please help a beginner
ED
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[i] = (char *) malloc(MAX_CHAR_LINE);
fgets(servers[i], MAX_CHAR_LINE, file);
printf("%s",servers[i]); //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[i] );
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
> 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
--
Due to hosting requirements I need visits to <url: http://alfps.izfree.com/>.
No ads, and there is some C++ stuff! :-) Just going there is good. Linking
to it is even better! Thanks in advance!
DerT...@web.de
> 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
>
> int read_server_list(char *config_file, char **servers)
> {
> for(i = 0; i < MAX_SERVERS; i++)
> {
> servers[i] = (char *) malloc(MAX_CHAR_LINE);
> fgets(servers[i], MAX_CHAR_LINE, file);
> printf("%s",servers[i]); //prints OK
> }
> }
>
> void cleanup_server_list(char **servers)
> {
> int i = 0;
> do {
> free( servers[i] );
> 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
Yannick Tremblay
post to a C specific group.
In article <4b67eba4-34f4-46d6...@a7g2000yqk.googlegroups.com>,
<DerT...@web.de> wrote:
>On Mar 30, 2:03 am, ED <edym...@gmail.com> wrote:
>> 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]
>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).
I'll give you another hint extremely useful for new programmers and
IMO useful for experinced programmers too:
Initialise all of your variable at decaration time especially
pointers.
So that should give:
char *servers = 0; // NULL if you prefer
// ...
FILE * pFile = 0; // NULL if you prefer
// ...
int status = -1;
Etc.
This is a very good habit to take but is unfortunately not very
popular with C programmers but will save you lots of hassle in your
life as a programmer.
Yannick
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);
}
}