function find_young() have to receive pointer p as an actual parameter.
And p have to point a struct s which has youngest person.
There is no error message but the program isn't work.
Please give me some advice.
typedef struct
{
char *name;
int age;
} PERSON;
void find_young(PERSON **ip)
{
PERSON s[3] = {{"John", 25}, {"Brandon", 28}, {"Alex", 30}};
int i;
int min = s[0].age;
for(i = 0; i < 3; i++)
{
if(min > s[i].age)
**ip = s[i];
}
}
int main()
{
PERSON *p;
find_young(&p);
printf("The youngest person is %s.\n", p->name);
}
You need to make a number of changes in your code to make it work
1) You need to allocate memory to the struct, and for the char* name in the struct
int main()
{
PERSON *p = malloc(sizeof(PERSON));
p->name = malloc(100);
find_young(&p);
printf("The youngest person is %s with age %d.\n", p->name, p->age);
free(p->name);
free(p);
}
2) Then you need to copy both the name and age values to the passed struct properly
void find_young(PERSON **ip)
{
PERSON s[3] = {{"John", 25}, {"Brandon", 28}, {"Alex", 30}};
int i;
int min = s[0].age; //copying the first value
(**ip).age = s[0].age;
strcpy((**ip).name, s[0].name);
for(i = 1; i < 3; i++) //starting from the next values to compare
{
if(min > s[i].age)
{
(**ip).age = s[i].age;
strcpy((**ip).name, s[i].name);
}
}
}
In your code,
either, for p, before passing &p to the function
or, for *ip, inside the function
you need to allocate memory. Other wise, you'll end up dereferencing invalid pointer, which invokes undefined behavior.
Related
#include <stdio.h>
#include <string.h>
typedef struct birth{
char *name;
char time[12];
}birth;
void swap(struct birth *a, struct birth *b){
struct birth tmp;
tmp = *a;
*a = *b;
*b = tmp;
}
int main(){
int n;
birth list[100], *p, *q;
scanf("%d", &n);
getchar();
for(p = list; p < list + n; p++){
scanf("%s %s", &p->name, &p->time);
}
for(p = list; p < list + n - 1; p++){
for(q = p + 1; q < list + n; q++){
if(strcmp(p->time, q->time) > 0){
swap(p ,q);
}
else if(strcmp(p->time, q->time) == 0){
if(strcmp(p->name, q->name) > 0){
swap(p ,q);
}
}
}
}
for(int i = 0; i < n; i++){
printf("%s %s\n", list[i].name, list[i].time);
}
return 0;
}
I am solving the problem of receiving n, which means the number of students, repeating the number of students, receiving the student's name and date of birth, and printing the names in advance if the date of birth is the same.
However, there was no answer, so I checked using the debugger in vcode, and when I received the input, the date of birth was well entered, but the name was not.
You are trying to read a string using a char pointer that was never initialized.
typedef struct birth{
char *name;
char time[12];
}birth;
...
scanf("%s %s", &p->name, &p->time); // error, &p->name points to nowhere
You should either allocate memory yourself or declare it as a fixed size char array. It would be best to check the string boundaries too:
#define S_NAME 12
#define S_TIME 12
typedef struct birth{
char name[S_NAME];
char time[S_TIME];
}birth;
...
// read string with safety guard
if (fgets(p->name, S_NAME, stdin) != NULL) {
// read name successfully
}
if (fgets(p->time, S_TIME, stdin) != NULL) {
// read time successfully
}
I need to have a global dynamic array of pointers, in which I will store my structs, beacuse later I will need to iterate through this array to list all the stored information, I also need to be able to read the name, age and job variables from the console, and store them in a person_t in the iterator array.
