Changing Memory Protection

Read-only	By making pages read-only, you cause a SIGSEGV signal to be generated in any process that tries to modify them. You could do this as a debugging measure, to trap an intermittent program error. You can change read-only pages back to read-write.
Read-write	You can put read-write protection on pages of program text, but this is bad idea except in unusual cases. For example, a debugging tool makes text pages read-write in order to set breakpoints.
Executable	Normal data pages cannot be executed. This is a protection against program errors--wild branches into data are trapped quickly. If your program constructs executable code, or reads it from a file, the protection must be changed to executable before the code can be executed.
No access	You can make pages inaccessible while retaining them as part of the address space.

Note: The mprotect() function changes the access rights only to memory image of a mapped file. You can apply it to the pages of a mapped file in order to control access to the file image in memory. However, mprotect() does not affect the access rights to the file itself, nor does it prevent other processes from opening and using the file as a file.