Strange behavior for libpq with PQconnectdb - c

Question

when calling PQconnectdb in the main program all run very well, but if i call it inside a function a seg fault appears.
here the code that run
#include <stdio.h>
#include <stdlib.h>
#include <postgresql/libpq-fe.h>
#define PG_HOST "127.0.0.1"
#define PG_USER "postgres"
#define PG_DB "postgres"
#define PG_PASS "postgres"
#define PG_PORT 5432
static void
exit_nicely(PGconn *conn)
{
PQfinish(conn);
exit(1);
}
int main( void )
{
char conninfo[250];
PGconn *conn = NULL;
PGresult *pgres = NULL;
sprintf(conninfo, "user=%s password=%s dbname=%s hostaddr=%s port=%d", PG_USER, PG_PASS, PG_DB, PG_HOST, PG_PORT);
conn = PQconnectdb(conninfo);
if (PQstatus(conn) != CONNECTION_OK)
{
fprintf(stderr, "ERROR: Connection to database failed: %s", PQerrorMessage(conn));
exit_nicely(conn);
}
PQfinish(conn);
return 0;
}
this code run very well.
but when i put PQconnect inside a function, the program will generate a seg fault
int connect(char* conninfo, PGconn* conn)
{
conn = PQconnectdb(conninfo);
if (PQstatus(conn) != CONNECTION_OK)
{
fprintf(stderr, "ERROR: Connection to database failed: %s", PQerrorMessage(conn));
exit_nicely(conn);
}
return 1;
}
int main( void )
{
char conninfo[250];
PGconn *conn = NULL;
PGresult *pgres = NULL;
sprintf(conninfo, "user=%s password=%s dbname=%s hostaddr=%s port=%d", PG_USER, PG_PASS, PG_DB, PG_HOST, PG_PORT);
connect(conninfo, conn);
if(!conn)
fprintf(stderr, "conn is null.\n");
PQfinish(conn);
return 0;
}
herein the crash stack
(gdb) where
#0 __strlen_sse2 () at ../sysdeps/x86_64/multiarch/../strlen.S:32
#1 0x00007ffff7893086 in __GI___strdup (s=0x7 <Address 0x7 out of bounds>) at strdup.c:42
#2 0x00007ffff7bbbd87 in ?? () from /usr/lib/libpq.so.5
#3 0x00007ffff7bbc2a5 in ?? () from /usr/lib/libpq.so.5
#4 0x00007ffff7bbe389 in PQconnectStart () from /usr/lib/libpq.so.5
#5 0x00007ffff7bbe416 in PQconnectdb () from /usr/lib/libpq.so.5
#6 0x0000000000400912 in connect (conninfo=0x7 <Address 0x7 out of bounds>, conn=0x60a630) at pqconnect.c:25
#7 0x00007ffff7bbcadb in PQconnectPoll () from /usr/lib/libpq.so.5
#8 0x00007ffff7bbd77e in ?? () from /usr/lib/libpq.so.5
#9 0x00007ffff7bbe3b4 in PQconnectStart () from /usr/lib/libpq.so.5
#10 0x00007ffff7bbe416 in PQconnectdb () from /usr/lib/libpq.so.5
#11 0x0000000000400912 in connect (conninfo=0x7fffffffe600 "user=btel_user password=JwN5K9e18PsTb dbname=ULIC hostaddr=127.0.0.1 port=5432", conn=0x0) at pqconnect.c:25
#12 0x00000000004009e3 in main () at pqconnect.c:49
When i declare my function connect as static, the seg fault error doesn't occur but the returned pointer for the variable conn is NULL
WHY? :(

Answer 1

I think you have a mixup between your connect and the standard library connect. Your connect takes precedence, so when PQconnectdb tries to call connect, things would go bad.
Try to rename the function.
Making connect static also prevents the mixup, which explains why the crash is removed.
Also, you pass the conn parameter to connect incorrectly. It's passed by value, so the variable in main isn't changed, and stays NULL.
You need to pass it by reference.

Answer 2

Note: #Aleix ==> This code should run well.
int pg_connect(char* conninfo, PGconn** conn)
{
*conn = PQconnectdb(conninfo);
if (PQstatus(*conn) != CONNECTION_OK) {
fprintf(stderr,
"ERROR: Connection to database failed: %s",
PQerrorMessage(*conn));
return 0;
}
return 1;
}
/* set correctely your values here */
#define PG_HOST "127.0.0.1"
#define PG_USER "postgres"
#define PG_DB "postgres"
#define PG_PASS "postgres"
#define PG_PORT 5432
int main(int argc, char *argv[])
{
char conninfo[250];
PGconn *conn = NULL;
PGresult *pgres = NULL;
sprintf(conninfo,
"user=%s password=%s dbname=%s hostaddr=%s port=%d",
PG_USER, PG_PASS, PG_DB, PG_HOST, PG_PORT);
if (!pg_connect(conninfo, &conn)) {
goto end;
}
/*
Here fit your staff
*/
end:
PQfinish(conn);
return 0;
}