How to create hard/soft links in C? - c

Question

I need to create a hard link to an existing file or directory.
I also need to create a soft/symbolic link to an existing file or directory. This is part of a larger program which is shown below. Commands are executed by typing "-f pathname linkname" and each break argument is a different command. I believe the functions -f, -d and -h have been created correctly so far. However I am having trouble creating these links.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>
#include <errno.h>
#include <sys/types.h>
#include <string.h>
#include <fcntl.h>
//Handles filename, absolute pathname
//Need relative pathname for -f
int main(int argc, char **argv)
{
char command[200], flag[20], pathname[100], linkname[100];
struct stat st = {0};
char cmd[200];
char *token; //Pointer
int counter = 1; //Counter variable
FILE *fp;
char mode2[] = "0750"; //To set the permission of a file/path
long j;
char mode[] = "0640"; //To set the permission of a file/path
long i;
printf("Enter command: ");
fgets(cmd, 420, stdin);
//User input is tokenized to determine the proper commands are entered and executed
token = strtok(cmd, " "); //Input is tokenized by white spaces.
if(token == NULL)
{
printf("Error with command input.\n");
exit(EXIT_FAILURE);
}
strcpy(command, token);
token = strtok(NULL, " ");
if(token != NULL)
strcpy(flag, token);
token = strtok(NULL, " ");
if(token != NULL)
strcpy(pathname, token);
token = strtok(NULL, " ");
if(token != NULL)
strcpy(linkname, token);
//Switch statement to determine which command the user is choosing and execute that command.
switch(flag[1]) {
//The f case will create a file who's name is chosen by the user.
case 'f':
fp=fopen(pathname,"w");
fclose(fp);
char mode[] = "0640"; //Sets the permission of file to 0640.
i = strtol(mode, 0, 8);
if (chmod(pathname, i) < 0)
{
fprintf(stderr, "%s: error in chmod(%s, %s) - %d (%s)\n",
argv[0], pathname, mode, errno, strerror(errno));
return -1;
}
return 0;
break;
//The d case will create a new directory chosen by the user.
case 'd':
if (stat(pathname, &st) == -1) {
mkdir(pathname, 0750); //Directory is given permission 0750.
}
j = strtol(mode, 0, 8);
if (chmod (pathname,j) < 0)
{
fprintf(stderr, "%s: error in chmod(%s, %s) - %d (%s)\n",
argv[0], pathname, mode, errno, strerror(errno));
return -1;
}
return 0;
break;
//The h case will create a hardlink to an existing file.
case 'h':
char *pathname; //Existing file
char *linkname; //Name of desired hardlink
int hlink; //Stores path
hlink = link(pathname, linkname); //Links linkname to pathname
if (chmod (pathname,j) < 0)
{
fprintf(stderr, "%s: error linking(%s, %s) - %d (%s)\n",
argv[0], pathname, mode, errno, strerror(errno));
return -1;
}
return 0;
break;
//The s case will create a symbol link to an existing file.
case 's':
char *pathname;
char *linkname;
int slink;
slink = symlink(pathname, linkname); //Using the symlink function to create a symbolic link of pathname
if (chmod (pathname,j) < 0)
{
fprintf(stderr, "%s: error linking(%s, %s) - %d (%s)\n",
argv[0], pathname, mode, errno, strerror(errno));
return -1;
}
return 0;
break;
}
}

Answer 1

POSIX
http://pubs.opengroup.org/onlinepubs/009695399/functions/link.html
http://pubs.opengroup.org/onlinepubs/009695399/functions/symlink.html
#include <unistd.h>
char *path1 = "/home/cnd/mod1";
char *path2 = "/modules/pass1";
int status;
...
status = link (path1, path2);
Windows
https://learn.microsoft.com/en-us/windows/desktop/api/winbase/nf-winbase-createhardlinka
https://learn.microsoft.com/en-us/windows/desktop/api/winbase/nf-winbase-createsymboliclinka
BOOL fCreatedLink = CreateHardLink( pszNewLinkName,
pszExistingFileName,
NULL ); // reserved, must be NULL
if ( fCreatedLink == FALSE )
{
;// handle error condition
}