How to read an integer from a socket in C

Question

I am trying to read in the char array from the socket and get the integer value which can then be used in a for loop. Sadly I am getting a segmentation fault at atoi(). What am I doing wrong?

bytesRead = read(sock, buffer, 1024);
buffer[bytesRead] = '\0';
char tmp[bytesRead];                // I suspect creating this shorter 
strncpy(tmp, buffer, bytesRead);    // array is not necessary... but not sure.
int num = atoi(tmp);

Answer 1

To make sure tmp is a C-"string", that is carries the 0 terminator. change the following:

char tmp[bytesRead]; 

to be

char tmp[bytesRead + 1] = "";

The modifications above do two things:

1. Allocate one more bytes then you will use.
2. Set all bytes to zero.

So if you overwrite the 1 ^st bytesRead bytes by the call to strncpy() the last byte stays untouched and with this continues to be '\\0' , that is it 0 -terminates the char -array and with this make it a C-"string".

---

Btw, this line:

buffer[bytesRead] = '\0';

requires buffer to refer to at least 1024 + 1 bytes ...

---

Introducing the usage of tmp however isn't necessary. The code also might look like this:

char buffer[1024 + 1];
ssize_t result = read(sock, buffer, sizeof buffer - 1);
if (-1 == result) 
{
  perror("read() failed");
}
else
{
  size_t bytesRead = result;
  buffer[bytesRead] = '\0';
  int num = atoi(buffer);
  if (0 == num)
  {
    fprintf(stderr, "atoi() (might have) failed");
  }
  ...
}