开发那些事--不要过度依赖snprintf/sprintf

开发那些事–不要过度依赖snprintf/sprintf

将数据按照指定format输出到buffer中，往往会用snprintf/sprintf（推荐前者）。但各个场景都习惯性的用snprintf/sprintf却并不是什么好事。

snprintf/sprintf性能问题

在分析团队项目性能时候，发现将大量数据以文本TEXT方式返回给客户端时，耗时非常多，且和类型有关，DATETIME > INT > varchar。perf图分析后发现，snprintf占用了很多时间。因为DATETIME调用4次，INT调用1次，varchar 0次。DATETIME和INT类型调用都主要是将INT转换为TEXT文本方式。

对于INT值打印，写ltoa函数和snprintf做性能对比。

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <sys/time.h>

char *ltoa10(int64_t val,char *dst, const bool is_signed)
{
  char buffer[65];
  uint64_t uval = (uint64_t) val;

  if (is_signed)
  {
    if (val < 0)
    {
      *dst++ = '-';
      uval = (uint64_t)0 - uval;
    }
  }

  register char *p = &buffer[sizeof(buffer)-1];
  *p = '\0';
  int64_t new_val= (int64_t) (uval / 10);
  *--p = (char)('0'+ (uval - (uint64_t) new_val * 10));
  val = new_val;

  while (val != 0)
  {
    new_val=val/10;
    *--p = (char)('0' + (val-new_val*10));
    val= new_val;
  }
  while ((*dst++ = *p++) != 0) ;
  return dst-1;
}

const int64_t INT_NUM = 10000;
int main()
{
  //init num
  int value[INT_NUM];
  for (int64_t idx = 0; idx < INT_NUM; ++idx) {
    value[idx] = random();
  }
  const int64_t MAX_CONST_LENGTH = 22;
  char str[MAX_CONST_LENGTH];

  struct timeval t_start, t_end;
  long start, end;

  //get snprintf time
  gettimeofday(&t_start, NULL);
  start = t_start.tv_sec * 1000000 + t_start.tv_usec;
  for (int64_t idx = 0; idx < INT_NUM; ++idx) {
    snprintf(str, MAX_CONST_LENGTH, "%ld", value[idx]);
  }
  gettimeofday(&t_end, NULL);
  end = t_end.tv_sec * 1000000 + t_end.tv_usec;
  printf("snprintf time:%ld\n", end - start);

  //get ltoa10 time
  gettimeofday(&t_start, NULL);
  start = t_start.tv_sec * 1000000 + t_start.tv_usec;
  for (int64_t idx = 0; idx < INT_NUM; ++idx) {
    ltoa10(value[idx], str, true);
  }
  gettimeofday(&t_end, NULL);
  end = t_end.tv_sec * 1000000 + t_end.tv_usec;
  printf("ltoa time:%ld\n", end - start);
  return 0;
}

O2编译执行，实验结果如下，ltoa性能是snprintf的1倍以上：

snprintf time:2053
ltoa time:833

（一篇integer to string conversion in C++各种方法比较的文章 http://zverovich.net/2013/09/07/integer-to-string-conversion-in-cplusplus.html）

对于DATETIME类型，其实只是需要输出xxxx-mm-dd hh-mm–ss.uuuuuu格式的数据。要输出的位数已经确定，可以使用更简单的方式例如两位的moth/day，或者可能3位的数字：

//should guarantee buff have two bytes
//snprintf(buff, "%02d", num) num is between 0 and 100
#define PRINTF_2D_WITH_TWO_DIGIT(buff, num)     \
{                                               \
  int32_t tmp2 = (num) / 10;                    \
  int32_t tmp = (num) - tmp2 * 10;              \
  *buff++ = (char) ('0' + tmp2);                \
  *buff++ = (char) ('0' + tmp);                 \
}

 //snprintf(buff, "%02d", num), num is between 0 and 1000
 #define PRINTF_2D_WITH_THREE_DIGIT(buff, num)   \
 {                                               \
   int32_t m = (num) / 10;                       \
   int32_t l = (num) - m * 10;                   \
   int32_t h = m / 10;                           \
   m = m - h * 10;                               \
   if (h > 0) {                                  \
     *buff++ = (char) ('0' + h);                 \
   }                                             \
   *buff++ = (char) ('0' + m);                   \
   *buff++ = (char) ('0' + l);                   \
 }

