Using a character pointer array to store multiple char arrays

Question

I have the following code in which I'm reading lines from a file and want to save them using a character pointer array. As I'm using one buffer inside my file read loop all my pointers in the character array end up pointing towards the last line read from file as the last line is the one that is currently held in the buffer when the loop terminates. How can I store them such that each pointer in the character array points to different char arrays in the order they were read.

int num_clients_to_start = 0;
char *token1, *token2,  *str;
FILE* fp;
char bufr[256];
char testchar[255] = {}; 
char *start_client[10];

while(fgets(bufr, 256, fp) != NULL){

    if(bufr[0] == '#'|| bufr[0] == '\n')
    continue;

    str = bufr;

    token2 = "";  /* initializing an empty token 2 */
        for(str = bufr;  ;str = NULL){
    token1 = strtok(str, " ");
    if(strcmp(token2, "client_name") == 0){
        sprintf(testchar,"%s", token1);
            start_client[num_clients_to_start] = testchar;
        num_clients_to_start++;
    }                   

    token2 = token1;

    if(str == NULL){
            break;
    }
  }//end of for loop

}//end of while loop

printf("client1 = %s client2 = %s client3 = %s",start_client[0],start_client[1],start_client[2]);

My input file is the following:

client_name abc
client_name def
client_name xyz

And print statement outputs:

client1 = xyz
client2 = xyz
client3 = xyz

David Ranieri · Accepted Answer · 2013-10-21 06:26:25Z

3

Note that start_client[0], [1], [2] are all pointers to the last string (readed by fgets)

Use strdup in order to allocate them:

start_client[num_clients_to_start] = strdup(testchar);

As strdup can be an external identifier, use a prototype

#include <string.h>

char *strdup(const char *s);

And don't forget to free() at the end

edited Oct 21, 2013 at 6:26

answered Oct 21, 2013 at 6:08

David Ranieri

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Comments

Some programmer dude · Accepted Answer · 2013-10-21 06:04:44Z

1

You assign the same pointer to all entries in the start_client array. The array testchar will get different contents, but the pointer to it will always be the same. You might want to make start_client an array of arrays of char, and copy the string instead.

Like

char start_client[10][256];

And

strcpy(start_client[num_clients_to_start++], token1);

answered Oct 21, 2013 at 6:04

Some programmer dude

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6 Comments

Zahaib Akhtar Over a year ago

sorry, I think i must have missed the assignment to token1 while copying my code. Have made the edit

Zahaib Akhtar Over a year ago

Yes thats the case, the pointer to testchar is the same hence at the end all prints statements are printing the same thing. Thats what I want to know, how to avoid this?

Some programmer dude Over a year ago

@ZahaibAkhtar Yes. Either that or use strdup (but then you have to remember to free the pointers when you're done with them).

Zahaib Akhtar Over a year ago

Right, so in your solution above basically start_client[0] will then be the first array, start_client[1] be the second and so on?

Zahaib Akhtar Over a year ago

Thanks, that added to my knowledge. I'm a little confused now if I should accept your answer or @AlterMann's. Both have covered the same solution.

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