Question

I’m new to C and learning using Zed Shaw’s Learn C the Hard Way. I’m having some trouble finding out why I’m getting the following memory error. I’m pretty sure that I’m not leaking memory since I used Valgrind to check my code.

Here are the problematic lines of my code:

 :  int main(int argc,char *argv[]){

        if (argc<3) die ("USAGE : ex17 <dbfile> <action> [action params]");
       ...char *filename=argv[1];// how are we able to get away with one dimension?
        char action =argv[2][0];
        /*Line 166*/:struct Connection *conn=Database_open(filename,action);// error throwing line

The function Database_open is defined as :

/*Line 61*/:struct Connection *Database_open(const char *filename, char mode)
{
    /* data */
    struct Connection *conn= malloc(sizeof(struct Connection));

    if(!conn->db) die("Memory Error");//?

    if(mode=='c'){
        conn->file=fopen(filename,"w");
    }
    else{
        conn->file=fopen(filename,"r+");


    if(conn->file){
        Database_load(conn);
    }
    }


        if(!conn->file) die("Failed to open file");
        return conn;
    };

And here’s the die function that Im using :

void die (const char *message)
{
        if(errno){
            perror(message);
        }
        else{
            printf("ERROR :%s\n", message);
        }
        exit(1);
}

ANd when I run the code using Valgrind I get:

==6112== Command: ./ex17 db.dat c
==6112== 
==6112== Conditional jump or move depends on uninitialised value(s)
==6112==    at 0x80487B3: Database_open (ex17.c:61)//Marked above
==6112==    by 0x8048BAA: main (ex17.c:166)// Line numbers marked above
==6112== 
ERROR :Memory Error
==6112== 
==6112== HEAP SUMMARY:
==6112==     in use at exit: 8 bytes in 1 blocks
==6112==   total heap usage: 1 allocs, 0 frees, 8 bytes allocated
==6112== 
==6112== LEAK SUMMARY:
==6112==    definitely lost: 0 bytes in 0 blocks
==6112==    indirectly lost: 0 bytes in 0 blocks
==6112==      possibly lost: 0 bytes in 0 blocks
==6112==    still reachable: 8 bytes in 1 blocks
==6112==         suppressed: 0 bytes in 0 blocks
==6112== Rerun with --leak-check=full to see details of leaked memory
==6112== 
==6112== For counts of detected and suppressed errors, rerun with: -v
==6112== Use --track-origins=yes to see where uninitialised values come from
==6112== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

EDIT

Struct Connection

struct Connection
{
    /* data */
    FILE *file;
    struct Database *db;
};

Answer 1

Thank you providing the details regarding the structure Connection. As is seen, the structure contains two pointer elements as members.

After allocating the structure, you have to specifically malloc the element Database *db inside it.

Something like:

struct Connection *conn= malloc(sizeof(struct Connection));
conn->db = malloc(sizeof(struct Database));

Hope this helps.