C programming help needed : pointers and malloc

Question

The output should be "the wallet belongs to Warren Buffett with balance of 85500000000" but I am getting "The wallet belongs to Warren Buffett with balance of 0.000000". Can you please let me know where am I making the mistake?

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

typedef struct {
    char *owner;
    long double balance;
} Wallet;

int main(int argc, char **argv) {
    const char *owner = "Warren Buffett";
    Wallet *wallet = (Wallet *) malloc(sizeof(Wallet));
    wallet->owner = (char *) malloc(strlen(owner) * sizeof(char));
    strcpy(wallet->owner, "Warren Buffett");
    wallet->balance = 85500000000;
    printf("The wallet belongs to %s with balance of %Lf",
           wallet->owner, wallet->balance);

    return 0;
}

You don't malloc enough space - strings have a null terminator — M.M
– M.M, Commented Aug 9, 2021 at 23:14
@Axifive: Try with AddressSanitizer and you'll see not so much. Can onlinegdb enable ASan? — Nate Eldredge
– Nate Eldredge, Commented Aug 9, 2021 at 23:33
An off-topic warning about using floating point numbers for currency: you shouldn't. Floating point numbers are approximations with unexpected edge cases (e.g (0.1 + 0.2) == 0.3 is false). Better to use the smallest quantity (e.g. cents or 1/00 of a cent or whatever is appropriate) and use well established banking rounding rules when multiplying or dividing. — John Bayko
– John Bayko, Commented Aug 9, 2021 at 23:59
Remember, for future sake, to always free() what you malloc(). In the code above (no recursion, no loops, no functions except main(), ...) there's no big harm by leaving the memory marked as "in use" when the program terminates. — pmg
– pmg, Commented Aug 10, 2021 at 8:30

dbush · Accepted Answer · 2021-08-09 23:17:42Z

3

You're not allocating enough space for wallet->owner.

Strings in C consist of a sequence of characters followed by a terminating null byte. You're allocating strlen(owner) * sizeof(char) bytes for wallet->owner which is enough for the characters in the string but not for the terminating null byte. As a result, you're writing past the end of allocated memory. This triggers undefined behavior, which can manifest as unexepected output.

Add 1 to the allocated space:

wallet->owner = malloc(strlen(owner) + 1);

Also, sizeof(char) is guaranteed to be 1 so the multiplication by that can be omitted, and you shouldn't cast the return value of malloc.

answered Aug 9, 2021 at 23:17

dbush

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