File content as of revision 2:1dbaef45ae46:

#include "mbed.h"
#include "string.h"
#include "SDFileSystem.h"

#define BUFFNUM 50
#define CHARNUM 16

SDFileSystem sd(dp2, dp1, dp6, dp14, "sd");
Serial mainCP(dp16, dp15);

bool row = false;
int8_t col=0, cnt=0;
char data[2][BUFFNUM][CHARNUM] = {};
FILE *fp;

void _log();

int main() {
  printf("Hello World!\r\n");
  mkdir("/sd/mydir", 0777);
  fp = fopen("/sd/mydir/log.txt", "w");
  if(fp == NULL) error("Could not open file for write\r\n");
  fprintf(fp, "Hello fun SD Card World!\n");
  fclose(fp);
  mainCP.attach(&_log, Serial::RxIrq);
  while(1);
}

void _log(){//受信割り込み用．\n区切りで保存する．
  data[row][col][cnt] = mainCP.getc();
  cnt++;
  if(data[row][col][cnt]=='\n'){
    col++;
    cnt = 0;
  }else if(cnt==CHARNUM){
    memset(&data[row][col][0],'\0',CHARNUM);
    cnt = 0;
  }

  if(col==BUFFNUM){
    col = 0;
    row = !row;
    fp = fopen("/sd/mydir/log.txt", "a");
    for (int i = 0; i < BUFFNUM; i++){
      fprintf(fp,&data[!row][i][0]);
    }
    fclose(fp);
    memset(&data[row][0][0],'\0',BUFFNUM*CHARNUM);
  }
}