使用strtok（）从字符串解析令牌

Question

char line[] = "COPY\tSTART\t0\tCOPY";
char *tmp;

tmp = strtok(line, "\t");
printf("%s", tmp);

This code's output is COPY . And when

char line[] = "\tSTART\t0\tCOPY";

Output is START .

But! I want to check there is nothing in front of string START. That is I think \\t is first delimiter so output of strtok(line, "\\t") is NULL . But real output is START .

Is there any misunderstanding? What can I do?

Answer 1

As per the man page of strtok() (emphasis mine)

A sequence of two or more contiguous delimiter bytes in the parsed string is considered to be a single delimiter. Delimiter bytes at the start or end of the string are ignored. Put another way: the tokens returned by strtok() are always nonempty strings.

So, what you're experiencing is the right behaviour of strtok() .

OTOH, strtok() will return NULL if there is no more tokens, so as per you have expected, returning NULL for the initial delimiter will convey wrong message and it will be confusing. So, the bottom line is,

• if a token is present

  the tokens returned by strtok() are always nonempty strings.

• if a token is not present

  strtok() will return NULL.

Note: it is useful to mention that before using the retured token , always check for NULL.

Answer 2

What can I do?

Build your own function, not exactly how strtok works but you can get some idea:

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

char *scan(char **pp, char c)
{
    char *s, *p;

    p = strchr(*pp, c);
    if (p) *p++ = '\0';
    s = *pp;
    *pp = p;
    return s;
}

int main(void)
{
    char line1[] = "COPY\tSTART\t0\tCOPY";
    char line2[] = "\tSTART\t0\tCOPY";
    char *p;

    puts("Line 1");
    p = line1;
    while (p) {
        printf("%s\n", scan(&p, '\t'));
    }
    puts("Line 2");
    p = line2;
    while (p) {
        printf("%s\n", scan(&p, '\t'));
    }
    return 0;
}

Output:

Line 1
COPY
START
0
COPY
Line 2

START
0
COPY