//deal year.year[0000-9999]
int32_t high = parts[DT_YEAR] / 100;
int32_t low = parts[DT_YEAR] - high * 100;
PRINTF_2D_WITH_TWO_DIGIT(buf_t, high);
PRINTF_2D_WITH_TWO_DIGIT(buf_t, low);
if (with_delim) {
  *buf_t++ = '-';
}

上面的方式还需要做计算，如果使用200字节的字符串记录0~99的对应字符，那么对于2位数字的转换就可以直接用int64_t的赋值操作，性能对比代码如下：

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>

char int_c[201] =
"000102030405060708091011121314151617181920212223242526272829303132333435363738394041424344454647484950"
"51525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899";

const int64_t INT_NUM = 10000;
int main()
{
  //init data
  int value[INT_NUM];
  for (int64_t idx = 0; idx < INT_NUM; ++idx) {
    value[idx] = random() % 100;
  }
  char buff[20001];
  buff[20000] = '\0';
  char *buf_t = buff;
  struct timeval t_start, t_end;
  long start, end;

  //get direct assign time
  gettimeofday(&t_start, NULL);
  start = t_start.tv_sec * 1000000 + t_start.tv_usec;
  for (int64_t idx = 0; idx < INT_NUM; ++idx) {
    int val = value[idx];
    *(int16_t*)buf_t = *((int16_t*)(int_c) + val);
    buf_t +=2;
  }
  gettimeofday(&t_end, NULL);
  end = t_end.tv_sec * 1000000 + t_end.tv_usec;
  printf("assign time:%ld\n", end - start);

  //get calc and assign time
  start = t_start.tv_sec * 1000000 + t_start.tv_usec;
  buf_t = buff;
  for (int64_t idx = 0; idx < INT_NUM; ++idx) {
    int tmp = value[idx];
    int tmp2 = tmp / 10; tmp = tmp - tmp2 * 10;
    *buf_t++ = (char) ('0' + tmp2);
    *buf_t++ = (char) ('0' + tmp);
  }
  gettimeofday(&t_end, NULL);
  end = t_end.tv_sec * 1000000 + t_end.tv_usec;
  printf("calc and assign time:%ld\n", end - start);
}

得到性能结果，直接赋值的性能达到之前计算每个字符方式的4倍：

assign time:20
calc and assign time:85

即用下面代码做替换可以得到更好的性能。

#define PRINTF_2D_WITH_TWO_DIGIT(buff, num)     \
{                                               \
  *(int16_t*)buf = *((int16_t*)(int_c) + val);  \
  buf += 2；                                    \
}

snprintf/sprintf细节理解不够

snprintf/sprint会在结尾补’\0’

一个底层to_hex_str函数将输入指定data_length的in_data按字节转换为HEX值，在下面的代码中检查buff_size至少是data_length的2倍，但是sprintf会在末尾补’\0’，会导致内存写越界。

unsigned const char *p = NULL;
int32_t i = 0;
if (in_data != NULL && buff != NULL && buff_size >= data_length * 2) {
  p = (unsigned const char *)in_data;
  for (; i < data_length; i++) {
    sprintf((char *)buff + i * 2, "%02X", *(p + i));
  }
}

snprintf/printf %02d打印代表至少2位；%X打印char是按整数打印。

一个类似to_hex_str的代码，使用如下方式打印。

//const char* in_buf, int64_t in_len,
//char *buffer, int64_t buf_len, int64_t &pos
for (int64_t i = 0; OB_SUCC(ret) && i < in_len; ++i) {
  if (FAIL(databuff_printf(buffer, buf_len, pos, "%02X",  *(in_buf + i)))) {
  } else {}
} // end for   

这里就出现一个错误，in_buf是char，是有符号的，在使用%02X打印的时候，按整数方式打印，char的范围是[-128,127)，但2个16进制仅能表示[0,255]。下面的例子中，buff中得到的就是FFFFFFFF。其内容显然不是希望的。

char c = char(-1);
snprintf(buff, BUFF_SIZE, "%02X", c);

这里如果使用snprintf，应该将char*转换为unsinged char* 。
实际上，每个字节打印16进制方式可以用如下代码：

static const char *HEXCHARS = "0123456789ABCDEF";

for (int64_t i = 0; i < data_length; ++i) {
  *dst++ = HEXCHARS[*in_data >> 4 & 0xF];
  *dst++ = HEXCHARS[*in_data & 0xF];
  in_data++;
}

snprintf/sprintf虽然用起来方便，但一定要分析好使用场景和功能，防止出现性能问题或者正确性问题。