#include <sys/types.h>
#include <sys/mman.h>
int mprotect( void *addr,
size_t len,
int protection);
Service Program Name: QP0LLIB1The mprotect() function is used to change the access protection of a memory mapping to that specified by protection. All whole pages of the process's address space, that were established by the mmap() function, addressed from addr continuing for a length of len will be affected by the change of access protection. You may specify PROT_NONE, PROT_READ, PROT_WRITE, or the inclusive or of PROT_READ and PROT_WRITE as values for the protect parameter.
The addr argument must be a multiple of the page size. The
sysconf() function may be used to determine the system page size.
No access through the memory mapping will be permitted if PROT_NONE is specified.
Storage associated with the mapping cannot be altered unless the PROT_WRITE value is specified.
For shared mappings, PROT_WRITE requires that the file descriptor used to establish the map had been opened for write access. A shared mapping is a mapping created with the MAP_SHARED value of the flag parameter of the mmap() function.
Since private mappings do not alter the underlying file, PROT_WRITE may be specified for a mapping that had been created MAP_PRIVATE and had been opened for read access.
The following table shows the symbolic constants allowed for the protection argument.
| Symbolic Constant | Decimal Value |
Description |
|---|---|---|
| PROT_WRITE | 2 | Write access allowed. |
| PROT_READ | 2 | Read access allowed. |
| PROT_NONE | 8 | No access allowed. |
No authorization is required.
Upon successful completion, the mprotect() function returns 0. Upon failure, -1 is returned and errno is set to the appropriate error number.
When the mprotect() function fails, it returns -1 and sets there errno variable as follows.
| Error condition | Additional information |
|---|---|
| [EACCES] |
If you are accessing a remote file through the Network File System, update operations to file permissions at the server are not reflected at the client until updates to data that is stored locally by the Network File System take place. (Several options on the Add Mounted File System (ADDMFS) command determine the time between refresh operations of local data.) Access to a remote file may also fail due to different mappings of user IDs (UID) or group IDs (GID) on the local and remote systems. The protection argument specifies a protection that violates the access permission the process has to the underlying mapped file. If the QSHRMEMCTL system value was 0 at the time the mapping was created, then this continues to limit the allowed access until the mapping is destroyed. An attempt to change the protection of a shared mapping to PROT_WRITE when the QSHRMEMCTL system value had been zero at the time of map creation will result in an errno of EACCES. |
| [EINVAL] |
For example, the addr argument is not a multiple of the page size. This error number also may indicate that the value of the len argument is 0. |
| [ENOMEM] |
The addr argument is out of the allowed range. |
| [ENOTAVAIL] | |
| [ENOTSUP] |
For mprotect() this can be caused by an invalid combination of access requests on the protection parameter. |
The following messages may be sent from this function.
| Message ID | Error Message Text |
|---|---|
| CPE3418 E | Possible APAR condition or hardware failure. |
| CPFA0D4 E | File system error occurred. Error number &1. |
| CPF3CF2 E | Error(s) occurred during running of &1 API. |
| CPF9872 E | Program or service program &1 in library &2 ended. Reason code &3. |
If the DTAMDL(*LLP64) parameter is used when compiling an ILE C program, this limitation does not apply as the pointers will be 8 byte pointers, and their pointer attribute will be preserved.
The following example creates a file, produces a memory mapping of the file using mmap(), and then changes the protection of the file using mprotect().
Note: By using the code examples, you agree to the terms of the Code license and disclaimer information.
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/mman.h>
main(void) {
size_t bytesWritten =0;
int fd;
int PageSize;
char textÝ = "This is a test";
if ( (PageSize = sysconf(_SC_PAGE_SIZE)) < 0) {
perror("sysconf() Error=");
return -1;
}
fd = open("/tmp/mmprotectTest",
(O_CREAT | O_TRUNC | O_RDWR),
(S_IRWXU | S_IRWXG | S_IRWXO) );
if ( fd < 0 ) {
perror("open() error");
return fd;
}
off_t lastoffset = lseek( fd, 0, SEEK_SET);
bytesWritten = write(fd, text, strlen(text));
if (bytesWritten != strlen(text) ) {
perror("write error. ");
return -1;
}
lastoffset = lseek( fd, PageSize-1, SEEK_SET);
bytesWritten = write(fd, " ", 1); /* grow file to 1 page. */
if ( bytesWritten != 1 ) {
perror("write error. ");
return -1;
}
/* mmap the file. */
void *address;
int len;
off_t my_offset = 0;
len = PageSize; /* Map one page */
address =
mmap(NULL, len, PROT_NONE, MAP_SHARED, fd, my_offset);
if ( address == MAP_FAILED ) {
perror("mmap error. " );
return -1;
}
if ( mprotect( address, len, PROT_WRITE) < 0 ) {
perror("mprotect failed with error:");
return -1;
}
else (void) printf("%s",address);
close(fd);
unlink("/tmp/mmprotectTest");
}