Why fread doesn't work right?

Question

this program should take in input a number n and so create n triangles. After, I want to write this triangles in a file and to print this file on standard output. I don't understand why if I input n = 2 the program prints only one triangle's coordinates. Moreover if I input coordinates: 1.0,1.1-1.5,1.6-2.4,2.5 for the triangle's vertices, output is 2.4,2.5-0.0,0.0-0.0,0.0. Can someome help me?

#include <stdio.h> #include <stdlib.h> #include <math.h> typedef struct{ float x; float y; } Punto; typedef struct{ Punto p1; Punto p2; Punto p3; } Triangolo; Punto creaPunto(); void creaTriangolo(Triangolo *); void stampaPunto(Punto); void stampaTriangolo(Triangolo); void creaFileTriangoli(char[], int); void stampaFileTriangolo(char[]); Punto creaPunto(){ Punto p; printf("Inserire la coordinata x: "); scanf("%f", &p.x); printf("Inserire la coordinata y: "); scanf("%f", &p.y); return p; } void creaTriangolo(Triangolo *t){ printf("Triangolo.\n"); printf("Vertice p1:\n"); t->p1 = creaPunto(); printf("Vertice p2:\n"); t->p2 = creaPunto(); printf("Vertice p3:\n"); t->p3 = creaPunto(); } void stampaPunto(Punto v){ printf("Coordinata x: %f\n", v.x); printf("Coordinata y: %f\n", v.y); } void stampaTriangolo(Triangolo t){ printf("Triangolo\n"); printf("Vertice p1:\n"); stampaPunto(t.p1); printf("Vertice p2:\n"); stampaPunto(t.p2); printf("Vertice p3:\n"); stampaPunto(t.p3); } void creaFileTriangoli(char f[], int n){ FILE *ptr; Triangolo t; int i; ptr = fopen(f, "wb"); if(ptr == NULL){ printf("File inesistente.\n"); } else{ for(i=0; i<n; i++){ creaTriangolo(&t); fwrite(&t, sizeof(Triangolo), 1, ptr); } } } void stampaFileTriangolo(char f[]){ FILE *ptr; Triangolo t; ptr = fopen(f, "rb"); if(ptr == NULL){ printf("File inesistente.\n"); } else{ fseek(ptr, sizeof(Triangolo), SEEK_SET); while(!feof(ptr)){ fread(&t, sizeof(Triangolo), 1, ptr); stampaTriangolo(t); } fclose(ptr); } } int main(int argc, char *argv[]) { FILE *ptr; char f[300] = "C:\\sole.dat"; Triangolo t; int n; float perimetro; printf("Inserire il numero di triangoli da analizzare: "); scanf("%d", &n); creaFileTriangoli(f,n); stampaFileTriangolo(f); return 0; }

See Why is “while ( !feof (file) )” always wrong?. You also fail to check any return values from fwrite() or fread() - and you also need to check the return value from fclose(). How do you know the fwrite() calls succeeded? — Andrew Henle
– Andrew Henle, Commented Jun 22, 2016 at 11:44
What is the purpose of fseek(ptr, sizeof(Triangolo), SEEK_SET);? This line causes you to skip the first triangle. — s7amuser
– s7amuser, Commented Jun 22, 2016 at 11:51
@s7amuser i thought that I could read the file from the start with that fseek . I try to delete it,but output is the same. — Malgiolio
– Malgiolio, Commented Jun 22, 2016 at 11:54

Weather Vane · Accepted Answer · 2016-06-22 12:14:50Z

There are two errors in stampaFileTriangolo, the first is that you skip the first record by using fseek. Note that when a file is opened, the file pointer is at the start of the file, and after you use fread, the file pointer is advanced automatically. So you don't need to call fseek to read a file sequentially.

The second is the abuse of feof, which does not do what many people think. It tells you after you have read beyond the file, not when there is no more data. I suggest using the return value from fread to control the loop, like this

void stampaFileTriangolo(char f[]){ FILE *ptr; Triangolo t; ptr = fopen(f, "rb"); if(ptr == NULL) { printf("File inesistente.\n"); } else { while (fread(&t, sizeof(Triangolo), 1, ptr) == 1) { stampaTriangolo(t); } fclose(ptr); } }

There is an error also in creaFileTriangoli - you have not closed the file. This may have caused you to think you needed an fseek here.

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