Segmentation fault and realloc(): invalid next size: [duplicate] - c

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free char*: invalid next size (fast) [duplicate]
(1 answer)
Closed 8 years ago.
I have a program and gets information from the keyboard and puts them into a struct and then write the struct to a file.
However, when I'm reallocating memory for the second time it seems to fail for no reason. Also, if I enter more than 1 person's information, the program fails in the end with a seg fault. The program runs fine if I enter just 1 person's information.
Thanks.
// Program
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
typedef struct person person;
struct person {
char fname[20];
char lname[20];
int num;
};
int main(void){
int size = 0;
int count = 0;
person* listofperson = NULL;
char answer = 'n';
FILE* myfile;
do{
char* buf = (char*)malloc(sizeof(char)*50);
printf("Please enter the person's first name: \n");
fgets(buf, 50, stdin);
if(count == size){
size += 2;
listofperson = (person*)realloc(listofperson, (size_t)(sizeof(person)*size));
}
strncpy((listofperson+count)->fname, buf, 50);
printf("Please enter the person's last name: \n");
fgets(buf, 50, stdin);
strncpy((listofperson+count)->lname, buf, 50);
printf("Please enter the person's number: \n");
fgets(buf, 50, stdin);
sscanf(buf, "%d", &((listofperson+count)->num));
free(buf);
count++;
printf("Do you want to enter another one?\n");
answer = getchar();
getchar();
}while(tolower(answer) != 'n');
myfile = fopen("myfile", "a");
for(int i = 0; i < count; i++){
fprintf(myfile, "%s", (listofperson+i)->fname );
fprintf(myfile, "%s", (listofperson+i)->lname );
fprintf(myfile, "%d\n", (listofperson+i)->num );
}
fclose(myfile);
myfile = NULL;
free(listofperson);
}

Answer 1

For one thing, the people who are saying that realloc() doesn't work with a NULL pointer aren't speaking the truth. The behavior is documented here for C++ and here for C, it just works like malloc() in the case of a NULL pointer passed. Although I would agree that it is bad practice to assign memory that way.
You are not checking for errors in your malloc() and realloc() calls, they are not guaranteed to succeed, so you shouldn't assume they will.
In this case you shouldn't name your point to a person node "list of people", as this convention may be confused with a linked list. I would strongly reccomend you attempt to implement a linked lists for this programming case, since that is basically how you are working with your data anyways. For a tutorial on linked lists, see this link.
You should change the name of the file in fopen() to include a .txt extension. Otherwise the system will not know what the file type is.

Answer 2

change
struct person {
char fname[20];
char lname[20];
int num;
};
to
struct person {
char fname[50];
char lname[50];
int num;
};
Which is too small, it's already pointed out by ouah. It is necessary to align the value of one.
The error message indicates that the memory is destroyed beyond the area secured.