Dynamically allocate an array of unspecified size in C

Question

I want to take an input in c and don't know the array size. please suggest me the ways how to do this..

hello this is
a sample
string to test.

Answer 1

malloc is one way:

char* const string = (char*)malloc(   NCharacters   ); // allocate it
...use string...
free(string); // free it

where NCharacters is the number of characters you need in that array.

Answer 2

If you're writing the code yourself, the answer will involve malloc() and realloc(), and maybe strdup(). You're going to need to read the strings (lines) into a large character array, then copy the strings (with strdup()) into a dynamically sized array of character pointers.

char line[4096];
char **strings = 0;
size_t num_strings = 0;
size_t max_strings = 0;

while (fgets(line, sizeof(line), stdin) != 0)
{
    if (num_strings >= max_strings)
    {
        size_t new_number = 2 * (max_strings + 1);
        char **new_strings = realloc(strings, new_number * sizeof(char *));
        if (new_strings == 0)
            ...memory allocation failed...handle error...
        strings = new_strings;
        max_strings = new_number;
    }
    strings[num_strings++] = strdup(line);
}

After this loop, there's enough space for max_strings, but only num_strings are in use. You could check that strdup() succeeded and handle a memory allocation error there too, or you can wait until you try accessing the values in the array to spot that trouble. This code exploits the fact that realloc() allocates memory afresh when the 'old' pointer is null. If you prefer to use malloc() for the initial allocation, you might use:

size_t num_strings = 0;
size_t max_strings = 2;
char **strings = malloc(max_strings * sizeof(char *));

if (strings == 0)
    ...handle out of memory condition...

If you don't have strdup() automatically, it is easy enough to write your own:

char *strdup(const char *str)
{
    size_t length = strlen(str) + 1;
    char  *target = malloc(length);
    if (target != 0)
        memmove(target, str, length);
    return target;
}

If you are working on a system with support for POSIX getline(), you can simply use that:

char   *buffer = 0;
size_t  buflen = 0;
ssize_t length;
while ((length = getline(&buffer, &buflen, stdin)) != -1)  // Not EOF!
{
    …use string in buffer, which still has the newline…
}
free(buffer);  // Avoid leaks

Answer 3

Thank you for the above answers. I have found out the exact answer that I wanted. I hope it will help other people's questions also.

while ((ch == getchar()) != '$')
{
    scanf("%c", &ch);
}