#include <stdio.h>
#include <stdlib.h>
typedef struct Person
{
char name[30];
int age;
char job[30];
} person_t;
person_t **iterator;
int capacity = 10;
int size = 0;
int main()
{
int i;
*iterator = (person_t *)malloc(capacity * sizeof(person_t));
for (i = 0; i < capacity; ++i)
{
person_t p;
p.age = i;
*iterator[i] = p;
}
return 0;
}
I get no errors/warnings compiling this code (gcc -ansi -pedantic -Wall -Wextra), but when I try to run it, I get a Segmentation fault immediately.
When you do this:
*iterator = (person_t *)malloc(capacity * sizeof(person_t));
You're deferencing iterator, however as a file-scope pointer variable it's initialized to NULL. Attempting to dereference a NULL pointer invokes undefined behavior.
I suspect what you really want is an array of structs, not an array of pointers to structs. That being the case, define iterator as:
person_t *iterator;
Then you allocate memory for it like this:
iterator = malloc(capacity * sizeof(person_t));
Then assign to array elements like this:
iterator[i] = p;
Your stated purpose is to create a "global dynamic array of pointers, in which I will store my structs". The following modification of your code (see comments) will do this:
person_t p[10] = {0};
int main()
{
int i;
// with declaration: person_t **iterator = NULL;,
//following is all that is needed to create an array of pointers:
iterator = malloc(capacity * sizeof(person_t *));//no need to cast return of malloc
for (i = 0; i < capacity; ++i)
{
//person_t p;//moved to scope that will exist outside of main()
p[i].age = i;
iterator[i] = &p[i];//assign the address of the object to the pointer
//iterator[i] is the ith pointer in a collection of
//pointers to be assigned to point to
//instances of struct person_t
}
//Once all fields are populated (to-do), the following will display the results:
for (i = 0; i < capacity; ++i)
{
printf("%d) Name: %s Age: %d Job: %s\n", i, iterator[i]->name,iterator[i]->age,iterator[i]->job);
}
return 0;
}
you are not allocating memory correctly
First you need to allocate memory for a pointer which can store capacity number of address i.e done through iterator = malloc(capacity * sizeof(person_t*)); and then you need to allocate memory for holding each structure element i.e iterator[i] = malloc(sizeof(person_t));
all the malloc'ed memory should be free'd once we are done with it.
Also, have not done the error check for malloc's , that is left as an exercise for you.
int main()
{
int i;
// test data
char *names[] = {"ABC", "DEF"};
char *jobs[] = {"Accountant", "Security"};
int ages[] = {50, 60};
// first allocate memory for iterator , which can hold pointers to store iterator poniters
iterator = malloc(capacity * sizeof(person_t*));
for (i = 0; i < capacity; ++i)
{
// now allocate memory for individual iterator
iterator[i] = malloc(sizeof(person_t));
strcpy(iterator[i]->name,names[i]);
iterator[i]->age = ages[i];
strcpy(iterator[i]->job, jobs[i]);
}
for (i = 0; i < capacity; ++i)
{
printf("name = %s ", iterator[i]->name);
printf("Age = %d ", iterator[i]->age);
printf("Job = %s\n", iterator[i]->job);
}
return 0;
}
So I've written this program to represent a car park as a bitset, each space in the car park being one bit. I have a checkSpace function to check if a space is occupied or not and for some reason the pointer to my car park bitset changes or the data changes after I pass it into the function. To test it I set up the car park, I checked a space, then checked it again immediately after and for some reason the return value is changing when it shouldn't be. Any help would be appreciated!
