Separate connect function for libmodbus - c

Question

I am trying to group the operation under libmodbus for Mod-bus connection and get-value into two simpler function as below.
However, it always cause Segmentation fault (core dumped) when I try to get value from the device.(get_float, modbus_read_registers)
Can anyone tell me how to fix it?
int connect(char *ip_addr, struct timeval timeout, modbus_t *ctx){
int fail = 0;
ctx = modbus_new_tcp(ip_addr, MODBUS_SERVER_PORT);
modbus_set_slave(ctx, MODBUS_DEVICE_ID);
modbus_set_debug(ctx, MODBUS_DEBUG);
timeout.tv_sec = MODBUS_TIMEOUT_SEC;
timeout.tv_usec = MODBUS_TIMEOUT_USEC;
modbus_get_byte_timeout(ctx, &timeout.tv_sec, &timeout.tv_usec);
timeout.tv_sec = MODBUS_TIMEOUT_SEC;
timeout.tv_usec = MODBUS_TIMEOUT_USEC;
modbus_set_response_timeout(ctx, timeout.tv_sec, timeout.tv_usec);
fail = modbus_connect(ctx);
if (fail == -1) {
fprintf(stderr, "Connection failed: %s\n",
modbus_strerror(errno));
modbus_free(ctx);
fail = -1;
}
return fail;
}
int get_float(modbus_t *ctx, uint16_t addr, float *val){
int fail = 0;
__uint16_t value[2];
printf("1\n");
fail = modbus_read_registers(ctx, (addr-1), 2, value);
printf("2\n");
if(fail <= 0) {
fprintf(stderr, "Reading error(%d): %s\n", addr, modbus_strerror(errno));
} else {
*val = modbus_get_float_abcd(value);
}
return fail;
}
Besides, I can successfully run the similar code when I put them in same function as below:
int connect_n_getFloat(char *ip_addr, uint16_t addr, float *val){
int fail = 0;
modbus_t *ctx = modbus_new_tcp(ip_addr, MODBUS_SERVER_PORT);
ctxConfig(ctx);
if (modbus_connect(ctx) == 0) {
__uint16_t value[2];
if(modbus_read_registers(ctx, (addr-1), 2, value) > 0) {
*val = modbus_get_float_abcd(value);
} else {
fprintf(stderr, "Reading error(%d): %s\n", addr, modbus_strerror(errno));
fail = -1;
}
} else {
fprintf(stderr, "Connection failed: %s\n",
modbus_strerror(errno));
modbus_free(ctx);
fail = -1;
}
return fail;
}

Answer 1

You're passing a context pointer to the connect function, but should be passing a pointer to a pointer, so you can return the allocated context and continue using it in further calls.
Change the function signature, and ctx usage, from
int connect(char *ip_addr, struct timeval timeout, modbus_t *ctx) {
int fail = 0;
ctx = modbus_new_tcp(ip_addr, MODBUS_SERVER_PORT);
to
int connect(char *ip_addr, struct timeval timeout, modbus_t **ctx) {
int fail = 0;
*ctx = modbus_new_tcp(ip_addr, MODBUS_SERVER_PORT);
This also explains why it works when you put them in the same function.