C - Dynamic Memory Allocation

Question

#include<stdio.h>
#include<malloc.h>

int main()
{
    char *string1;
    int length;

    scanf("%d", &length);

    string1 = (char *)malloc(sizeof(char) * length);


    printf("\n Enter the First String : ");
    fgets(string1, length, stdin);

    printf("\n The First String : %s ",string1);

    free(string1);      

    return 0;
}

Can someone help me on the above code ? I trying to get the length of a string and the string as inputs. But, I am able to enter only Length of the string. After that it skips the string input part.

This is the output I am getting :

sh-4.3$ main
10

Enter the First String :
The First String :
sh-4.3$

Yeah, this is what is happening. Use `scanf` instead of `fgets`. — AhmadWabbi, Oct 24 '16 at 07:47
Add a `getchar();` after the scanf to discard the new line character you left behind in stdin. — Lundin, Oct 24 '16 at 07:58
-> Scanf() won't consider whitespace. Hence I used fgets(). -> after getting input, I did press enter. -> I want to follow Standard C. -> Thanks for the stdlib.h suggestion/correction. — k.s aravind, Oct 24 '16 at 08:23
@Lundin : Thanks for getchar() suggestion. It worked. So is there a way to skip this new line character ? — k.s aravind, Oct 24 '16 at 08:46
@k.saravind See [The C FAQ](http://c-faq.com/stdio/gets_flush2.html). — Lundin, Oct 24 '16 at 09:35

mch · Accepted Answer · 2016-10-24T08:58:17.643

1

After typing "10<enter>" the <enter> or "\n" will remain in the stdin buffer, so you have to use getchar() after the scanf to remove it.
Also you should #include <stdlib.h> instead of malloc.h.
You malloc 1 character too less, because of the 0-terminator. string1 = malloc(length + 1); will do the job, the cast is not necessary and sizeof(char) is always 1.

edited Oct 24 '16 at 08:58

answered Oct 24 '16 at 08:33

mch

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Added a getchar() after scanf(). It worked. Thanks To all for taking time to help. – k.s aravind Oct 24 '16 at 08:43

score 0 · Answer 2 · answered Oct 24 '16 at 07:55

0

If you need to use stdin for string input you can use fgetln. I edited you example and now it looks like that:

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

int main()
{
    char *string1;
    size_t length;

    //scanf("%d", &length);

    //string1 = (char *)malloc(sizeof(char) * length);


    printf("\n Enter the First String : ");
    //fgets(string1, length, stdin);
    string1 = fgetln(stdin, &length);
    string1[length] = '\0';

    printf("\n The First String : %s ",string1);

    free(string1);

    return 0;
}

Note: fgetln returns not a C string, you should add a NULL character to the end.

answered Oct 24 '16 at 07:55

Roman Podymov

4,168
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3

`fgetln` isn't standard. Use `fgets` instead. – Lundin Oct 24 '16 at 07:57
Author of the question did not wrote nothing about the compiler. I used Apple LLVM. – Roman Podymov Oct 24 '16 at 07:59
2

Since it is tagged C with no mentioning of what system that's used, you should try to answer with standard C. Particularly, there is no reason to use non-standard functions that have standard equivalents. – Lundin Oct 24 '16 at 08:02

score 0 · Answer 3 · answered Oct 24 '16 at 09:50

0

The most simple answer when reading from STDIN/keyboard with the newline etc. is to just add the "\n" to the scanf, ie:

scanf("%d\n", &length);

Solves the problem ;)

PS: Beware of all the other security/buffer overflow issues of scanf

answered Oct 24 '16 at 09:50

Hvisage

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C - Dynamic Memory Allocation

3 Answers3