struct carPark{
int spaces, levels;
unsigned char * park;
};
struct carPark * emptyCarPark(int levels, int spaces){
int chars = (spaces*levels)/8;
if((spaces*levels)%8 != 0){
chars++;
}
unsigned char park[chars];
for (int i = 0; i < chars; ++i){
park[i] = 0;
}
unsigned char * ptr = &park[0];
struct carPark * myPark = malloc(sizeof(struct carPark));
myPark->park = ptr;
myPark->spaces = spaces;
myPark->levels = levels;
return myPark;
}
int checkSpace(int level, int spaceNum, struct carPark * carpark){
int charPosition = ((level*carpark->spaces) + spaceNum)/8;
int bitPosition = ((level*carpark->spaces) + spaceNum)%8;
if(carpark->park[charPosition]&&(1<<bitPosition) != 0){
return 1;
}
return 0;
}
int main(int argc, char const *argv[]){
struct carPark * myPark = emptyCarPark(5,20);
printf("1st check: %d\n",checkSpace(1,1,myPark));
printf("Second check: %d\n",checkSpace(1,1,myPark));
return 0;
}
So when I run the program I get:
1st check: 0
Second check: 1
Look at the code below - in emptyCarPark() you are allocating the park array on the stack, and then returning a pointer to it. As soon as the function returns, the park array is no longer allocated and you have a dangling pointer - for more information, see: Cause of dangling pointers (Wikipedia)
unsigned char park[chars];
for (int i = 0; i < chars; ++i){
park[i] = 0;
}
// This is the pointer to an object on the stack.
unsigned char * ptr = &park[0];
struct carPark * myPark = malloc(sizeof(struct carPark));
myPark->park = ptr;
I'm new to C, so this may be a silly question to ask:
What I want to do here is to input the data to the array of pointers to a structure and then print it out. But I get a segmentation fault when running into the insert function.
Below is my code
common.h
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct book * Book;
struct book{
int id;
char *name;
};
extern int b_insert(Book *b, int id, char *name);
extern int b_print(Book books[], int len);
insert.c
#include "common.h"
int b_insert(Book *b, int id, char *name){
Book p;
p = (Book)malloc(sizeof(struct book));
p->id = id;
strcpy(p->name, name);
*b = p;
printf("success insert book:\n");
printf("\tID: %d Name: %s\n", (*b)->id, (*b)->name);
return 0;
}
int b_print(Book books[], int len){
int i;
printf("Book List\n");
for(i=0; i<len; i++){
printf("books[%d] = ID: %d, Name: %s\n", i, books[i]->id, books[i]->name);
}
return 0;
}
main.c
#include "common.h"
#define MAX 2
int main(){
Book books[MAX];
Book *b=books;
int i;
int id;
char name[10];
for(i=0; i<MAX; i++){
printf("please input new books info\n");
printf("ID: ");
scanf("%d", &id);
printf("Name: ");
scanf("%s", name);
if(b_insert(b, id, name) == -1){
printf("fail to insert\n");
}
b++;
}
b_print(books, MAX);
return 0;
}
Main problem:
Allocate memory for p->name before using
strcpy(p->name, name);
using malloc:
p->name = malloc(10); //Or some other size
Other problems:
Remove the cast here:
p = (Book)malloc(sizeof(struct book));
Why? Here is the answer
if(b_insert(b, id, name) == -1){ will never be true.
Check the result of malloc to check if it was successful in allocating memory.
Check the return value of all the scanfs to see if it was successful in scanning data.
Add a length modifier to the second scanf to prevent buffer overflows:
scanf("%9s", name); /* +1 for the NUL-terminator */
You're not allocating space for name:
int b_insert(Book *b, int id, char *name){
Book p;
p = malloc(sizeof(struct book));
if (p != NULL)
{
p->name = malloc(strlen(name)+1); // It allocates space where the input name will be copied.
if (p->name != NULL)
{
p->id = id;
strcpy(p->name, name);
*b = p;
printf("success insert book:\n");
printf("\tID: %d Name: %s\n", (*b)->id, (*b)->name);
}
else return -1; // No space to allocate string
}
else return -1; // No space to allocate struct
return 0;
}
As mentioned before, allocate space for p->name. You should probably also use something different to read the book title, either scanf format %ms with a pointer to a char pointer, or %9s with your buffer, otherwise the title "war or peace" will also result in a segfault.
Here you create a static variable and the space for it is allocated automatically.
