c中字符串的动态内存分配

Question

I found this code working perfectly.

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




int main(int argc,char *argv[])
{
    char* s;        /* input string */
    s=malloc(sizeof(s));    

    int c;

    if(argc==1){ // if file name not given


        while (gets(s)){
            puts(s);
        }
    }
}

What I don't understand is, how is the string s stored in memory.i am allocating memory only for the pointer s, which is of 4 bytes.Now where does the input string given by the user get stored in?

Answer 1

Rather than this you should do

s=malloc(sizeof(*s)*(number_of_chars+1)); 

You set number_of_chars to appropriate value, so that you allocate memory to store those many characters. +1 is for last '\\0' character.

With your approach you are allocating 4 bytes so you can store usually those many characters.

Answer 2

You've allocated sizeof(void*) bytes of memory and filling it with user-provided data. You have an address and writing to it, it's ok from compiler's point of view (maybe it's really what you want, who knows). Even if you program didn't crash when you exceed it - it's still an error. It's just a memory, something else could be stored in this area, and you'll overwrite it - so expect heavy trouble if you'll ever do that.

Answer 3

It's only safe for the first four bytes. The fifth byte will overrun the allocated data and tramp on something else which will yield undefined behaviour (might crash, might not).

Also, you don't null terminate the string with '\\0' after you finish writing the chars, so you'll probably introduce another crash when you try and call a string routine (strcpy) on it - unless the memory after your string happened to contain zeros anyway, but naturally you shouldn't rely on this chance!

Answer 4

It's possible as compiler assigns two bytes. now you give 10 bytes in input, so your allocated memory overflows and data stored beyond your allocated memory only if its available.

It might give error if the data you want to store is greater then available and not give error if the data you want to store is greater then allocated.

puts will print data until it gets '\\0'.

So this is expected behavior!!