CODEWITHSUNDEEP
cfunctionpointersreturn
AdamSMith
AdamSMith

Reputation: 25

Malloc in C - return value

I have this code sample, and generally understand its logic, but am stuck in some details.

#define LSH_RL_BUFSIZE 1024
char *lsh_read_line(void) {
  int bufsize = LSH_RL_BUFSIZE;
  int position = 0;
  char *buffer = malloc(sizeof(char) * bufsize);
  int c;

  if (!buffer) {
    fprintf(stderr, "lsh: allocation error\n");
    exit(EXIT_FAILURE);
  }

  while (1) {
    // Read a character
    c = getchar();

    // If we hit EOF, replace it with a null character and return.
    if (c == EOF || c == '\n') {
      buffer[position] = '\0';
      return buffer;
    } else {
      buffer[position] = c;
    }
    position++;

    // If we have exceeded the buffer, reallocate.
    if (position >= bufsize) {
      bufsize += LSH_RL_BUFSIZE;
      buffer = realloc(buffer, bufsize);
      if (!buffer) {
        fprintf(stderr, "lsh: allocation error\n");
        exit(EXIT_FAILURE);
      }
    }
  }
}

I cannot understand two things: Firstly, what exactly does this line do?

char *buffer = malloc(sizeof(char) * bufsize)

And secondly, how does the following line work? How is it that a pointer can be returned?

return buffer;

Upvotes: 1

Views: 2442

Answers (1)

A.Rao
A.Rao

Reputation: 51

what exactly does this line do?

malloc means "memory allocation" malloc function is used to dynamically create a memory block, it allocates a memory block of size specified in bytes (as a parameter). It returns a pointer to the beginning of that block.

So the specified size here is 'sizeof(char) * bufsize' i.e. a block of characters of length 'bufsize' is being requested. To get a size of 1 character you use the operator sizeof (sizeof is an operator not a function)

how does the following line work?

the function lsh_read_line returns pointer to a memory block, which is the memory allocated by malloc here - and it is buffer.

How is it that a pointer can be returned?

Since it is dynamic memory from the heap - it is valid memory block even after the function returns.

As a side note - 'lsh_read_line' caller must free this buffer otherwise there is a memory leak!

For further details on C Function stack please refer to https://en.wikipedia.org/wiki/Call_stack

Upvotes: 3

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