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FreeBSD 11.0 - man page for mprotect (freebsd section 2)

MPROTECT(2)						      BSD System Calls Manual						       MPROTECT(2)

mprotect -- control the protection of pages
Standard C Library (libc, -lc)
#include <sys/mman.h> int mprotect(const void *addr, size_t len, int prot);
The mprotect() system call changes the specified pages to have protection prot. Not all implementations will guarantee protection on a page basis; the granularity of protection changes may be as large as an entire region. A region is the virtual address space defined by the start and end addresses of a struct vm_map_entry. Currently these protection bits are known, which can be combined, OR'd together: PROT_NONE No permissions at all. PROT_READ The pages can be read. PROT_WRITE The pages can be written. PROT_EXEC The pages can be executed.
The mprotect() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indi- cate the error.
The mprotect() system call will fail if: [EINVAL] The virtual address range specified by the addr and len arguments is not valid. [EACCES] The calling process was not allowed to change the protection to the value specified by the prot argument.
madvise(2), mincore(2), msync(2), munmap(2)
The mprotect() system call first appeared in 4.4BSD.
June 9, 1993 BSD