Book p;
You can allocate a space manually when you assign it to pointer, in this line it's not pointer but static variable.
p = (Book)malloc(sizeof(struct book));
What's more if you want to refer to attribute of static variable you should use "." instead of "->". So you have two option. Create a pointer and allocate a space for the structure and then you "->" oraz create static variable.
p->id = id;
I am trying to make a dynamic array of structs, and I can successfully add one struct to it. But any more structs I add cause a segmentation fault. Here is my code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define PEOPLE_BLOCK 4
struct Person {
char *first_name;
char *last_name;
unsigned int age;
};
int add_person(struct Person **people, size_t *people_size, size_t *population, struct Person p) {
if ((sizeof(struct Person) * *population) > *people_size) {
return -1;
}
if ((sizeof(struct Person) * (*population + 1)) >= *people_size) {
*people_size = *people_size + sizeof(struct Person) * PEOPLE_BLOCK;
*people = realloc(*people, *people_size);
if (!*people) {
return -1;
}
}
*people[*population] = p;
++*population;
return 0;
}
int main(int argc, char const *argv[]) {
size_t population;
size_t people_size;
struct Person *people, timn, batman;
population = 0;
people_size = sizeof(struct Person) * PEOPLE_BLOCK;
people = malloc(people_size);
timn.first_name = "Timn";
timn.last_name = "Timothy";
timn.age = 38;
add_person(&people, &people_size, &population, timn);
printf("Person 0's first name: %s\n", people[0].first_name);
batman.first_name = "Bat";
batman.last_name = "Man";
batman.age = 42;
add_person(&people, &people_size, &population, batman);
printf("Person 1's first name: %s\n", people[1].first_name);
free(people);
return 0;
}
I'd appreciate any help on why this is happening, thanks!
The problem resides with this line :
*people[*population] = p;
Change it to:
(*people)[*population] = p;
Why are the parenthesis requried?
The compiler has rules of operator precedence. When applying them, it sees your code as this:
*(people[*population]) = p;
which is not what you intended. Given a pointer-to-pointer Type **pp,
*pp[n] = value;
means "take the n'th pointer starting at pp, and assign value at the location dereferenced from the address that pointer holds. In other words, it means essentially this:
Type *p = pp[n];
*p = value;
What you really want is something that does this:
Type *p = *pp;
p[n] = value;
and that is what (*pp)[n], distinguishing the dereference of the pointer to pointer, gives you. Without that, you're using an invalid pointer, leading to your fault.
Not sure whether this answer will help, but anyway.
I don't understand your code, what you are trying to do.
You directly use the number of elements, a pointer to the first person, and the maximum number of elements. You'll probably have a lot of problems passing that all around.
You're storing literal strings directly in your structs, which means that in a real case (using no literals) that would result in memory leaks.
Here is my take. I've made PEOPLE_BLOCK smaller for testing reasons.
Hope this helps.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define PEOPLE_BLOCK 2
typedef struct _Person {
char *first_name;
char *last_name;
unsigned int age;
} Person;
typedef struct _VectorPeople {
Person * people;
size_t num;
size_t max;
} VectorPeople;
void init(VectorPeople *v)
{
v->max = PEOPLE_BLOCK;
v->num = 0;
v->people = (Person *) malloc( sizeof(Person) * v->max );
}
void clear(VectorPeople *v)
{
// Clear persons
Person * it = v->people;
while( ( it - v->people ) < v->num ) {
free( it->first_name );
free( it->last_name );
++it;
}
// Clear vector
v->max = v->num = 0;
free( v->people );
v->people = NULL;
}
void add(VectorPeople *v, Person *p)
{
// Reserve
if ( v->num >= v->max ) {
v->max += PEOPLE_BLOCK;
// Realloc
v->people = realloc( v->people, v->max * sizeof(Person) );
if ( v->people == NULL ) {
exit( -1 );
}
}
// Copy strings
p->first_name = strdup( p->first_name );
p->last_name = strdup( p->last_name );
// Insert
v->people[ ( v->num )++ ] = *p;
}
int main(int argc, char const *argv[]) {
VectorPeople vp;
Person timn;
Person batman;
Person bond;
Person superman;
init( &vp );
timn.first_name = "Timn";
timn.last_name = "Timothy";
timn.age = 38;
add( &vp, &timn );
batman.first_name = "Batn";
batman.last_name = "Man";
batman.age = 42;
add( &vp, &batman );
bond.first_name = "James";
bond.last_name = "Bond";
bond.age = 45;
add( &vp, &bond );
superman.first_name = "Super";
superman.last_name = "Man";
superman.age = 45;
add( &vp, &superman );
int i = 0;
for(; i < vp.num; ++i ) {
printf( "Person: %s, %s.\n", vp.people[ i ].last_name, vp.people[ i ].first_name );
}
clear( &vp );
return 0;
}
There were a number of errors in your code. One thing to keep in mind, when you dynamically allocate memory, you are responsible for keeping track of it and freeing it when you no longer need it (otherwise, you will leak memory like a sieve).
