C programming - why garbage data inside my struct?

Question

I have the following program in C:

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <string.h>

typedef struct str{
    char * s;
    int len;
}string;

typedef struct stud{
    unsigned int id;
    string name;
    char gender;
}student;

student* addstud(const int id, const char name[64], const char gender);

student* addstud(void){
    char buf[64]; 
    struct stud *sb;
    sb = (struct stud*)malloc(sizeof(struct stud));

    printf("Enter student ID:\n");
    scanf("%d",&sb->id);

    printf("Enter student name:\n");
    scanf("%s", buf);
    sb->name.s = buf;
    sb->name.len = (int)strlen(buf);

    printf("Enter student gender:\n");
    scanf(" %c", &sb->gender);
    printf("Student ID: %d, name: %s, gender: %c\n", sb->id, sb->name.s, sb->gender);

    return sb;
}

int main(){
    student *mystudent;
    mystudent=addstud();
    printf("Student ID: %d, name: %s, gender: %c\n", mystudent->id, mystudent->name.s, mystudent->gender);
    getchar();
    return 0;
}

In the addstud() function, everthing is fine but in main() when I try to print out the student name from mystudent->name.s it just garbage data. I don't understand why this could happened as it was clearly fine in addstud() function and both pointers point to the same memory address :( Could anyone please shred some light what I did wrong here, please? Thank you!

`sb->name.s = buf;` buf is on the stack you need to dup the string — Ôrel, Aug 26 '15 at 15:39
[don't cast the return value of `malloc()`](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — The Paramagnetic Croissant, Aug 26 '15 at 15:45

score 2 · Accepted Answer · answered Aug 26 '15 at 15:41

2

you can duplicate the buf.

sb->name.s = strdup(buf);

answered Aug 26 '15 at 15:41

Ôrel

7,044
3
27
46

score 0 · Answer 2 · answered Aug 26 '15 at 15:40

0

char buf[64];

Here buf is local to function addstud() so once you exit the function this array is out of scope and accessing it will lead to undefined behavior.

answered Aug 26 '15 at 15:40

Gopi

19,784
4
24
36

score -1 · Answer 3 · answered Aug 26 '15 at 15:42

-1

sb->name.s = buf;

In this, buf is a local variable to the function addstud(). So, when the function exits, accessing the variable is UB.

You have 2 options,

1)Either allot memory for buf using malloc()

2) Use strcpy() to copy the contents of the variable buf to sb->name.s

answered Aug 26 '15 at 15:42

Haris

12,120
6
43
70

Very detail and precise answer, thank you very much! – osappuk Aug 26 '15 at 16:08

C programming - why garbage data inside my struct?

3 Answers3