Dynamic memory allocation with char

Question

I'm trying to allocate memory only if i need it for the next while .

char *str = malloc(sizeof(char));
int i = 0;

while(something == true){
    str[i] = fgetc(fp);
    str = realloc(str, strlen(str)+1);
    i++;
}

free(str);

But for some reason the code above give me an "Invalid read of size 1" at strlen().

Answer 1

strlen will not determine the size of the allocated char array even if it contains a null terminated string. See proposed fix although I do not like the code structure overall: You will always end up with an extra allocated character.

char *str = malloc(sizeof(char));
int i = 0;

while(something == true){
    str[i] = fgetc(fp);
    str = realloc(str, (i+2)*sizeof(char));
    i++;
}
// str[i*sizeof(char)]='\0'; <-- Add this if you want a null terminated string

free(str);

I would propose the following code that would avoid allocating the extra character:

char *str = NULL;
int i = 0;

while(something == true){
    str = realloc(str, (i+1)*sizeof(char));
    str[i] = fgetc(fp);
    i++;
}

free(str);

As per documentation, "In case that ptr is a null pointer, the function behaves like malloc, assigning a new block of size bytes and returning a pointer to its beginning."

This is in case you are not reading text and not planning to use such functions as strlen, strcat...

Chunk at a time allocation:

char *str = malloc(sizeof(char));
int i = 0;
const int chunk_size = 100;

while(something == true){
    str[i] = fgetc(fp);
    if (i % chunk_size == 0)
        str = realloc(str, (i+1+chunk_size)*sizeof(char));
    i++;
}
// str[i*sizeof(char)]='\0'; <-- Add this if you want a null terminated string

free(str);