Dynamic memory allocation and strings.

[Sefster300]

Hi,
I am trying to ask the user for an input (a string), then store the input in a string variable with the same length of the string in-putted.
Is this possible? I have some code below but this requires the user to input the string twice.

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

int main(void){
int num_char=1, i;
char holder=' ';
char *string;
printf("Please enter a sentence:\n");
while (holder != '\n'){
	holder = getchar();
	printf("%c ",holder);
	num_char++;
}
printf("\n%d\n",num_char);
string = (char*) malloc(sizeof(char)*num_char);
gets(string);
for (i=0; i<num_char; i++){
	printf("%c",string);}
return 0;
}

Thanks

Sef

[itsme86]

You're not allocating enough space. Make sure you allocate enough room to store the '\0':

string = (char*) malloc(sizeof(char)*(num_char + 1));

Also, please don't ever use gets(). It's not safe.

And to the meat of the matter, there's no way to portably peak into the input buffer, so the only way you're going to pull off what you're trying to accomplish is by slowly building up the buffer using realloc():

char *string = NULL;
size_t bytes_allocated = 0;
puts("Please enter a sentence:");
do
{
  holder = getchar();
  if(holder != '\n')
  {
    if(string != NULL)
      string = malloc(2);
    else
      string = realloc(string, bytes_allocated + 2);  // + 2 to make room for the input character plus a '\0'
    string[bytes_allocated++] = holder;
  } while(holder != '\n');
}
string[bytes_allocated] = '\0';

Something like that anyway.

[anduril462]

I can think of two basic options.
1. There is a maximum you expect from the user and will discard anything else after that:

char buffer[MAX_LEN+1];  // one extra space for the null character
char *input;
read string into buffer using fgets, scanf, etc
allocate strlen(buffer) + 1 bytes into input 
copy the contents of buffer into input

2. There is no maximum, or you like causing yourself undue suffering.

char c;
char *input = malloc(1);  // initial allocation so we can just realloc in our loop
int n_char = 0;
input[0] = '\0';  // null terminate input in case they enter no data
do
    c = read a character
    if c is not EOF
        reallocate n_char+1 bytes into input
        copy c onto the end of input
        null terminate input
while c is not EOF

There is a third method that uses a combination of 1 and 2 so you read in, say, 1000 chars at a time to a buffer and reallocate if need be. This would be my preferred method only if option 1 is not sufficient.

[anduril462]

...
      string = realloc(string, bytes_allocated + 2);  // + 2 to make room for the input
...

You only need to add one more byte each time, since your previous instance already had a null, and you're only adding one character of input in each iteration.

[itsme86]

You only need to add one more byte each time, since your previous instance already had a null, and you're only adding one character of input in each iteration.

Look at my code again. I'm only adding the '\0' once, after the loop.

[anduril462]

Duh...yeah, I see that now. Besides, you only increment bytes_allocated by 1 each time, so realloc will only go up by one from it's previous value. I'll take that foot out of my mouth and remove my head from my...

[iMalc]

You appear to be using gets. gets is bad and can never be used safely. Do not use it ever!
Use fgets instead.

[Sefster300]

Ahh it all makes sense now, also I realise not to use gets. haha

Thanks