Get home directory in Linux

You need getuid to get the user id of the current user and then getpwuid to get the password entry (which includes the home directory) of that user:

#include <unistd.h>
#include <sys/types.h>
#include <pwd.h>

struct passwd *pw = getpwuid(getuid());

const char *homedir = pw->pw_dir;

Note: if you need this in a threaded application, you'll want to use getpwuid_r instead.

You should first check the $HOME environment variable, and if that does not exist, use getpwuid.

#include <unistd.h>
#include <sys/types.h>
#include <pwd.h>

const char *homedir;

if ((homedir = getenv("HOME")) == NULL) {
    homedir = getpwuid(getuid())->pw_dir;
}

Also note, that if you want the home directory to store configuration or cache data as part of a program you write and want to distribute to users, you should consider following the XDG Base Directory Specification. For example if you want to create a configuration directory for your application, you should first check $XDG_CONFIG_HOME using getenv as shown above and only fall back to the code above if the variable is not set.

If you require multi-thread safety, you should use getpwuid_r instead of getpwuid like this (from the getpwnam(3) man page):

struct passwd pwd;
struct passwd *result;
char *buf;
size_t bufsize;
int s;
bufsize = sysconf(_SC_GETPW_R_SIZE_MAX);
if (bufsize == -1)
    bufsize = 0x4000; // = all zeroes with the 14th bit set (1 << 14)
buf = malloc(bufsize);
if (buf == NULL) {
    perror("malloc");
    exit(EXIT_FAILURE);
}
s = getpwuid_r(getuid(), &pwd, buf, bufsize, &result);
if (result == NULL) {
    if (s == 0)
        printf("Not found\n");
    else {
        errno = s;
        perror("getpwnam_r");
    }
    exit(EXIT_FAILURE);
}
char *homedir = result.pw_dir;