c - 从字符串中提取子字符串(使用字符串库在 C 中从 ping 回复中获取时间)

Question

只是想获得时间值。查看 C 库表没有直接的答案。

64 bytes from 58.27.61.104: icmp_seq=3 ttl=59 time=45.5 ms

Answer 1

使用标准string.h函数strstr来定位字符串中的子字符串。当找到时，这将返回指向所找到的子字符串的第一个字符的指针。添加子字符串长度，使其指向紧随该子字符串的字符。

然后使用strtod将ASCII字符串转换为 double 值。 strtod 忽略前导空格并使用当前区域设置:

.. leading white-space characters in the string (as defined by the isspace(3) function) are skipped. The decimal point character is defined in the program's locale (category LC_NUMERIC).
(from man strtod on OS X)

一个健壮的程序应该检查无效的输入(即time=后面没有数字，或者该值不能用double表示)，但错误检查已经为了清楚起见，此处省略。

在代码中:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main (void)
{
    char *input = "64 bytes from 58.27.61.104: icmp_seq=3 ttl=59 time=45.5 ms";
    char *looking_for = "time=";
    char *pos_of_time;
    double the_time;

    pos_of_time = strstr (input, looking_for);
    if (pos_of_time == NULL)
    {
        printf ("No match for 'time=' found\n");
    } else
    {
        /* pos_of_time points to the 't' */
        /* we need to look beyond the string */
        pos_of_time += strlen(looking_for);
        the_time = strtod (pos_of_time, NULL);
        printf ("time is %f\n", the_time);
    }

    return 0;
}

显示了预期的输出:

time is 45.500000