C - Dynamically allocating array for stdin without realloc

Question

I have a project in which I'm working on, that's gonna take an input that comes out from another program ran on the terminal, like so:

./other_program | ./project

so I'm taking the output from other_program and using it on project with

read(0, buffer, BUFF_SIZE);

But if imagine that's not the best way to do that. I know I can iterate through stdin and just use realloc to increase the buffer size, but I'm forbidden from using realloc, due to project specifications, which say I can only use malloc, read, write, open and free.

Is there any other way out? thanks!

Answer 1

Repeatedly read data into a local buffer and append it to a big buffer.

After reading, memory allocated to bigbuf will be right-sized to the data read.

A more robust solution would use an exponentially (maybe 1.5x to 3x) growing bigbufsize.

#define BUFF_SIZE 1024
char buffer[BUFF_SIZE];
char *bigbuf = NULL;
size_t bigbufsize = 0;

ssize_t len;
while( (len = read(0, buffer, sizeof buffer)) > 0) {
  size_t newbigbufsize = bigbufsize + len;
  char *newbigbuf = malloc(newbigbufsize);
  if (newbigbuf == NULL) exit(1);  //Handle OOM

  memcpy(newbigbuf, bigbuf, bigbufsize);
  memcpy(&newbigbuf[bigbufsize], buffer, len);
  free(bigbuf);
  bigbuf = newbigbuf;
  bigbufsize = newbigbufsize;
}

// Use data
foo(bigbuf, bigbufsize);

// clean-up
free(bigbuf);
bigbuf = NULL;
bigbufsize = 0;