please help a beginner

[ED]

Hey people... I'm new in C programing, and i'm having some problem.
What's wrong with the code below. I got no error compiling the file,
but when executing i get a *** glibc detected *** ./test: double free
or corruption (out): 0xbfe83468 *** error:

It's a simple code:

//file config.c

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

#include "config.h"

int read_server_list(char *config_file, char **servers)
{
FILE *file;

int i, status;
file = fopen(config_file, "r+");

if(file != NULL)

{
printf("File opened.\n");
status = SUCCESS;

servers = (char **) malloc(MAX_SERVERS * sizeof(char *));

if(servers == NULL)

{
printf("Memory out.\n");
status = OUT_OF_MEM;
}

else
{
for(i = 0; i < MAX_SERVERS; i++)
{
servers = (char *) malloc(MAX_CHAR_LINE);
fgets(servers, MAX_CHAR_LINE, file);
printf("%s",servers); //prints OK
}
}

fclose(file);
}
else
{
printf("Error openig the file.\n");
status = FILE_ERROR;
}

return status;
}


void cleanup_server_list(char **servers)
{
int i = 0;
do {
free( servers );
servers[i++] = NULL;
} while( i < MAX_SERVERS );

free( servers );
servers = NULL;
}


and then

//file main.c

#include <stdio.h>
#include "config.h"

int main()
{

int status;
char *file = "text.txt";
char **servers;
status = read_server_list(file, servers);

cleanup_server_list(servers); // HERE I GET THE ERROR

return 0;
}

 

[Alf P. Steinbach]

* ED:

Hey people... I'm new in C programing, and i'm having some problem.

Aha. You're new to C programming. That's why you're posting to a C++ group.

#include <stdio.h>
#include "config.h"

int main()
{

int status;
char *file = "text.txt";
char **servers;
status = read_server_list(file, servers);

cleanup_server_list(servers); // HERE I GET THE ERROR

return 0;
}

The 'servers' variable is uninitialized both in the call to 'read_server' and in
the call to 'cleanup_server_list'.

Why don't you use C++ instead, it helps you do cleanup etc. automatically.


Cheers & hth.,

- Alf

 

[DerTopper]

Hey people... I'm new in C programing, and i'm having some problem.
What's wrong with the code below. I got no error compiling the file,
but when executing i get a *** glibc detected *** ./test: double free
or corruption (out): 0xbfe83468 *** error:

It's a simple code:

//file config.c
[snip]
int read_server_list(char *config_file, char **servers)
{ [snip]
                        for(i = 0; i < MAX_SERVERS; i++)
                        {
                                servers = (char *) malloc(MAX_CHAR_LINE);
                                fgets(servers, MAX_CHAR_LINE, file);
                                printf("%s",servers); //prints OK
                        } [snip]
}

void cleanup_server_list(char **servers)
{
        int i = 0;
        do {
            free( servers );
            servers[i++] = NULL;
        } while( i < MAX_SERVERS );

        free( servers );
        servers = NULL;

}

and then

//file main.c

#include <stdio.h>
#include "config.h"

int main()
{

        int status;
        char *file = "text.txt";
        char **servers;
        status = read_server_list(file, servers);

        cleanup_server_list(servers); // HERE I GET THE ERROR
}

Let's explain Alf's answer a bit more detailed: You should pass the
address of a pointer as second argument to read_server_list.
Replace
char **servers;
status = read_server_list(file, servers);
by
char *servers;
status = read_server_list(file, &servers);
and you'll do just fine. The difference between those two is that the
variable servers in the second version is a pointer will point to the
newly allocated memory after read_server_list has been called.

The first version will pass a pointer to a pointer to the function.
Since this pointer is not initialized, but de-referenced inside the
function, you'll get undefined behaviour (most time you'll be lucky,
but your code may produce an Access Violation from time to time).

Regards,
Stuart

 

[Juha Nieminen]

Hey people... I'm new in C programing, and i'm having some problem.

Since this is a C++ group, I recommend you ditch C and do it in C++
instead. It becomes much easier and safer:

#include <fstream>
#include <string>
#include <vector>

int main()
{
std::vector<std::string> servers;
std::ifstream file("text.txt");
if(file)
{
std::string server;
while(std::getline(file, server))
servers.push_back(server);
}
}