************************************************/

int _close_r (struct _reent *r, int file)
{ return 0;
}

/***************************************************************************/

/* Register name faking - works in collusion with the linker. */
register char * stack_ptr asm ("sp");

caddr_t _sbrk_r (struct _reent *r, int incr)
{ extern char end asm ("end"); /* Defined by the linker. */ static char * heap_end; char * prev_heap_end;

if (heap_end == NULL) heap_end = & end;

prev_heap_end = heap_end;

if (heap_end + incr > stack_ptr) { /* Some of the libstdc++-v3 tests rely upon detecting out of memory errors, so do not abort here. */
#if 0 extern void abort (void);

_write (1, "_sbrk: Heap and stack collision\n", 32);

abort ();
#else errno = ENOMEM; return (caddr_t) -1;
#endif }

heap_end += incr;

return (caddr_t) prev_heap_end;
}

/***************************************************************************/

int _fstat_r (struct _reent *r, int file, struct stat * st)
{ r = r; file = file;

memset (st, 0, sizeof (* st)); st->st_mode = S_IFCHR; return 0;
}

/***************************************************************************/

int _isatty_r(struct _reent *r, int fd)
{ r = r; fd = fd;

return 1;
}

/*** EOF ***/