hello

i build two separate software simples , one for write one file that i called persona.txt and another for read it



=================================
#include <stdio.h>
#include <stdlib.h>

struct persona {
int numero ;
char *nome ;
char cognome [10] ;
} ;

typedef struct persona persona ;

int main ()


{

FILE *Jtr ;


persona persona ;

fgets (persona.cognome , 9 , stdin) ;

persona.nome = (char*) malloc (sizeof (char ) * 6 ) ;
persona.nome = "tony" ;

fprintf (stdout , "%s" , persona.nome) ;





if ( (Jtr = fopen ("persona.txt" , "wb" ) ) != NULL )
{

fwrite (&persona , sizeof (struct persona ) , 1 , Jtr);



}


fclose (Jtr ) ;
system ("PAUSE") ;

return 0 ;
}

==========================================================


after i try to read that file "persona.txt"


with it



#include <stdio.h>
#include <stdlib.h>

struct persona {
int numero ;
char *nome ;
char cognome [10] ;
} ;

typedef struct persona persona ;

int main ()


{


FILE *Jtr ;
char *Ptr ;

persona persona ;




if ( (Jtr = fopen ("persona.txt" , "rb" ) ) != NULL )
{



persona.nome = (char*) malloc ( sizeof (char) * 10 ) ;
fread (&persona , sizeof (struct persona ) , 1 , Jtr ) ;





fprintf (stdout , "%s\n" , persona .cognome ) ;
fprintf (stdout , "%s\n", persona .nome ) ;


}



fclose (Jtr ) ;
system ("PAUSE") ;

return 0 ;
}



but this software tell me this Error :

"Unhandled exception at 0x64f3984f (msvcr90d.dll) in lettura.c.exe: 0xC0000005: Access violation reading location 0x00f55758"


i thinked about problem of allocation with function malloc and
the pointer of string "persona.nome" , but i can't resolve!

somebody could help me'

thanks

It is a standard issue with misunderstanding of pointers. Your structure contains a pointer. Pointer is a variable, which contains an address to OTHER location in memory. In other words, when you fwrite your structure to the file - only the value of pointer is written and its contents is not written:

YOUR STRUCTURE:
+-----------+-----+-----+
| . . . .   | PTR | ... | (THIS IS WRITTEN TO FILE)
+-----------+-----+-----+
               |
               |     +------+
               +---> | TONY | (THIS IS NOT WRITTEN TO FILE)
                     +------+

Also, this is incorrect (if nome is allocated):

persona.nome="tony";

Proper way in this case is copying the text:

strcpy (persona.nome, "tony");

Or, even better, use strdup to allocate memory AND copy text at the same time.

A proper code to write a structure will be this way:

//
// First write whole structure:
//
fwrite (&persona, 1, sizeof (persona), file);
//
// Then write length of text at nome (including \0 terminator).
// Without this step - not possible to read data back from file
//
int length = 1 + strlen (persona.nome);
fwrite (&length, 1, sizeof (length), file);
//
// Finally, write text itself (including \0 terminator)
//
fwrite (persona.nome, 1, length, file);

To get the data back you need the following code:

//
// Read whole structure back. At this point persona.nome will
// contain a garbage pointer, saved into file by OTHER process.
//
fread (&persona, 1, sizeof (persona), file);
//
// Read the length of text at nome
//
int length;
fread (&length, 1, sizeof (length), file);
//
// Allocate memory for persona.nome (we know the length now)
//
persona.nome = malloc (length);
//
// Finally, read text with \0 terminator
//
fread (persona.nome, 1, length, file);