C-language cutting multilayer strings (Strtok_r strtok use method) _c language

1. Strtok Introduction

As is known to all, Strtok can be based on the user-supplied delimiter (which can also be plural for example). ”）
Splits a section of string until it encounters a "yes".

For example, the delimiter = "," string = "Fred,john,ann"

by Strtok, 3 strings of "Fred" "John" "Ann" can be extracted.

The C code above is

Copy Code code as follows:

int in=0;
Char buffer[]= "Fred,john,ann"
Char *p[3];
char *buff = buffer;
while ((P[in]=strtok (buf, ","))!=null) {
i++;
Buf=null; }

As the code above, the first execution of the strtok requires the address of the destination string to be the first parameter (Buf=buffer), after which the strtok needs to be NULL as the first parameter (buf=null). Pointer column p[], then stores the split result, p[0]= "John", p[1]= "John", p[2]= "Ann", and Buf becomes fred\0john\0ann\0.

2. Weaknesses of the Strtok

Let's change our plan: We have a string "Fred male 25,john male 62,anna female 16" We want to sort this string into a struct,

Copy Code code as follows:

struct Person {
Char [] name;
char [6] sex;
Char [4] age;
}

One way to do this is to first extract a string that is "," and then split it into "" (space).
For example: Intercept "Fred male 25" and split it into "Fred" "Male" "25"
I've written a little program to show this process:

Copy Code code as follows:

#include <stdio.h>
#include <string.h>
#define INFO_MAX_SZ 255
int main ()
{
int in=0;
Char buffer[info_max_sz]= "Fred male 25,john male 62,anna female 16";
Char *p[20];
Char *buf=buffer;

while ((P[in]=strtok (buf, ","))!=null) {
Buf=p[in];
while ((P[in]=strtok (buf, ""))!=null) {
in++;
Buf=null;
}
p[in++]= "* * *"; Performance segmentation
Buf=null; }

printf ("Here we are have%d strings\n", in);
for (int j=0; j<in; j + +)
printf (">%s<\n", P[j]);
return 0;
}

The output of this program is:
Here we have 4 strings
>Fred<
>male<
>25<
>***<
This is just a little piece of data, not what we need. But why is that? This is because Strtok uses a static (static) pointer to manipulate the data, let me analyze the above code to run the process:

Red is the location of the strtok's built-in pointer, and the blue is strtok to the string modification

1. "Fred male 25,john male 62,anna female 16"//outer circulation

2. "Fred male 25\0john male 62,anna female 16"/into the inner loop

3. "Fred\0male 25\0john male 62,anna female 16"

4. "Fred\0male\025\0john male 62,anna female 16"

5 "Fred\0male\025\0john male 62,anna female 16"//Inner Loop Encounter "" "back to outer loop

6 "Fred\0male\025\0john male 62,anna female 16"//Outer loop encountered "" "Run end.

3. Use of Strtok_r

In this case we should use Strtok_r, Strtok reentrant.
Char *strtok_r (char *s, const char *delim, char **ptrptr);

Relative strtok we need to provide strtok with a pointer to operate, rather than using matching pointers like strtok.
Code:

Copy Code code as follows:

#include <stdio.h>
#include <string.h>
#define INFO_MAX_SZ 255
int main ()
{
int in=0;
Char buffer[info_max_sz]= "Fred male 25,john male 62,anna female 16";
Char *p[20];
Char *buf=buffer;

Char *outer_ptr=null;
Char *inner_ptr=null;

while ((P[in]=strtok_r (buf, ",", &outer_ptr))!=null) {
Buf=p[in];
while ((P[in]=strtok_r (buf, "", &inner_ptr))!=null) {
in++;
Buf=null;
}
p[in++]= "* * *";
Buf=null; }

printf ("Here we are have%d strings\n", in);
for (int j=0; j<in; j + +)
printf (">%s<\n", P[j]);
return 0;
}

This time the output is:
Here we have strings
>Fred<
>male<
>25<
>***<
>John<
>male<
>62<
>***<
>Anna<
>female<
>16<
>***<

Let me analyze the running process of the above code:

Red is the position of the Strtok_r Outer_ptr Point,
Purple is the position of the inner_ptr point of Strtok_r,
Blue for strtok modification of strings

1. "Fred male 25,john male 62,anna female 16"//outer circulation
2. "Fred male 25\0john male 62,anna female 16"/into the inner loop
3. "Fred\0male 25\0john male 62,anna female 16"
4 "Fred\0male\025\0john male 62,anna female 16"
5 "Fred\0male\025\0john male 62,anna female 16"//Inner Loop Encounter "" "back to outer loop
6 "Fred\0male\025\0john male 62\0anna female 16"//Into the inner loop

Originally, the function modifies the original string.

Therefore, when you pass in a char *test2 = "Feng,ke,wei" as the first parameter, a memory error occurs at the location ① because the contents of the Test2 point are saved in the literal constant area and the contents of the area cannot be modified. and char test1[] = "Feng,ke,wei" in the Test1 point to the content is stored in the stack area, so you can modify