The kernel has to initialize the virtual address space of a process, creating the .text, .bss, .data, heap, and stack segments.

If a process creates a thread, a new segment must be added to the virtual address space for a stack for the thread.

The mmap system call can create new mappings; that is, can add a new segment at some virtual address space of the process.

#include <unistd.h>
#include <sys/mman.h>

void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset);
int munmap(void *start, size_t length);

 prot: PROT_EXEC, PROT_READ, PROT_WRITE, PROT_NONE
 flags: MAP_PRIVATE, MAP_SHARED, MAP_ANON

• PROT_EXEC Pages may be executed.
• PROT_READ Pages may be read.
• PROT_WRITE Pages may be written.
• PROT_NONE Pages may not be accessed.

• MAP_PRIVATE Private copy-on-write mapping
• MAP_SHARED Share with all other processes
• MAP_ANONYMOUS Not associated with a file (only used with MAP_PRIVATE)

MAP_PRIVATE | MAP_ANON could be used to map an area of virtual memory for a stack segment for a new thread.

Copy-on-write is also used in the implementation of fork.