C passing string between functions

Question

I'm writing simple program in C and got stucked. I have three functions:

void changeStatusOfBook(User users[], Book books[]) {
  char *id= askForBookID();
  Book book = getBookById(books, id);
  .
  .
  .
}

char * askForBookID() {
  char id[6];
  printf("Tell me which book (ID)\n");
  scanf_s("%5s",id, 6);
  return id;
}

Book getBookById(Book books[], char bookID[]) {
  int counter = 0;
  //bookID becomes a trash here
  .
  .
  .
}

The problem is: In the first function I get correct user string input, but when I pass it to third function I'm getting some trash in it. How to fix it?

Compile with all warnings & debug info. Notice that arrays are decayed into pointers when passed as arguments. — Basile Starynkevitch
– Basile Starynkevitch, Commented Feb 18, 2017 at 12:47

Seek Addo · Accepted Answer · 2017-02-18 19:25:09Z

3

You can't return a local variable char id[] from a function.It's memory is on the stack and when the function returns all stack memory for that function and its local variables are no more accessible.

when memory is need on the stack for another program it overrides the memory space for char id, and this becomes a problem for your program.

 char * askForBookID() {
      //char id[6];
        char *id = malloc(sizeof(char)*6);
        if(!id){ //check if malloc got some memory for id
           printf("Sorry not enough memory"); return NULL;

         }

       printf("Tell me which book (ID)\n");
       scanf_s("%5s",id, 6);

       return id;
}

edited Feb 18, 2017 at 19:25

answered Feb 18, 2017 at 12:56

Seek Addo

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4 Comments

Bathsheba Over a year ago

I prefer my answer as it doesn't make any assumptions about the implementation of c on the platform, but this answer is probably more useful. Have an upvote.

Seek Addo Over a year ago

@Bathsheba i agree with you.

MrKaszu Over a year ago

Thanks it worked - I was trying to use malloc in changeStatusOfBook() but dunno why I didn't think about askForBookID().

Seek Addo Over a year ago

@MrKaszu oh i see, i hope i could help. When you need memory in a function you need to use malloc. Sometimes making the variable static also but i prefer malloc. don't forget askForBookID() is now responsible for freeing memory use free(askForBookID()) when done with it.

Bathsheba · Accepted Answer · 2017-02-18 12:47:05Z

3

The function askForBookID returns the address of the first element of an array with automatic storage duration.

The behaviour on your using that pointer oncce the function has been called is undefined.

Use malloc instead.

answered Feb 18, 2017 at 12:47

Bathsheba

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