Need helps on an array of pointers to structure

[Ying]

I have problems to access a member in an array of pointers to structure.
I declare the following structure

struct {
char name[80];
float mark;
}*rec[10];

I would like to put data to rec[0]. So I use following codes

gets(rec[0]->name);
scanf("%f", rec[0]->mark);

the gets command is working fine, But the command for rec[0]->mark generate an error saying that the floting point doesn't linked.

Can any one help me?

More.... it is working fine if I declare an extra floating var, eg. float a and do the follwoing
scanf("%f", &a);
rec[0]->mark = a;

[koder]

i got a differnt warnig from GCC

the `gets' function is dangerous and should not be used.

why is it so?

[apiplani]

Try using this :

scanf("%f", &rec[0]->mark);

[apiplani]

I am not sure why but I tried this piece of code and it worked fine for me :
----------------------------------------------------------------------
#include <iostream.h>
#include <stdio.h>

struct Rec {
char name[80];
float mark;
};



void main ()
{

printf("Entering prog...\n");
Rec * rec[10];

/*gets(rec[0]->name);*/
scanf("%s", &rec[0]->name);
printf("After reading name...\n");
scanf("%f", &rec[0]->mark);

printf("Name - <%s>: Marks - <%f>", rec[0]->name, rec[0]->mark);

}

------------------------------------------------------------------------------------

I am not sure if this explanation sounds reasonable to you or not but I think it happened because when we said :

scanf("%f", rec[0]->mark);

The rec[0]->mark still didnt return the reference to the variable "mark" whereas scanf expects a reference to the variable's memory location as in :

scanf("%f", &a);

[Ying]

It tried what you suggested, but it doesn't work.
It still said the floating point doesn't linked for

scanf("%f", &rec[0]->mark);

but it works fine for

scanf("%s", &rec[0]->name);

This is so strange. Huh!

I am not sure why but I tried this piece of code and it worked fine for me :
----------------------------------------------------------------------
#include <iostream.h>
#include <stdio.h>

struct Rec {
char name[80];
float mark;
};



void main ()
{

printf("Entering prog...\n");
Rec * rec[10];

/*gets(rec[0]->name);*/
scanf("%s", &rec[0]->name);
printf("After reading name...\n");
scanf("%f", &rec[0]->mark);

printf("Name - <%s>: Marks - <%f>", rec[0]->name, rec[0]->mark);

}

------------------------------------------------------------------------------------

I am not sure if this explanation sounds reasonable to you or not but I think it happened because when we said :

scanf("%f", rec[0]->mark);

The rec[0]->mark still didnt return the reference to the variable "mark" whereas scanf expects a reference to the variable's memory location as in :

scanf("%f", &a);

[Banfa]

i got a differnt warnig from GCC


why is it so?

The reason gets is dangerous and it is recomend that you dont use it (and the same goes for scanf too if it is used to enter a string) is that you provide a pointer to an array of characters but you do not tell the function how long the array is. This means that the user can enter data that is longer than the array of characters and force an overrun error where gets overwrites the variables following the array. This is a technique used by hackers to break into systems.

the safe version of gets is fgets

char * fgets (char * string , int num , FILE * stream);

This is safe because you pass the size of the array you have passed a pointer to and the function ensures that only that amount of data is written no matter how much data the user inputs preventing over run errors. Although this takes a FILE * for the input stream you can still use this function by using the system defined stream stdin

char text[20];
 
gets(text);  // <-- bad user may enter more than 19 characters
 
fgets(text, sizeof text, stdin);  // <-- good array text is proctected 
                                  // from over run errors
 

[Banfa]

It tried what you suggested, but it doesn't work.
It still said the floating point doesn't linked for

scanf("%f", &rec[0]->mark);

but it works fine for

scanf("%s", &rec[0]->name);

This is so strange. Huh!

This is more than strange it is wrong. Here are some of your errors

1. You declare Rec * rec[10]; then you start accesing the members pointed to by rec[0] without allocating any memory to that pointer. As soon as you do this you invoke undefined behaviour. Once undefined behaviour has been invoked anywhere in your code all bets are off. The code may work as expected or it may crash or it may perform some completely unexpected behaviour. Don't do it.

2. You have to pass pointers to basic types to scanf to the locations to write data back to so

scanf("%f", rec[0]->mark);

rec[0]->mark is not a pointer it is a float, this wont work

scanf("%s", &rec[0]->name);

rec[0]->name is already a pointer, &rec[0]->name is a pointer to a pointer to char (char **), scanf does not understand this, it wont work

These are the working version of these lines


scanf("%f", &rec[0]->mark);
scanf("%s", rec[0]->name);

3. I very much doubt it said "floating point doesn't linked", most compilers manage to use resonable English which this isn't. If you are getting an error post the actual error not your interpretation of it.

[zahidkhan]

This is more than strange it is wrong. Here are some of your errors

1. You declare Rec * rec[10]; then you start accesing the members pointed to by rec[0] without allocating any memory to that pointer. As soon as you do this you invoke undefined behaviour. Once undefined behaviour has been invoked anywhere in your code all bets are off. The code may work as expected or it may crash or it may perform some completely unexpected behaviour. Don't do it.

2. You have to pass pointers to basic types to scanf to the locations to write data back to so

scanf("%f", rec[0]->mark);

rec[0]->mark is not a pointer it is a float, this wont work

scanf("%s", &rec[0]->name);

rec[0]->name is already a pointer, &rec[0]->name is a pointer to a pointer to char (char **), scanf does not understand this, it wont work

These are the working version of these lines


scanf("%f", &rec[0]->mark);
scanf("%s", rec[0]->name);

3. I very much doubt it said "floating point doesn't linked", most compilers manage to use resonable English which this isn't. If you are getting an error post the actual error not your interpretation of it.

Hi ,
include <float.h> this will solve your problem
and make it sure that you are allocating the memory
for each individual pointers
If not so then your application may crash or core dump.