Error in printing output when using fscanf in C-CodePudding

Here's a code I have been working on C.

char Host_ip[256];
char *Database_name;
char *Username;
int listen_port;

FILE *myfile = fopen("Coin_Converter.config", "r");
fscanf(myfile, "Host=%s Database=%s Username=%s listenport=%d", Host_ip, Database_name,
                                                Username, &listen_port);
fclose(myfile);

printf("Host = %s", Host_ip);
printf("Username= %s", Username);
printf("Database= %s", Database_name);
printf("listenport = %d", listen_port);

My config file looks like this:

Host = 127.0.0.1
Database = base20
Username = John
listenport = 8080

I don't have any issue with compiling the code but I am not able to print anything. Infact, when I only print Host_ip it's working but otherwise I am not getting anything. I tried few things but I can't figure out what I am doing wrong.

CodePudding user response：

Two main mistakes in your code are

Using unallocated pointers instead of buffers
Not filtering whitespace from the input stream.

With some changes (commented) the code becomes

#include <stdio.h>

int main()
{
    char Host_ip[256];
    char Database_name[256];        // array instead of pointer
    char Username[256];             // array instead of pointer
    int listen_port;

    FILE *myfile = fopen("Coin_Converter.txt", "r");
    if(myfile == NULL)
        return 1;
    fscanf(myfile, "Host = %5s Database = %5s Username = %5s listenport = %d",
                Host_ip, Database_name, Username, &listen_port);    // added spaces
    fclose(myfile);

    printf("Host = %s\n", Host_ip);             // added newlines
    printf("Username= %s\n", Username);
    printf("Database= %s\n", Database_name);
    printf("listenport = %d\n", listen_port);
}

and outputs

Host = 127.0.0.1
Username= John
Database= base20
listenport = 8080

The added spaces in the fscanf format string are to remove any whitespace (or none) from the input stream.