Read / write char arrays in binary files

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Read / write char arrays in binary files

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#1 by (3 votes)

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I am trying to binary save strings from static character arrays and then print them. The point is that, in spite of the fact that I assign them the value in a static way (so when creating them, they should be created with the %code% at the end, so I should not put it at hand). The biggest problem is that these arrays are part of a structure, so I have given a maximum value in the declaration of the same and then assign values. This structure seems to be saved correctly, but when reading the arrays, they do not print correctly. Here is the code:

The structure:

typedef struct
{
    char P [250];
    char R1 [250] ;
    char R2 [250] ;
    char R3[250]  ;

} t_p_r;

The assignment of values to their arrays and the attempt to print them:

arrPreg =(t_p_r*)malloc(sizeof(t_p_r));

        t_p_r preg;

        strcpy(preg.P,"hola");
        strcpy(preg.R1,"1");
        strcpy(preg.R2, "2");
        strcpy(preg.R3 ,"3");

        arrPreg[0] = preg;
        guardarPR(arrPreg,1);

        pS = (t_p_r*)malloc(sizeof(t_p_r));
        size = leerPR(pS);


        for(int i=0; i<strlen(pS[0].P);i++)
        {
            printf("%c\n", pS[0].P[i] );
        }

Methods for reading / writing:

void guardarPR(t_p_r P[], int numP)
{
  FILE* fichero = fopen("PR.dat", "wb");  
  fputc(numP, fichero);
  fwrite(&P, sizeof( t_p_r), numP, fichero);
  fclose(fichero);
}

int leerPR(t_p_r* PL)
{
    int numElem;

  FILE* fichero = fopen("PR.dat", "rb");

  numElem = fgetc(fichero);
  PL= (t_p_r*) malloc((numElem) * sizeof(t_p_r));
  fread(&PL, sizeof(t_p_r) , numElem, fichero);
  fclose(fichero);

  return numElem;
}

EDIT

Likewise, I have done another test, this time saving the structures individually. Although it gives a better result, it still does not work:

    guardarSingular(preg);
    t_p_r preg2;
    leerSingular(&preg2);
        printf(" %s\n", preg2.P);
        printf(" %s\n", preg2.R1);
        printf(" %s\n", preg2.R2);
        printf(" %s\n", preg2.R3);


void guardarSingular(t_p_r PG)
{
  FILE* fichero = fopen("PR.dat", "wb"); 
  fputc(1, fichero);
  fwrite(&PG, sizeof(t_p_r), 1, fichero);
  fclose(fichero);
}

void leerSingular(t_p_r* PL)
{
  FILE* fichero = fopen("PR.dat", "rb");
  fread(PL, sizeof(t_p_r) , 1, fichero);
  fclose(fichero);
}

Apparently, in view of the result, the structures seem to keep well, but at the time of reading and wanting to show them, they show strange characters different from those introduced at the beginning. How can I store this structure in a binary file in order to retrieve its strings as I saved them?

string c archivo

asked by pepito 05.04.2018 в 10:54

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1 answer

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score 3 · Accepted Answer

This structure seems to be saved correctly

Well, I would say no:

void guardarPR(t_p_r P[], int numP)
//                   ^^^ P es un puntero
{
  FILE* fichero = fopen("PR.dat", "wb");  
  fputc(numP, fichero);
  fwrite(&P, sizeof( t_p_r), numP, fichero);
  //     ^^ En el fichero guardas la referencia al puntero
  fclose(fichero);
}

fwrite asks you for a pointer to the information to save ... and you are passing the reference to a pointer ... that is, a double pointer ...

Try to pass the pointer directly:

void guardarPR(t_p_r P[], int numP)
{
  FILE* fichero = fopen("PR.dat", "wb");  
  fputc(numP, fichero);
  fwrite(P, sizeof( t_p_r), numP, fichero); // <<--- Linea modificada
  fclose(fichero);
}

Although the code will be more readable if you change the signature of the function:

void guardarPR(t_p_r *P, int numP)

And now let's review the reading:

int leerPR(t_p_r* PL)
{
  int numElem;
  FILE* fichero = fopen("PR.dat", "rb");
  numElem = fgetc(fichero);
  PL= (t_p_r*) malloc((numElem) * sizeof(t_p_r)); // <<--- cambio local
  fread(&PL, sizeof(t_p_r) , numElem, fichero);
  fclose(fichero);
  return numElem;
}

If you have a code such that:

void func (int var)
{ var = 10; }

int main ()
{
  int v;
  func (v);
  printf ("%d\n",v);
}

What value does it print?

The correct answer is an indeterminate number and it is easy to see that the assignment made in func is local ... with the value of the pointers the exact same thing happens ... the pointers are used to modify a memory common ... not to use a different memory ... if you want that effect you must use double pointers:

int leerPR(t_p_r** PL)
{
  int numElem;
  FILE* fichero = fopen("PR.dat", "rb");
  numElem = fgetc(fichero);
  *PL= (t_p_r*) malloc((numElem) * sizeof(t_p_r));
  fread(*PL, sizeof(t_p_r) , numElem, fichero);
  fclose(fichero);
  return numElem;
}