Dynamic memory allocation code

Question

I am learning the basics of memory allocation in C(C++).

#include "stdio.h"
#include "string.h"
#include "stdlib.h"
void main(){

    char *str;
    char *input;
    int *ilist;
    int i, size1, size2;

    printf("Number of letters in word: ");
    scanf("%d", &size1);
    printf("Number of integers: ");
    scanf("%d", &size2);                  

    str = (char *)malloc(size1*sizeof(char) + 1); 

    ilist = (int *)malloc(size2*sizeof(int)); 
    if (str == NULL || ilist == NULL){ 
        printf("Lack of memory");
    }
    printf("Word: ");
    int k = size1;
    // the following line is done to prevent memory bugs when the amount of 
    letters in greater than size1.
    scanf("%ks", &str); //I guess something is wrong with this line
    /* user inputs a string */
    for (i = 0; i < size2; i++) {
        printf("Number %d of %d: ", i + 1, size2);
        //this scanf is skipped during the execution of the program
        scanf("%d", ilist + i);         
    }


    free(str);
    free(ilist);
    system("pause");

}

The program asks user to write the amount of letters in the word and the amount of digits in the number. Then user writes the word. Then he writes integer one-by-one depending on what number was typed before. The problem I have is when the user writes the whole word, next scanf is skipped. Thank you. P.S. can there be other kind of memory bugs in this code?

First lesson c != c++ and in c you don't cast `malloc()` to the target pointer type because you don't need to. — Iharob Al Asimi, Jan 02 '16 at 16:32
I suggest installing Valgrind http://valgrind.org/ to check for memory leaks. — birdoftheday, Jan 02 '16 at 16:32
Why are "stdio.h" in quotation marks instead of brackets? It should be `#include ` instead of `#include "stdio.h"`. — birdoftheday, Jan 02 '16 at 16:32
`scanf("%ks", &str);` doesn't do what you think it does. It tells `scanf` to scan a `%`,a `k` and a `s` and not to scan a string of maximum length `k`. — Spikatrix, Jan 02 '16 at 16:33
@ilent2 - That is my job to type that in :-) - anyway why think that the users are that good to input the correct values for the size of wiords and the length of the list? — Ed Heal, Jan 02 '16 at 16:34
@CoolGuy how can I do so that it scans till the length of str? — Бейбут Тукибаев, Jan 02 '16 at 16:37
All that comes to my mind is `int c, counter = 0; while(counter < size1 && (c = getchar()) != '\n' && c != EOF) str[counter++] = c; str[counter] = '\0';` — Spikatrix, Jan 02 '16 at 16:40
`scanf("%99s", str)` to read a string of up to 99 characters, the array needs to accommodate the null character - i.e. be 100 characters long — Ed Heal, Jan 02 '16 at 16:46
It still works for historical reasons - https://ideone.com/c0yBkK — Ed Heal, Jan 02 '16 at 17:10
@EdHeal: For a hosted env/ it's UB as per the C11 Standard appendix J2 along with 5.1.2.2.1. — alk, Jan 02 '16 at 17:11

score 2 · Answer 1 · edited May 23 '17 at 12:31

Regarding //I guess something is wrong with this line...
The format specifier "%ks" in scanf("%ks", &str); contains a k, which is not a valid scanf() format specifier.

Excerpt from link:

For user input value in the width specifier, you can create a format buffer:

char format[10];
int k = size1;//assume size1 == 10
sprintf(format, "%c%d%c", '%', k, 's');
//assuming k == 10, format contains "%10s"
scanf(format, &str); //now, there is nothing wrong with this line

Other Observations
There are several recommended prototypes for the C main function. void main() is not one of them.

This line: str = (char *)malloc(size1*sizeof(char) + 1);
Could be written as:

str = malloc(size1 + 1); //removed sizeof(char) as it is always 1
                         //removed cast, not recommended in C

Similar for for: ilist = (int *)malloc(size2*sizeof(int));

ilist = malloc(size2*sizeof(int));//remove cast, but keep sizeof(int) 
                                  //unlike sizeof(char), sizeof(int) is never 1

A few Basics to consider for dynamic memory allocation in C:

1) In C, do not cast the output of calloc(), malloc() or realloc(). (do however cast in C++.)
2) For each call to calloc(), malloc() or realloc(), there must be a corresponding call to free()
3) While automatic memory is sourced from the stack, dynamic memory comes from the heap
4) If speed efficiency is important, favor the stack over the heap.

haccks · Answer 2 · 2016-01-02T17:15:41.110

1

Instead of using scanf use fgets. But, you need to clear the input buffer first to consume the \n left behind by previous scanfs.

int c;
while((c = getchar()) != '\n' && c != EOF); // Clear input buffer  
fgets(str, k, stdin);

Note that if a '\n' is read then it will be stored in str. You should have to take care of that.

edited Jan 02 '16 at 17:15

answered Jan 02 '16 at 17:12

haccks

104,019
25
176
264

@BasileStarynkevitch; Not a standard C library function. – haccks Jan 02 '16 at 17:16
1

@haccks: But it's POSIX. – alk Jan 02 '16 at 17:21

Dynamic memory allocation code

2 Answers2