How do I create a soft link programmatically in C/C++?

You can call symlink()

int  symlink(const char *name1, const char *name2);

A symbolic  link name2 is created to name1 (name2 is the name of the file
created, name1 is the string used in creating the symbolic  link).  Either
name may be an arbitrary path name; the files need  not be on the same
file system.

The system call you want is symlink(2).

#include <unistd.h>

int symlink(const char  *name1, const char *name2);

A symbolic link name2 is created to name1