What is malloc doing in this code?

Question

Could you explain following code?

str = (char *) malloc (sizeof(char) * (num+1));

1. What is malloc doing here?

Answer 1

Malloc allocates memory, in this case for the string str of length num. (char *) is the type for str sizeof(char) is the number of bytes required for each character. The +1 is for the trailing null character in the string, normally zero.

Answer 2

This code tries to allocate a chunk of memory that can hold num + 1 values of type char. So if a chat equals one byte and num is 10 it will try to allocate 11 bytes of memory and return a pointer to that memory.

+1 is likely used because the programmer wanted to store a string (character array) of num characters and needs an extra char to store the terminating '\0' (null) character. In C/C++. chracater strings are, by convention, terminated by a null-character.

Answer 3

malloc allocates memory from the heap and returns a pointer to it. It's useful when you don't know how much memory you are going to need at compile time.

As for why (num+1), it really depends on what the code is doing... perhaps num is the number of characters in the string, and the +1 is for the NUL terminator byte at the end. I don't know what the sizeof(char) would be for in that case, though.

Answer 4

malloc allocates memory.

num+1 as num is the number of characters in the string and +1 for the null terminator

Answer 5

malloc is a function that allocates a chunk of memory on the heap and returns a pointer to it. It's similar to the new operator in many languages. In this case, it's creating a block of memory that can survive for an arbitrary length of time and have an arbitrary size. That in itself is fairly in-depth stuff, which is somewhat difficult to explain and warrants a separate question.

The num + 1 compensates for the null terminator on the end of strings. Strings often need to know how long they are, and the C tradition is to allocate room for an additional character on the end of the string, which will always be the special character \0. That allows functions that deal with the string to determine the size of the string automatically. For example, if you wanted to do something to every character of a string without knowing how long the string is, you could do something like this:

const char *ptr = str;
while (*ptr != '\0') {
    process(*ptr);
    ptr++;
}

Answer 6

malloc allocates an array of char (in this case) on the heap.

array will be num+1 long, but the longest string it can possibly hold is 'num' long, because string in C need a ending null-byte.