fread fails to read an unsigned int? [C] - c

Question

I have a Person Struct as the following:
typedef struct Person {
char name[NUM_CHARS];
unsigned int age;
} Person;
where #define NUM_CHARS 20.
I want to write this structure to a binary file, so I wrote two function to handle that:
int writePerson(Person* person, FILE* _fp) {
int i = 0;
int count = 0;
int len = strlen(person->name);
// Write name
for(i = 0; i < len+1; i++) {
count += fwrite(&(person->name[i]), sizeof(char), 1, _fp);
// fseek(_fp, 1, SEEK_CUR);
}
// Continue
// Write age
count += fwrite(&(person->age), sizeof(unsigned int), 1, _fp);
return count;
}
int readPerson(Person* person, FILE* _fp) {
int i = 0;
int count = 0;
// Write name
for(i = 0;person->name[i] != NULL;i++) {
count += fread(&(person->name[i]), sizeof(char), 1, _fp);
}
// Continue
// Write age
count += fread(&(person->age), sizeof(unsigned int), 1, _fp);
return count;
}
So I wrote a Person to a blank file Px.bin that I've created well in advance:
int main() {
FILE* fp = fopen("Px.bin", "r+b");
Person person = {"Billie", 40};
// Person y ;
int x = writePerson(&person, fp);
printf("%d", x);
// printPerson(&y);
getchar();
fcloseall();
return 0;
}
Seems to work well, prints 8.
but when I try to read this file:
int main() {
FILE* fp = fopen("Px.bin", "r+b");
// Person person = {"Billie", 40};
Person y ;
int x = readPerson(&y, fp);
printf("%d\n", x);
printPerson(&y);
getchar();
fcloseall();
return 0;
}
I'm getting this result:
11
Billie
-858993460
Where printPerson is:
void printPerson(Person* p) {
printf("%s\n%d\n", p->name, p->age);
}
What is the problem?

Answer 1

This bit
for(i = 0;person->name[i] != NULL;i++) {
count += fread(&(person->name[i]), sizeof(char), 1, _fp);
}
is going to use uninitialized (IOW, containing garbage) person->name[] as the condition for reading. And it may so happen that the wrong number of bytes will get read here.
After that the following
count += fread(&(person->age), sizeof(unsigned int), 1, _fp);
can read the integer from an incorrect location within the file, not where said integer has been stored.
The fix would be to first read a character and then see if it's '\0' or not. If it is, the string's been fully read. If it's not, keep reading characters.

Answer 2

Your code that reads name does check for likely non-initialized (or zeroed out) person->name[i] elements. As result the loop end at some non-predictable time (i.e. if it is zeroed out loop will not fread even single character).
for(i = 0;person->name[i] != NULL;i++) {
count += fread(&(person->name[i]), sizeof(char), 1, _fp);
}
You want to change it to do/while loop or instead of reading till 0 prefix data in the file string with length and read that many characters.