Structures binary file into array C - c

Question

I need to put structs from a binary file into an array of structs, one struct per key.
Here is my struct:
struct candidate{
char inscr[10];
char name[44];
int year;
int position;
char curse[30];
};
typedef struct candidate Candidate;
Here's what I tried to do:
void ordenafile(char fname[13]){
FILE *f = fopen (fname, "rb");
if(f==NULL){
printf("Error.");
return;
}
fseek(f, 0, SEEK_END);
int sz = ftell(f);
Candidate *p, *aux,*arr[sz/sizeof(Candidate)];
p=(Candidate*)malloc(sizeof(Candidate));
int i = 0;
while(fread(p,sizeof(candidate),1,f)>0){
arr[i]=p;
i++;
}

Answer 1

Having found the end of the file with fseek(), you need to rewind before reading from it.
This part of the code doesn't do what you want:
Candidate *p, *aux,*arr[sz/sizeof(Candidate)];
p=(Candidate*)malloc(sizeof(Candidate));
int i = 0;
while(fread(p,sizeof(candidate),1,f)>0){
arr[i]=p;
i++;
}
You have an array of structure pointers, and you allocate one structure, pointed at by p. Your loop then reads a value into the one structure, and copies the pointer into an element of the array of pointers. So, all the elements of the array point to the same structure. The variable aux is also unused.
You need one of two solutions:
Candidate arr[sz/sizeof(Candidate)];
int i;
for (i = 0; fread(&arr[i], sizeof(Candidate), 1, f) == 1; i++)
;
This assumes you are going to be able to use the array in the current function and functions it calls, and don't need to return it to the calling code.
Alternatively:
Candidate *arr = malloc(sz);
if (arr == 0)
return 0;
int i;
for (i = 0; fread(&arr[i], sizeof(Candidate), 1, f) == 1; i++)
;
return arr;
You can return this array from the function.
Working code
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct candidate
{
char inscr[10];
char name[44];
int year;
int position;
char curse[30];
};
typedef struct candidate Candidate;
int write_sample_data(const char *filename);
Candidate *read_sample_data(const char *filename, int *number);
void print_sample_data(int num, Candidate *candidates);
int main(void)
{
const char filename[] = "practice.data";
if (write_sample_data(filename) != 0)
fprintf(stderr, "Failed to write sample data\n");
else
{
int number = 0;
Candidate *clist = read_sample_data(filename, &number);
if (number == 0)
fprintf(stderr, "Failed to read sample data\n");
else
{
print_sample_data(number, clist);
free(clist);
}
}
return 0;
}
int write_sample_data(const char *filename)
{
FILE *fp = fopen(filename, "wb");
if (fp == NULL)
{
fprintf(stderr, "Failed to open file %s for writing\n", filename);
return -1;
}
Candidate c;
for (int i = 0; i < 5; i++)
{
char inscr[10];
char name[40];
char curse[30];
sprintf(inscr, "I-%.4d", i);
sprintf(name, "Name %.4d Surname %.4d", rand() % 1000, rand() % 1000);
sprintf(curse, "Curse %.2d", rand() % 100);
// Using null-filling property of strncpy()
strncpy(c.inscr, inscr, sizeof(c.inscr));
strncpy(c.name, name, sizeof(c.name));
strncpy(c.curse, curse, sizeof(c.curse));
c.year = 2010 + i;
c.position = 1000 * i + (99 - i);
if (fwrite(&c, sizeof(Candidate), 1, fp) != 1)
{
fclose(fp);
fprintf(stderr, "Failed to write candidate %d\n", i);
return -1;
}
}
fclose(fp);
return 0;
}
Candidate *read_sample_data(const char *filename, int *number)
{
FILE *fp = fopen(filename, "rb");
if (fp == NULL)
{
fprintf(stderr, "Failed to open file %s for reading\n", filename);
