Segmentation fault using strcpy - c

Question

I have some troubles when using strcpy to copy an array of string inside a double pointer with allocated memory, but i can't understand why i get segmentation fault even if i have previously allocated memory.
Here is the code:
#include <stdio.h>
#include <string.h>
typedef struct Students {
int q_exams;
char **done_exams;
}Students;
int main() {
Students a;
int i;
char support[30];
printf("how many exams have you done ?\n");
scanf("%d",&(a.q_exams));
a.done_exams=malloc(sizeof(char*)*a.q_exams);
if(a.done_exams==NULL)
{
printf("out of memory\n");
return 0;
}
for(i=0;i<a.q_exams;i++)
{
printf("Insert the name of the exam\n");
scanf("%28s",support);
a.done_exams[i]=malloc(strlen(support)+1);
if(a.done_exams[i]==NULL)
{
printf("out of memory\n");
return 0;
}
strcpy(a.done_exams[i][0],support);
fflush(stdin);
}
return 0;
}

Answer 1

You need to pass an address of the initial character to strcpy, either like this
strcpy(&a.done_exams[i][0],support);
// ^
// Add an ampersand
or equivalently like this:
strcpy(a.done_exams[i] , support);
// ^
// Remove the second index
Currently, your code passes the value* of the initial character, rather than its address.
* The value is undefined at the time as well, but it is not the primary cause, because you should not be passing value at all.

Answer 2

This code is fixed
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
typedef struct Students {
int q_exams;
char **done_exams;
} Students;
int main()
{
Students a;
int i;
char support[49];
printf("how many exams have you done ?\n");
scanf("%d",&(a.q_exams));
a.done_exams = malloc(sizeof(char*) * a.q_exams);
if(a.done_exams==NULL)
{
printf("out of memory\n");
return 0;
}
for(i = 0 ; i < a.q_exams ; i++)
{
printf("Insert the name of the exam\n");
scanf("%48s",support);
a.done_exams[i] = malloc(strlen(support)+1);
if(a.done_exams[i] == NULL)
{
printf("out of memory\n");
return 0;
}
strcpy(a.done_exams[i]/*[0]*/, support);
/* ^ ^- this is wrong
* + pass the address to the array not the first element value
*
* if you had warnings turned on you would have seen this
*/
fflush(stdin);
}
return 0;
}
notice that
scanf("%48s", support);
requires
char support[49];
which is also fixed in the code.

Answer 3

See the man page of strcpy().
The first argument should be of type char *.
As per your code, the argument [a.done_exams[i][0]] is of type char. You need to pass a char * [starting address of the destination] actually.
Change your code to
strcpy(a.done_exams[i],support);

Answer 4

The
strcpy(a.done_exams[i][0],support);
should be
strcpy(a.done_exams[i],support);
or
strcpy(&a.done_exams[i][0],support);
My advice would be to always compile with compiler warnings turned on. My compiler (gcc) does a very good job of catching the problem and telling you exactly what needs to be done to fix it:
test.c:37:12: warning: incompatible integer to pointer conversion passing 'char' to
parameter of type 'char *'; take the address with & [-Wint-conversion]
strcpy(a.done_exams[i][0],support);
^~~~~~~~~~~~~~~~~~
&
P.S. You are also missing some #includes:
#include <stdlib.h>
#include <string.h>