How to change size of a struct's attribute in C?

Question

I'm very new in C programming and I need to create a phone book. Each entry must have a name, a phone number and a notes section. The thing is that the size of notes section should be changeable according to input. For example, if the user enters a string with 30 characters, size of note should be 30. I couldn't figure out how to do it.

Here is my code so far:

#include <stdio.h>
#include <stdlib.h>
#include <note>

struct Entries {
    char name[50];
    char phoneNumber[11];
    char * note;
    note = (char *)malloc(5*sizeof(char));
}Entries;

int main()
{
int command;
int counter = 0;
char tempNote[50];
char tempName[50];
char tempPhoneNumber[11];

while(1){
    printf("Welcome to myPhoneBook! Please select an option:\n   1) New entry\n   2) List all entries\n   3) Edit\n   4) Delete\n   5) Search\n");
    scanf("%d", &command);

    if(command == 1){
        struct Entries entry;

        printf( "Enter a name:\n");
        scanf("%s",tempName);
        strcpy(entry.name, tempName);

        printf( "Enter a phone:\n");
        scanf("%s",tempPhoneNumber);
        strcpy(entry.phoneNumber, tempPhoneNumber);

        printf( "Enter a note:\n");
        scanf("%s",tempNote);
        entry.note = (char *)realloc(entry.note,strlen(tempNote));
        strcpy(entry.note, tempNote);

        counter++;
    }
}
return 0;
}

When the user enters name, phoneNumber and note for the first time, the program automatically stops working despite the fact that it should've asked to user the attributes forever.

You'll want the `malloc` to be done outside of the struct, with the size based on user input. — Matt Goodrich, Mar 09 '17 at 18:38
@SouravGhosh I've just tried to declare a first size. I don't even know why I did that but I thought it was necessary. — M.Soyturk, Mar 09 '17 at 18:39
Delete `note = (char *)malloc(5*sizeof(char));`. Replace `entry.note = (char *)realloc(entry.note,strlen(tempNote));` with `entry.note = malloc(strlen(tempNote)+1);` — BLUEPIXY, Mar 09 '17 at 18:42
@MattGoodrich I tried that but I got the same result. Once I give inputs for name, number and note, the program immediately stops. — M.Soyturk, Mar 09 '17 at 18:42
[Please see this discussion on why not to cast the return value of `malloc()` and family in `C`.](http://stackoverflow.com/q/605845/2173917). — Sourav Ghosh, Mar 09 '17 at 18:45
@BLUEPIXY Thanks a lot. It works now but still I couldn't quite get why we used malloc instead of realloc? I thought once we declare the size of an attribute, after that point we should use realloc.(Edit : I just saw the comment. I got it.) — M.Soyturk, Mar 09 '17 at 18:45
Since input size depends on input of `tempNote`(just `strlen(tempNote)+1`), it will never be higher, so `realloc`(and Initial size) is not necessary. — BLUEPIXY, Mar 09 '17 at 18:47

score 2 · Answer 1 · answered Mar 09 '17 at 18:40

You can make use of a flexible array member. That's much better approach.

Quoting from the C11 standard, chapter §6.7.2.1

[...] the last element of a structure with more than one named member may have an incomplete array type; this is called a flexible array member. In most situations, the flexible array member is ignored. In particular, the size of the structure is as if the flexible array member were omitted except that it may have more trailing padding than the omission would imply. However, when a . (or ->) operator has a left operand that is (a pointer to) a structure with a flexible array member and the right operand names that member, it behaves as if that member were replaced with the longest array (with the same element type) that would not make the structure larger than the object being accessed; the offset of the array shall remain that of the flexible array member, even if this would differ from that of the replacement array. If this array would have no elements, it behaves as if it had one element but the behavior is undefined if any attempt is made to access that element or to generate a pointer one past it.

and the example

EXAMPLE 2 After the declaration:
  struct s { int n; double d[]; };
the structure struct s has a flexible array member d. A typical way to use this is:
int m = /* some value */;

struct s *p = malloc(sizeof (struct s) + sizeof (double [m]));
and assuming that the call to malloc succeeds, the object pointed to by p behaves, for most purposes, as if p had been declared as:
struct { int n; double d[m]; } *p;

Now, following the above, you can scan an input, use the input length as m and then , allocate enough memory to hold the input.

VHS · Accepted Answer · 2017-03-09T18:45:24.790

0

Replace

entry.note = (char *)realloc(entry.note,strlen(tempNote));

with

entry.note = malloc(strlen(tempNote) + 1);

Also

Remove

note = (char *)malloc(5*sizeof(char)

from the struct definition. It doesn't belong there.

edited Mar 09 '17 at 18:45

answered Mar 09 '17 at 18:42

VHS

9,534
3
19
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`malloc(strlen(tempNote))` --> `malloc(strlen(tempNote)+1)`, +1 for null-terminator. – BLUEPIXY Mar 09 '17 at 18:44
[Please see this discussion on why not to cast the return value of `malloc()` and family in `C`.](http://stackoverflow.com/q/605845/2173917). – Sourav Ghosh Mar 09 '17 at 18:45
@BLUEPIXY. Good catch. I updated my answer accordingly. Thanks. – VHS Mar 09 '17 at 18:45
@SouravGhosh, that was a good link you shared. I just updated my answer. Thanks. – VHS Mar 09 '17 at 18:48

score 0 · Answer 3 · answered Mar 09 '17 at 18:46

I would suggest learning the basics of C first. C is really just a portable assembler with macros. You absolutely need to know how pointers and memory allocation work before you can get anything done :)

Btw., congratulations for beginning to learn C, this will give you a lot of great insights about the nature of both computers and programming languages :):)

score 0 · Answer 4 · answered Mar 09 '17 at 22:08

Close,

If the user enters a string with 30 characters, size of note should be 30.

Strings in C are null-terminated, thus you need to reserve space for the terminal character ('\0'). You could alternately store the string as both the length of the string and the characters, but then you would need to decide the maximum number of characters expressible.

The easiest is to use strdup() to allocate just enough space for the string and the null terminator.

if( entry.note ) free(entry.note); //avoid leaking memory
entry.note = strdup(tempNote);

Or you could use a struct containing length and data, allocating, assigning, and freeing using the following,

typedef struct {
    unsigned short int len;
    char str[1];
} strwlen_t;
strwlen_t* string_withlen_alloc(unsigned short int size) {
    strwlen_t* p = malloc( size + sizeof(strwlen_t) );
    p->str[0] = '\0';
    p->len = 0;
    return p;
}
strwlen_t* strwlen_dup(char* str) {
    unsigned short int len = strlen(str);
    strwlen_t* p = malloc( len + sizeof(strwlen_t) );
    memcpy(p->str,str);
    p->len = len;
    return p;
}
void strwlen_free(strwlen_t* p) {
    free(p);
}
strwlen_t* strwlen_cpy(strwlen_t* dp,char* sp) {
    strcpy(dp->str,sp);
    dp->len = len(sp);
    return(dp);
}

Declare your Entries struct,

struct Entries {
    char name[50];
    char phoneNumber[11];
    strwlen_t* note;
} Entries;

Then you could use,

if( entry.note ) free(entry.note);
entry.note = strwlen_dup(tempNote);

And print entry.note using,

printf("note: %s\n",entry.note.str);

How to change size of a struct's attribute in C?

4 Answers4