return 0;
}
fseek(fp, 0, SEEK_END);
size_t sz = ftell(fp);
rewind(fp);
Candidate *arr = malloc(sz);
if (arr == 0)
{
fprintf(stderr, "Failed to allocate %zu bytes memory\n", sz);
return 0;
}
int i;
for (i = 0; fread(&arr[i], sizeof(Candidate), 1, fp) == 1; i++)
;
*number = i;
return arr;
}
void print_sample_data(int number, Candidate *clist)
{
for (int i = 0; i < number; i++)
{
printf("%-10s %-30s %.4d %.4d %s\n",
clist[i].inscr, clist[i].name, clist[i].year,
clist[i].position, clist[i].curse);
}
}
Note that this produces the same data each time it is run, despite using the pseudo-random numbers from rand(). Note too that the code deliberately exploits the behaviour of strncpy() that pads a field with null bytes to the full length so that the data in the file doesn't have any extraneous matter.
Sample output:
I-0000 Name 0249 Surname 0807 2010 0099 Curse 73
I-0001 Name 0930 Surname 0658 2011 1098 Curse 72
I-0002 Name 0878 Surname 0544 2012 2097 Curse 23
I-0003 Name 0440 Surname 0709 2013 3096 Curse 65
I-0004 Name 0042 Surname 0492 2014 4095 Curse 87
Hex dump of data file:
0x0000: 49 2D 30 30 30 30 00 00 00 00 4E 61 6D 65 20 30 I-0000....Name 0
0x0010: 32 34 39 20 53 75 72 6E 61 6D 65 20 30 38 30 37 249 Surname 0807
0x0020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0x0030: 00 00 00 00 00 00 00 00 DA 07 00 00 63 00 00 00 ............c...
0x0040: 43 75 72 73 65 20 37 33 00 00 00 00 00 00 00 00 Curse 73........
0x0050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0x0060: 49 2D 30 30 30 31 00 00 00 00 4E 61 6D 65 20 30 I-0001....Name 0
0x0070: 39 33 30 20 53 75 72 6E 61 6D 65 20 30 36 35 38 930 Surname 0658
0x0080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0x0090: 00 00 00 00 00 00 00 00 DB 07 00 00 4A 04 00 00 ............J...
0x00A0: 43 75 72 73 65 20 37 32 00 00 00 00 00 00 00 00 Curse 72........
0x00B0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0x00C0: 49 2D 30 30 30 32 00 00 00 00 4E 61 6D 65 20 30 I-0002....Name 0
0x00D0: 38 37 38 20 53 75 72 6E 61 6D 65 20 30 35 34 34 878 Surname 0544
0x00E0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0x00F0: 00 00 00 00 00 00 00 00 DC 07 00 00 31 08 00 00 ............1...
0x0100: 43 75 72 73 65 20 32 33 00 00 00 00 00 00 00 00 Curse 23........
0x0110: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0x0120: 49 2D 30 30 30 33 00 00 00 00 4E 61 6D 65 20 30 I-0003....Name 0
0x0130: 34 34 30 20 53 75 72 6E 61 6D 65 20 30 37 30 39 440 Surname 0709
0x0140: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0x0150: 00 00 00 00 00 00 00 00 DD 07 00 00 18 0C 00 00 ................
0x0160: 43 75 72 73 65 20 36 35 00 00 00 00 00 00 00 00 Curse 65........
0x0170: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0x0180: 49 2D 30 30 30 34 00 00 00 00 4E 61 6D 65 20 30 I-0004....Name 0
0x0190: 30 34 32 20 53 75 72 6E 61 6D 65 20 30 34 39 32 042 Surname 0492
0x01A0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0x01B0: 00 00 00 00 00 00 00 00 DE 07 00 00 FF 0F 00 00 ................
0x01C0: 43 75 72 73 65 20 38 37 00 00 00 00 00 00 00 00 Curse 87........
0x01D0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0x01E0:
There are 4 bytes of padding in the structures; 2 after the name and 2 at the end, so its length is a convenient 96 bytes, which just happens to be a multiple of 16, so the hex dump aligns nicely.