Structer pointer and pointer char array malloc array - c

Question

I want to do structer array but I don't know structer array size therefore I need to use pointer structer and I want to do char array in the structer and I don't know char array size therefore I need to use pointer char in this structer but I don't understand malloc and realloc functions. How can I do this ?
#include <stdio.h>
#include <stdlib.h>
struct School{
char *school_name;
int student_size;
}*high_school;
void createSchool(struct School *s, char *schl_name, int student, int school_size)
{
int i = 0;
if(school_size == 1){
s = (struct School*) malloc(sizeof(struct School));
}
else{
s = (struct School*) realloc(s, (school_size*sizeof(struct School)));
}
(s+(school_size-1))->student_size = student;
(s+(school_size-1))->school_name = (char *) malloc(20); // 20 it is not important
(s+(school_size-1))->school_name = schl_name;
for(i; i<school_size; i++){
printf("%s\t%d\n",(s+i)->school_name, (s+i)->student_size);
}
printf("\n\n");
}
int main()
{
int i = 1;
createSchool(high_school, "Harvard", 50, i);
i++;
createSchool(high_school, "Oxford", 40, i);
i++;
createSchool(high_school, "MIT", 30, i);
}
I want to do screen shoot:
Harvard 50
Harvard 50
Oxford 40
Harvard 50
Oxford 40
MIT 30
but screen shoot of program :
Harvard 50
└1q 7405760
Oxford 40
7405760
(null) 0
MIT 30

Answer 1

Your pointer inside createSchool has local scope, so global pointer is not modified. Faster fix is to return new allocated memory back to caller.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct School{
char *school_name;
int student_size;
}*high_school;
struct School* createSchool(struct School *s, char *schl_name, int student, int school_size)
{
if(school_size == 1)
{
s = malloc(sizeof(struct School));
}
else
{
s = realloc(s, (school_size*sizeof(struct School)));
}
if (s != NULL)
{
s[school_size-1].student_size = student;
s[school_size-1].school_name = malloc(strlen(schl_name)+1);
strcpy(s[school_size-1].school_name, schl_name);
for(int i=0; i<school_size; i++)
{
printf("%s\t%d\n", s[i].school_name, s[i].student_size);
}
printf("\n\n");
}
return s;
}
int main(void)
{
int i = 1;
high_school = createSchool(high_school, "Harvard", 50, i);
i++;
high_school = createSchool(high_school, "Oxford", 40, i);
i++;
high_school = createSchool(high_school, "MIT", 30, i);
}
Minimal signature for main is int main (void)
Take note that malloc/realloc returned value have to be checked.
With your code, in case of realloc fails, you are loosing the pointer to the already allocated memory. So you should use a temp pointer to store the realloc result and check for integrity. After that you can reassign it ot your pointer.
struct School* createSchool(struct School *s, char *schl_name, int student, int school_size)
{
if(school_size == 1){
s = malloc(sizeof(struct School));
}
else
{
struct School *temp = realloc(s, (school_size*sizeof(struct School)));
if (temp == NULL)
return s;
s = temp;
}
if (s != NULL)
{
s[school_size-1].student_size = student;
s[school_size-1].school_name = malloc(strlen(schl_name)+1);
strcpy(s[school_size-1].school_name, schl_name);
for(int i=0; i<school_size; i++)
{
printf("%s\t%d\n", s[i].school_name, s[i].student_size);
}
printf("\n\n");
}
return s;
}
Different solution can be implemented using double pointer:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct School{
char *school_name;
int student_size;
}*high_school;
void createSchool(struct School **s, char *schl_name, int student, int school_size)
{
if(school_size == 1)
{
*s = malloc(sizeof(struct School));
}
else
{
struct School *temp = realloc(*s, (school_size*sizeof(struct School)));
if (temp == NULL)
return;
*s = temp;
}
if (*s != NULL)
{
(*s)[school_size-1].student_size = student;
(*s)[school_size-1].school_name = malloc(strlen(schl_name)+1);
strcpy((*s)[school_size-1].school_name, schl_name);
for(int i=0; i<school_size; i++)
{
printf("%s\t%d\n", (*s)[i].school_name, (*s)[i].student_size);
}
printf("\n\n");
}
}
int main(void)
{
int i = 1;
createSchool(&high_school, "Harvard", 50, i);
i++;
createSchool(&high_school, "Oxford", 40, i);
i++;
createSchool(&high_school, "MIT", 30, i);
}
Last thing take note that,to assign the name of school you can simply use:
(*s)[school_size-1].school_name = schl_name;