Segmentation fault with structures assignments C

Question

I get segmentation fault on v[cant]=completar(); I checked the function completar and it works ok, and the segmentation fault only appears if I do the assignment. So I really don't know what's going on.

#include <stdio.h>
#define cant 100
#define nom  20
struct detalles
{
    int l;
    char n[nom];
    float p;
    int I;
};

struct detalles completar();
main()
{ 
    int i;
    struct detalles v[cant];


    //trouble is here

    v[cant]=completar();

    for(i=0;i<(v[0].I);i++)
    {
        printf("\v\v\v\v\v\r");
        printf("legajo\t%d\n",v[i].l);
        printf("Nombre\t%s\n",v[i].n);
        printf("Promedio\t%f\n",v[i].p);
    }
    return 0;
}

struct detalles completar()
{

    int i=0;
    struct detalles temp[cant];
    printf("Ingrese cero cundo desee terminar\n");
    do
    {
        printf("Ingrese el numero de legajo de un estudiante\t");
        scanf("%d",&temp[i].l);
        while(temp[i].l<0)
        {
            printf("El numero ingresado no es valido, intente nuevamente\v\v\r");
            printf("Ingrese el numero de legajo de un estudiante\t");
            scanf("%d",&temp[i].l);
        }
        printf("ingrese el nombre del alumno\t");
        scanf("%s",temp[i].n);
        printf("Ingrese promedio del alumno\t");
        scanf("%f",&temp[i].p);
        i++;
        if(i==100)
        {
            printf("Se ha superado en numero maximo de estudiantes\n");
        }
    }
    while((temp[i-1].p!=0)&&(temp[i-1].l!=0)&&(i<100));
    temp[0].I=i-1;
    return temp[cant];
}

Your program invokes undefined behavior. You're accessing your v array out of range. It is only cant items in size, meaning it is only subscriptable by 0..(cant-1). Further v[0].I is indeterminate, yet you rely on its value in the condition of your `for(i-0;i<v[0].I;...) loop.; more undefined behavior. — WhozCraig
– WhozCraig, Commented Aug 15, 2016 at 18:04
not sure if this is the problem, but you are writing outside the bounds ofr your v array. Indices should be from 0 to cant - 1 — yano
– yano, Commented Aug 15, 2016 at 18:05
return temp[cant]; has the same problem. It doesn't refer to the whole array, it refers to an invalid position one past the end of the array. — aschepler
– aschepler, Commented Aug 15, 2016 at 18:08
it doesn't matter what the while loop is doing. cant is a defined constant. return temp[cant]; is the same as return temp[100];, which is outside the bounds of the array. If you want to return the last element in the array, return temp[cant-1]; instead. — yano
– yano, Commented Aug 15, 2016 at 18:37

Renzo · Accepted Answer · 2016-08-15 18:04:51Z

1

cant is equal to 100, you assign a value to v[cant], that is to v[100], but the array v, with length 100, is assignable only with an index from 0 to 99.

answered Aug 15, 2016 at 18:04

Renzo

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2 Comments

rocbl Over a year ago

I don't use index 100. Only on the while below I use the index and don't include the 100. Anyway, If I comment the line v[cant]=completar(); it works fine. So don't why there's a trouble if the function returners temp[cant]. ------------------------------------------------------------------------------------------------------------------- while((temp[i-1].p!=0)&&(temp[i-1].l!=0)&&(i<100)); temp[0].I=i-1; return temp[cant];

Renzo Over a year ago

@rocbl, the first command of the main is v[cant]=completar();, and cant is a constant with value 100, so the statement is actually assigning a value returned by completar() to v[100]. This is the reason of the segmentation, as you have discovered by commenting that line. The statement return temp[cant]; does not cause the fault since you are only reading v[100], not assigning it, but this means that the statement read a value outside of the vector space.

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