In your code, you attempt to create an array of structs holding pointer to an array of characters. The char * pointers are NOT allocated and cannot simply be assigned in the manner you attempt. strdup can help, but you have just allocated memory, so free it when you are done with it.
Attempting to allocate an array of structs with varying (unknown) lengths of first_name and last_name requires that you keep track of every allocation. In some sense, you are better off declaring people as pointer to pointer to Person This allows iteration over your people without having to store the population somewhere allowing you to iterate until the first NULL pointer is encountered.
Likewise, creating a typedef to your struct can greatly cut down on the number of times you write sizeof (struct Person). It keeps the code clean and helps you think though the pointer haze.
Here is an example using a pointer-to-pointer-to-struct of what I think you intended to do. It is followed below by an implementation using only a pointer to struct. Evaluate both and decide which implementation you prefer:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXPOP 128
typedef struct {
char *first_name;
char *last_name;
unsigned char age;
} Person;
Person *add_person (Person ***ppl, Person p, size_t *pop, size_t *max);
Person **realloc_person (Person **ppl, size_t *n);
void free_person (Person *p);
void free_person_names (Person *p);
int main (void) {
size_t population = 0;
size_t maxp = MAXPOP;
size_t i = 0;
Person timn, batman;
Person **people = calloc (MAXPOP, sizeof *people);
if (!people) {
fprintf (stderr, "error: virtual memory exhausted.\n");
return 1;
}
timn.first_name = strdup ("Timn");
timn.last_name = strdup ("Timothy");
timn.age = 38;
add_person (&people, timn, &population, &maxp);
free_person_names (&timn);
printf("\nPerson 0\n first name: %s\n last name : %s\n age : %hhu\n",
people[0]->first_name, people[0]->last_name, people[0]->age);
batman.first_name = strdup ("Bat");
batman.last_name = strdup ("Man");
batman.age = 42;
add_person (&people, batman, &population, &maxp);
free_person_names (&batman);
printf("\nPerson 1\n first name: %s\n last name : %s\n age : %hhu\n",
people[1]->first_name, people[1]->last_name, people[1]->age);
for (i = 0; i < population; i++)
free_person (people[i]);
free (people);
return 0;
}
/* add a person to an array of pointers to Person */
Person *add_person (Person ***ppl, Person p, size_t *pop, size_t *max)
{
if (*pop == *max)
*ppl = realloc_person (*ppl, max);
if (!((*ppl)[*pop] = malloc (sizeof ***ppl)))
return NULL;
size_t i = (*pop)++;
(*ppl)[i]-> first_name = strdup (p.first_name);
(*ppl)[i]-> last_name = strdup (p.last_name);
(*ppl)[i]-> age = p.age;
return (*ppl)[i];
}
/* realloc an array of pointers to Person setting memory to 0. */
Person **realloc_person (Person **ppl, size_t *n)
{
Person **tmp = realloc (ppl, 2 * *n * sizeof *ppl);
if (!tmp) {
fprintf (stderr, "Error: struct reallocation failure.\n");
// return NULL;
exit (EXIT_FAILURE);
}
ppl = tmp;
memset (ppl + *n, 0, *n * sizeof *ppl); /* memset new ptrs 0 */
*n *= 2;
return ppl;
}
/* free memory for a Person */
void free_person (Person *p)
{
if (!p) return;
if (p->first_name) free (p->first_name);
if (p->last_name) free (p->last_name);
free (p);
}
/* free only names of Person (for temp structs) */
void free_person_names (Person *p)
{
if (!p) return;
if (p->first_name) free (p->first_name);
if (p->last_name) free (p->last_name);
}
Note: updated to correct ppl start address on reallocation.
