Pointer issue not solved

Question

In the below code, the file test.txt has the following data :

192.168.1.1-90    
192.168.2.2-80

The output of this is not as expected. I expect the output to be

192.168.1.1    
90     
192.168.2.2   
80

The current output is

192.168.2.2    
80     
192.168.2.2     
80

I know that the pointer of str is pointing to the same address in the second iteration as well.

Im just not able to the problem. Any help would be appreciated.

#include <string.h> 
#include <stdio.h>
#include <stdlib.h>
int main() {
  FILE * fp;
  char * result[10][4];
  int i = 0;
  const char s[2] = "-";
  char temp[50];
  char * value, str[128], * string, t[20], x[29] = "192.168.2.2";
  fp = fopen("test.txt", "r");
  if (fp == NULL)
    printf("File doesn't exist\n");
  else {
    while (!feof(fp)) {

      if (fgets(str, sizeof(str), fp)) {

        /* get the first value */
        value = strtok(str, s);

        result[i][0] = value;
        printf("IP : %s\n", result[i][0]); //to be removed after testing


        /* get second value */
        value = strtok(NULL, s);

        result[i][1] = value;
        printf("PORT : %s\n", result[i][1]); //to be removed after testing
        i++;
      }
    }
    for (int k = 0; k < 2; k++) {
      for (int j = 0; j < 2; j++) {
        printf("\n%s\n", result[k][j]);
      }
    }

  }
  return (0);
}

Do you need char *result[10][4];? Why not just have an array of strings instead, like char *result[4]? — RoadRunner
– RoadRunner, Commented Nov 4, 2016 at 13:17
@RoadRunner, its like i want to store the IP and port number separately. — vatsal511
– vatsal511, Commented Nov 4, 2016 at 13:18
So, i'm trying to make a row and column scenario, where column one is IP and column 2 is port number — vatsal511
– vatsal511, Commented Nov 4, 2016 at 13:19
@JohnBollinger I thought about it again, and it does in fact introduce a bug: while double-checking for end-of-file is wasteful but harmless, if any other error occurs, the program will loop infinitely. Let me second your previous comment ;) — Quentin
– Quentin, Commented Nov 4, 2016 at 13:43

BLUEPIXY · Accepted Answer · 2016-11-04 13:38:17Z

0

I propose like this:

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

enum { IP = 0, PORT = 1};

int main(void){
    FILE *fp;
    char result[2][2][16];//2 lines, 2 kinds, 16:XXX.XXX.XXX.XXX+NUL
    const char *s = "-";//delimiter
    char *value, line[128];
    int i=0;

    fp = fopen("test.txt", "r");
    if (fp == NULL) {
        printf("File doesn't exist\n");
        exit(EXIT_FAILURE);
    }
    while(i < 2 && fgets(line, sizeof(line), fp)){
        value = strtok(line, s);
        strcpy(result[i][IP], value);
        printf("IP : %s\n",result[i][IP]);

        value = strtok(NULL, s);
        strcpy(result[i][PORT], value);
        printf("PORT : %s\n",result[i][PORT]);
        i++;
    }
    puts("");
    for (int k=0;k<2;k++){
        for (int j=0;j<2;j++){
            printf("%s\n",result[k][j]);
        }
    }
    fclose(fp);

    return 0;
}

answered Nov 4, 2016 at 13:38

BLUEPIXY

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2 Comments

vatsal511 Over a year ago

when im trying to compare result[k][j] (in the final loop) to some string like 192.168.1.1, i'm unable to do that. Why is that so.?

BLUEPIXY Over a year ago

@vatsal511 I don't get you. strcmp(result[k][j], "192.168.1.1") == 0 is true when k : 0, j : 0.

Enver Evci · Accepted Answer · 2016-11-04 13:38:57Z

0

What you are doing wrong is that you are assigning "value" pointer to elements of "result" array. In your implementation, all the elements of "result" just mirror the value of "value" pointer. Therefore, when you change the value of "value", you also change all the "result" elements.

Because of that, you should use strcpy function after allocating memory for the specific "result" element.

value = strtok(str, s);
result[i][0]=malloc(strlen(value) + 1);
strcpy(result[i][0], value);

answered Nov 4, 2016 at 13:38

Enver Evci

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1 Comment

vatsal511 Over a year ago

Not working... Tried what you asked me to. result[i][0]=malloc(strlen(value) + 1); is being shown illegal

elikatsis · Accepted Answer · 2016-11-04 13:34:43Z

-1

When you want to keep-copy strings you have to use the function strcpy()

Instead of result[i][x] = value you should do the following

strcpy(result[i][x], value);

Edit: Before the strcpy you have to use malloc to allocate memory for the result[i][x] string. eg:

result[i][0] = malloc(10 * sizeof(char));

edited Nov 4, 2016 at 13:34

answered Nov 4, 2016 at 13:20

elikatsis

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2 Comments

John Bollinger Over a year ago

Well, not exactly. All of the elements of result are uninitialized char pointers, so your suggested solution produces undefined behavior.

elikatsis Over a year ago

Oh true true, you have to malloc the result[i][x]. Adding it in the answer!

RoadRunner · Accepted Answer · 2016-11-04 14:12:21Z

-1

I suggest using malloc for allocating space for each ip and port, and freeing them at the end with free. Additionally, a struct might be handy here, if you have bigger text files in the future that you want to use.

The code:

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

#define COLS 2
#define MAXCHAR 10
#define BUFFSIZE 128

void exit_if_null(void *ptr, const char *msg);

int
main(void) {
    FILE *filename;
    char *result[COLS][MAXCHAR+1];
    char buffer[BUFFSIZE];
    char *ip, *port;
    int row, i = 0;

    filename = fopen("ips.txt", "r");

    if (filename == NULL) {
        fprintf(stderr, "%s\n", "Error reading file!\n");
        exit(EXIT_FAILURE);
    } else {
        while (fgets(buffer, BUFFSIZE, filename) != NULL && i < 2) {
            ip = strtok(buffer, "-");
            port = strtok(NULL, "\n");

            result[i][0] = malloc(strlen(ip)+1);
            exit_if_null(result[i][0], "Initial Allocation");

            result[i][1] = malloc(strlen(port)+1);
            exit_if_null(result[i][1], "Initial Allocation");

            strcpy(result[i][0], ip);
            strcpy(result[i][1], port);

            i++;
        }
    }

    for (row = 0; row < i; row++) {
        printf("%s\n", result[row][0]);
        printf("%s\n", result[row][1]);

        free(result[row][0]);
        free(result[row][1]);

        result[row][0] = NULL;
        result[row][1] = NULL;
    }

    return 0;
}

void
exit_if_null(void *ptr, const char *msg) {
    if (!ptr) {
        printf("Unexpected null pointer: %s\n", msg);
        exit(EXIT_FAILURE);
    }
}

edited Nov 4, 2016 at 14:12

answered Nov 4, 2016 at 13:42

RoadRunner

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3 Comments

RoadRunner Over a year ago

@vatsal511, what about this solution?

vatsal511 Over a year ago

Im getting the same error which i got in the above code given by @elikatsis void *__cdecl malloc(size_t _Size)

RoadRunner Over a year ago

you might need to #include <malloc.h>

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