使用链接列表时malloc中的分段错误

Question

The program reads from a file, stores the required data in some variables then push it to a stack. This is a small part of the input file, the file contains this data repeated several times( value of variables changed in each chunk of data ).

----------------------------------------------------------------------- 
Timestamp (Wed Mar 29 20:44:08 2017) 
[1] Received msg from node <00116 / fc:c2:3d:00:00:10:ab:35> 
RSSI -6 dBm / LQI 22
+[Node_Voltage]  <2.963000 Volts> 
+[P_MS5637]      <896 mbar> 
+[NTC_THERM (Murata NXFT15XH103)]    <27.755314 deg C> 
+[Temp_LM75B]    <27.620001 Deg C> 
+[RH_CC2D33S]    <33.000000 %> 
+[Temp_CC2D33S]  <27.000000 Deg C> 

The stored data is pushed to a stack where the segmentation fault is occurring. The program is able to store only one stack after that the fault occurs.

void create(stack **head){
    *head=NULL;
}

void copy_string(char arr[],char arr2[]){
    int i=0;

    for(i=0;i<strlen(arr2);i++){
        arr[i] = arr2[i];
    }
}

stack demo;          
stack *tracker;

// **head is used since this is an ADT, i've not pasted the code in source file here 

void push(stack **head,char date[],char time[],char month[],char year[],float pressure,float temprature1,float temprature2,float rel_humid,float node_voltage){
    stack *temp = malloc(sizeof(demo));   

    temp->pressure = pressure;
    temp->temprature1 = temprature1;
    temp->temprature2 = temprature2;
    temp->rel_humid = rel_humid;
    temp->node_voltage = node_voltage;

    printf("Inside push function\n");

    copy_string(temp->date, date);
    copy_string(temp->time, time);
    copy_string(temp->month, month);
    copy_string(temp->year, year);


    if(*head==NULL){
        temp->next = NULL;
        *head = temp;
        tracker = temp;
    }
    else{
        tracker->next = temp;
        tracker = tracker->next;
        tracker->next = NULL;
    }

    free(temp);     //on removing this, program runs infinitely instead of giving segmentation fault

    printf("%s %s %f %f ",tracker->date,tracker->year,tracker->pressure,tracker->node_voltage);
}

Using gdb( GNU Debugger ) i got this error message -

Inside push function

29 2017) 896.000000 2.963000 Done!!

Program received signal SIGSEGV, Segmentation fault.
__GI___libc_free (mem=0x11f1) at malloc.c:2949
2949    malloc.c: No such file or directory.

SOLVED : The reason for the whole problem was free(temp), it needed to be removed from the above code and also in the main file, file pointer was getting closed by mistake after running the code once so infinite loop was running while taking input again.

Answer 1

You are free() ing then accessing the same object when *head==NULL . Look at the lines marked **** below.

if(*head==NULL){
    temp->next = NULL;
    *head = temp;
    tracker = temp;//**** temp is assigned to tracker. They point to the same place.
}
else{
    //...
}

free(temp);     //**** temp is free()d but remember tracker==temp..

//**** Now you output tracker but the object at this location was just freed.
printf("%s %s %f %f ",tracker->date,tracker->year,tracker->pressure,tracker->node_voltage);

So remove free(temp) it is in the totally wrong place but you haven't given enough code to say where it goes.

The 'indefinite looping' is some other error but you haven't provided enough code to identify that.

Also notice the else part doesn't make much sense:

    tracker->next = temp;
    tracker = tracker->next;
    tracker->next = NULL;

It's not clear what tracker is going in but assuming it is valid this amounts to:

    tracker = temp;
    temp->next = NULL;