Realloc crash: cygwin_exception::open_stackdumpfile: Dumping stack trace to malloc.exe.stackdump

Question

I'm new with dynamic memory allocation and I tried to write a simple program to concatenate 2 strings (one initialized and one read from stdin) using realloc for the first string. But I receive this error:

cygwin_exception::open_stackdumpfile: Dumping stack trace to malloc.exe.stackdump

The code breaks at realloc and I do not know why, could you please help?

Input: man alive.

Expected output: The most beloved man alive.

I have tried replacing strlen with a number but with no avail.

Here's my code:

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

int main()
{
    char *str = "The most beloved";
    char str2[20];
    scanf("%[^\n]%*c", str2);
    //printf("%s %s\n", str, str2);

    str = (char *)realloc(str, strlen(str) + strlen(str2));

    strcat(str, str2);
    printf("%s\n", str);
    free(str);
}

Answer 1

You declared a pointer to a string literal

char *str = "The most beloved";

String literals have static storage duration. So they may not be reallocated dynamically.

Thus this statement

str = (char *)realloc(str, strlen(str) + strlen(str2));

invokes undefined behavior.

Pay attention to that you need to reserve memory for the terminating zero character '\0' of the result string.

What you need is the following

char *result = ( char * )malloc( strlen(str) + strlen(str2) + 1 );

strcpy( result, str );
strcat( result, str2 );

puts( result );

free( result );

Also it would be more safer to write

scanf("%19[^\n]", str2);