Обсуждение: Seg fault on completing query

Hi,

I'm working on a frontend to a Postgresql database.  I've chosen C in 
which to work, mainly because I wanted to learn the language.  So, I'm 
using the pqlib frontend library.  The C source I have is below.  I 
compile it (on linux) with gcc -g -Wall -lpq -oantibug antibug.c.  When 
I run the executable, it prints out the query to the screen ok, but also 
prints a "Segmentation Fault" on completion of the list.  Does anyone 
know why? How can I prevent this?

Thanks,

Chris J

Source:

/*************************************************************** A test C program to perform a SELECT query on
tblantibiotic**on the bugswat database.
***************************************************************/
#include <stdio.h>#include <libpq-fe.h>#include <stdlib.h>

void exit_nicely(PGconn * conn) {   PQfinish(conn);   printf("Abnormal exit");   exit(1);
}

int main() {   /* Declare variables for the connection and getting results */   int num_records;   PGconn *connect;
PGresult*res;   int i;   char *name;
 
   /* Connect to the database */   connect = PQconnectdb("host = linux.chrisdomain dbname=antibugdb 
user=cjewell");       if (PQstatus(connect) == CONNECTION_BAD) {           fprintf(stderr, "Could not connect to
database");          fprintf(stderr, "Error: %s\n", PQerrorMessage(connect));           exit_nicely(connect);       }
 
       /* Perform our query */   res = PQexec(connect, "SELECT * FROM tblactivity;");       if ((!res) ||
(PQresultStatus(res)!= PGRES_TUPLES_OK)) {           fprintf(stderr, "SELECT query failed, error: %d\n", 
 
PQresultStatus(res));           fprintf(stderr, "Server error: %s\n", PQerrorMessage(connect));           PQclear(res);
         exit_nicely(connect);       }
 
   num_records = PQntuples(res); /* put the number of tuples into 
num_records */
   /* Now print out the results to the screen */   for(i=0; i < num_records; ++i) {       sprintf(name, "%s",
PQgetvalue(res,i, 0));       printf("%s\n", name);   }
 
   PQclear(res);   PQfinish(connect);
   return(0);

}


Output:

[cjewell@chris cprogs]$ ./antibug
good
some
poor
none
unknown
Segmentation fault
[cjewell@chris cprogs]$

Chris Jewell <vs0u8055@liv.ac.uk> wrote:
>     Hi,
> 
> I'm working on a frontend to a Postgresql database.  I've chosen C in 
> which to work, mainly because I wanted to learn the language.  So, I'm 
> using the pqlib frontend library.  The C source I have is below.  I 
> compile it (on linux) with gcc -g -Wall -lpq -oantibug antibug.c.  When 
> I run the executable, it prints out the query to the screen ok, but also 
> prints a "Segmentation Fault" on completion of the list.  Does anyone 
> know why?

This is really a C/Linux/gcc question. I haven't looked at your source, so
just some general remarks:

You'll want to compile your program with "-g" as an additional compiler
switch. Then you also want core dumps to be written to disk
"ulimit -c unlimited" in your shell.

You can then analyze the core dump using gdb:

$ gdb /path/to/your/app
gdb> core /path/to/corefile # usually just 'core'
gdb> bt      # short for 'backtrace'

You'll then see where your program crashed.

Alternatively, run your app under gdb (or a GUI frontend like ddd/gvd/...)
in the first place to see where it crashes.

HTH,

-- Gerhard

On Thu, Mar 13, 2003 at 03:28:20PM +0000, Chris Jewell wrote:

>    char *name;
[...]
>        sprintf(name, "%s", PQgetvalue(res, i, 0));
[...]

So where do you allocate something for 'name' to point to?


Jeroen