[c] Segmentation Fault - C

Why does the following code return with a segmentation fault? When I comment out line 7, the seg fault disappears.

int main(void){
      char *s;
      int ln;
      puts("Enter String");
      // scanf("%s", s);
      gets(s);
      ln = strlen(s); // remove this line to end seg fault
      char *dyn_s = (char*) malloc (strlen(s)+1); //strlen(s) is used here as well but doesn't change outcome
      dyn_s = s;
      dyn_s[strlen(s)] = '\0';
      puts(dyn_s);
      return 0;
    }

Cheers!

char *s  does not have some memory allocated . You need to allocate it manually in your case . You can do it as follows
s = (char *)malloc(100) ;

This would not lead to segmentation fault error as you will not be refering to an unknown location anymore

Even better

#include <stdio.h>
int
main(void)
{
  char *line = NULL;
  size_t count;
  char *dup_line;

  getline(&line,&count, stdin);
  dup_line=strdup(line);

  puts(dup_line);

  free(dup_line);
  free(line);

  return 0;
}

Catastrophically bad:

int main(void){
      char *s;
      int ln;
      puts("Enter String");
      // scanf("%s", s);
      gets(s);
      ln = strlen(s); // remove this line to end seg fault
      char *dyn_s = (char*) malloc (strlen(s)+1); //strlen(s) is used here as well but doesn't change outcome
      dyn_s = s;
      dyn_s[strlen(s)] = '\0';
      puts(dyn_s);
      return 0;
    }

Better:

#include <stdio.h>
#define BUF_SIZE 80

int 
main(int argc, char *argv[])
{
      char s[BUF_SIZE];
      int ln;
      puts("Enter String");
      // scanf("%s", s);
      gets(s);
      ln = strlen(s); // remove this line to end seg fault
      char *dyn_s = (char*) malloc (strlen(s)+1); //strlen(s) is used here as well but doesn't change outcome
      dyn_s = s;
      dyn_s[strlen(s)] = '\0';
      puts(dyn_s);
      return 0;
    }

Best:

#include <stdio.h>
#define BUF_SIZE 80

int 
main(int argc, char *argv[])
{
      char s[BUF_SIZE];
      int ln;
      puts("Enter String");
      fgets(s, BUF_SIZE, stdin); // Use fgets (our "cin"): NEVER "gets()"

      int ln = strlen(s); 
      char *dyn_s = (char*) malloc (ln+1);
      strcpy (dyn_s, s);
      puts(dyn_s);
      return 0;
    }

Your scanf("%s", s); is commented out. That means s is uninitialized, so when this line ln = strlen(s); executes, you get a seg fault.

It always helps to initialize a pointer to NULL, and then test for null before using the pointer.