A C string is just a bunch of characters right next to each other in memory. The string ends when the character 0x00 is encountered.
Functions that take C strings expect the address of the string’s first character. strlen(), for example, will count the number of characters in a string. Try building a string and using strlen() to count the letters:
#include <stdio.h> // For printf
#include <stdlib.h> // For malloc/free
#include <string.h> // For strlen
int main (int argc, const char * argv[])
{
char x = '!'; // The character '!'
while (x <= '~') { // The character '~'
printf("%x is %cn", x, x);
x++;
}
// Get a pointer to 5 bytes of memory on the heap
char *start = malloc(5);
// Put 'L' in the first byte
*start = 'L';
// Put 'o' in the second byte
*(start + 1) = 'o';
// Put 'v' in the third byte
*(start + 2) = 'v';
// Put 'e' in the fourth byte
*(start + 3) = 'e';
// Put zero in the fifth byte
*(start + 4) = '0';
// Print out the string and its length
printf("%s has %zu charactersn", start, strlen(start));
// Print out the third letter
printf("The third letter is %cn", *(start + 2));
// Free the memory so that it can be reused
free(start);
start = NULL;
return 0;
}
Build and run it.
Notice the places where you added a pointer and a number together. start is declared to be a char *. A char is one byte. So start + 1 is a pointer one byte further in memory than start. start + 2 is two bytes further in memory than start.
This adding to a pointer and dereferencing the result is so common that there is a shorthand for it: start[2] is equivalent to *(start + 2). Change your code to use it:
char *start = malloc(5); start[0] = 'L'; start[1] = 'o'; start[2] = 'v'; start[3] = 'e'; start[4] = '0'; printf("%s has %zu charactersn", start, strlen(start)); printf("The third letter is %cn", start[2]); free(start); start = NULL; return 0; }
Build and run it.
It should be mentioned that this works with any data type. Here, for example, I can make a list of my favorite 3 floating point numbers and print them out:
int main (int argc, const char * argv[])
{
// Claim a chunk of memory big enough to hold three floats
float *favorites = malloc(3 * sizeof(float));
// Push values into the locations in that buffer
favorites[0] = 3.14158;
favorites[1] = 2.71828;
favorites[2] = 1.41421;
// Print out each number on the list
for (int i = 0; i < 3; i++) {
printf("%.4f is favorite %dn", favorites[i], i);
}
// Free the memory so that it can be reused
free(favorites);
favorites = NULL;
return 0;
}
The only interesting difference here is that favorites is typed as a float *. A float is 4 bytes. Thus favorites + 1 is 4 bytes further in memory than favorites.