Proper Use Of gethostname?

eric.r.turner · 01-22-2006, 12:30 PM

First, here's the function signature:

Code:

int gethostname(char *name, size_t len);

What shall we use for name and len? I've seen code that looks like:

Code:

#include <limits.h>
char hostname[HOST_NAME_MAX+1];
gethostname(hostname, sizeof(hostname));

According to the POSIX standard, HOST_NAME_MAX is a "possibly indeterminate" definition meaning that it

Quote:

...shall be omitted from <limits.h> on specific implementations where the corresponding value is equal to or greater than the stated minimum, but is unspecified.

HOST_NAME_MAX is not defined on my Linux box, probably because the kernel developers don't want to specify a maximum host name. I suppose I could throw in a

Code:

#ifndef HOST_NAME_MAX
#define HOST_NAME_MAX 256
#endif

but that doesn't seem right to me. So, what length for a host name do you specify on a system that has no defined maximum host name?

paulsm4 · 01-22-2006, 02:27 PM

My copy of "Unix Network Programming" (Stevens, 2nd ed) recommends using MAXHOSTNAMELEN, defined in sys/param.h. I tried it; it seems to work OK on my current Linux:

Code:

#include <stdio.h>
#include <unistd.h>
#include <sys/param.h>

int
main (int argc, char *argv[])
{
  char host[MAXHOSTNAMELEN];
  gethostname (host, sizeof host);
  return printf ("host: %s\n", host);
}

paulsm4 · 01-22-2006, 02:39 PM

Oh yeah - here's a version #ifdef'ed for Windows and *nix:

Code:

#include <stdio.h>
#if defined(WIN32)
  #include <windows.h>
#else
  #include <unistd.h>
  #include <sys/param.h>
#endif

int
main (int argc, char *argv[])
{
#if defined(WIN32)
  char host[MAX_COMPUTERNAME_LENGTH+1];
  int ilen = sizeof (host);
  GetComputerName (host, &ilen);
#else
  char host[MAXHOSTNAMELEN];
  gethostname (host, sizeof host);
#endif
  return printf ("host: %s\n", host);
}

JMerkle · 07-17-2019, 10:06 AM

1) If you have a Raspberry Pi with desktop image, open Geany IDE.
2) Copy the following into a new file:

// file: hostname.cpp

#include <unistd.h>
#include <stdio.h>
#include <limits.h>

int main(int argc, char *argv[])
{
char hostname[HOST_NAME_MAX];
int status = gethostname(hostname,sizeof(hostname));
printf("HOST_NAME_MAX: %u, Hostname: "%s", status: %d\n",HOST_NAME_MAX,hostname,status);

}

3) save file to "hostname.cpp"
4) click the "Build the current file" button
5) click the "run or view the current file" button

Resulting output:
HOST_NAME_MAX: 64, Hostname: "raspberrypi.frontiernet.net", status: 0

The API works great, but I thought I had already given my device a different hostname... Something else to investigate...

astrogeek · 07-17-2019, 12:58 PM

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