Using only Array of Person
While not inherently different than using a pointer to pointer to Person using a simple pointer to Person eliminates the ability to iterate over your array until a NULL or (empty) pointer is encountered. The following is an implementation of the same code using only an array of Person:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXPOP 128
typedef struct {
char *first_name;
char *last_name;
unsigned char age;
} Person;
Person *add_person (Person **ppl, Person p, size_t *pop, size_t *max);
Person *realloc_person (Person *ppl, size_t *n);
void free_person_names (Person p);
int main (void) {
size_t population = 0;
size_t maxp = MAXPOP;
size_t i = 0;
Person timn, batman;
Person *people = calloc (MAXPOP, sizeof *people);
if (!people) {
fprintf (stderr, "error: virtual memory exhausted.\n");
return 1;
}
timn.first_name = strdup ("Timn");
timn.last_name = strdup ("Timothy");
timn.age = 38;
add_person (&people, timn, &population, &maxp);
free_person_names (timn);
printf("\nPerson 0\n first name: %s\n last name : %s\n age : %hhu\n",
people[0].first_name, people[0].last_name, people[0].age);
batman.first_name = strdup ("Bat");
batman.last_name = strdup ("Man");
batman.age = 42;
add_person (&people, batman, &population, &maxp);
free_person_names (batman);
printf("\nPerson 1\n first name: %s\n last name : %s\n age : %hhu\n",
people[1].first_name, people[1].last_name, people[1].age);
for (i = 0; i < population; i++)
free_person_names (people[i]);
free (people);
return 0;
}
/* add a person to an array of pointers to Person */
Person *add_person (Person **ppl, Person p, size_t *pop, size_t *max)
{
if (*pop == *max)
*ppl = realloc_person (*ppl, max);
size_t i = (*pop)++;
(*ppl)[i].first_name = strdup (p.first_name);
(*ppl)[i].last_name = strdup (p.last_name);
(*ppl)[i].age = p.age;
return ppl[i];
}
/* realloc an array Person setting memory to 0. */
Person *realloc_person (Person *ppl, size_t *n)
{
Person *tmp = realloc (ppl, 2 * *n * sizeof *ppl);
if (!tmp) {
fprintf (stderr, "Error: struct reallocation failure.\n");
// return NULL;
exit (EXIT_FAILURE);
}
ppl = tmp;
memset (ppl + *n, 0, *n * sizeof *ppl); /* memset new ptrs 0 */
*n *= 2;
return ppl;
}
/* free only names of Person (for temp structs) */
void free_person_names (Person p)
{
if (p.first_name) free (p.first_name);
if (p.last_name) free (p.last_name);
}
Output
$ ./bin/struct_add_person
Person 0
first name: Timn
last name : Timothy
age : 38
Person 1
first name: Bat
last name : Man
age : 42
One problem is the last argument of add_person() to be specific, the argument '(struct Person) p'. When 'timn' and 'batman' are passed into the add_person() function, they are passed as a copy of the original structure. In the add_person() structure, that data is actually on the stack and is volatile outside the scope of the function. Try changing the last argument to a pointer.