I can't call function if i don't use printf or dynamic allocation in function. why?

Question

First issue : If I don't use "printf" function, I can't call fucntion.

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

char *m_strncpy(char *str1, int count) {
    int i;
    char *new_str;
    for (i = 0; i < count; i++) {
        new_str[i] = str1[i];
    }
    return new_str;
}

int main(void)
{

    char str[80];
    char *sp, e;
    int count;

    printf("Enter a string : ");
    fgets(str, sizeof(str) - 1, stdin);
    printf("The number of character : ");
    scanf("%d", &count);
    //printf("Input complete\n");
    sp = m_strncpy(str, count);
    printf("Cut string is %s\n", sp);
    return 0;
}

If I don't use printf("Input complete\n"); The m_strncpy function is not called.

Second issue : If I don't use dynamic allocation in m_strncpy function, I can't call function.

Visual Studio 2017 doesn't allow uninitialization. But Repl compiler allows.

So When I don't initialize char *new_str, it cannot be called. Why??

You need to allocate memory for `new_str` to point to. – Steve Summit Mar 29 '18 at 11:57 — Steve Summit, Mar 29 '18 at 11:57
Also in `m_strncpy` you need to add a terminating `'\0'`. – Steve Summit Mar 29 '18 at 11:58 — Steve Summit, Mar 29 '18 at 11:58
Because: undefined behavior bugs. Anything can happen. – Lundin Mar 29 '18 at 12:44 — Lundin, Mar 29 '18 at 12:44

Jabberwocky · Accepted Answer · 2018-03-29T12:04:34.850

0

Your original code exhibits undefined behaviour:

char *m_strncpy(char *str1, int count) {
    int i;
    char *new_str;                  // new_str is not initialized, it doesn't
                                    //  point anywhere
    for (i = 0; i < count; i++) {
        new_str[i] = str1[i];       // dereferencing an uninitialized pointer is undefined
                                    // behaviour, anything can happen.
    }
    return new_str;
}

You probably want this:

char *m_strncpy(char *str1, int count) {
  int i;
  char *new_str = malloc(count + 1);  // +1 for the NUL terminator

  if (new_str != NULL)
  {    
    for (i = 0; i < count; i++) {       // copy count first chars
      new_str[i] = str1[i];
    }

    new_str[i] = 0;                     // NUL terminate the string
  }

  return new_str;
}

edited Mar 29 '18 at 12:04

answered Mar 29 '18 at 11:59

Jabberwocky

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@ArndtJonasson answer edited, you can remove your comment – Jabberwocky Mar 29 '18 at 12:08
So you mean "uninitialization" can happen anything? – molamola Mar 29 '18 at 12:12
@haram "happen anything": not _really_ anything, for example it won't burn your computer. Google "undefined behaviour". – Jabberwocky Mar 29 '18 at 12:15
Thanks for your answer! – molamola Mar 29 '18 at 12:44

I can't call function if i don't use printf or dynamic allocation in function. why?

1 Answers1