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
– Ôrel, Commented Aug 26, 2015 at 15:39

Ôrel · Accepted Answer · 2015-08-26 15:41:06Z

2

you can duplicate the buf.

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

answered Aug 26, 2015 at 15:41

Ôrel

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Comments

Gopi · Accepted Answer · 2015-08-26 15:40:03Z

1

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, 2015 at 15:40

Gopi

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Comments

Haris · Accepted Answer · 2015-08-26 15:42:20Z

-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, 2015 at 15:42

Haris

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1 Comment

osappuk Over a year ago

Very detail and precise answer, thank you very much!

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