DESCRIPTION

memfd_create() creates an anonymous file and returns a file descriptor that refers to it. The file behaves like a regular file, and so can be modified, truncated, memory-mapped, and so on. However, unlike a regular file, it lives in RAM and has a volatile backing storage. Once all references to the file are dropped, it is automatically released. Anonymous memory is used for all backing pages of the file. Therefore, files created by memfd_create() have the same semantics as other anonymous memory allocations such as those allocated using mmap(2) with the MAP_ANONYMOUS flag.

The initial size of the file is set to 0. Following the call, the file size should be set using ftruncate(2). (Alternatively, the file may be populated by calls to write(2) or similar.)

The name supplied in name is used as a filename and will be displayed as the target of the corresponding symbolic link in the directory /proc/self/fd/. The displayed name is always prefixed with memfd: and serves only for debugging purposes. Names do not affect the behavior of the file descriptor, and as such multiple files can have the same name without any side effects.

The following values may be bitwise ORed in flags to change the behavior of memfd_create():

Unused bits in flags must be 0.

As its return value, memfd_create() returns a new file descriptor that can be used to refer to the file. This file descriptor is opened for both reading and writing (O_RDWR) and O_LARGEFILE is set for the file descriptor.

With respect to fork(2) and execve(2), the usual semantics apply for the file descriptor created by memfd_create(). A copy of the file descriptor is inherited by the child produced by fork(2) and refers to the same file. The file descriptor is preserved across execve(2), unless the close-on-exec flag has been set.

RETURN VALUE

On success, memfd_create() returns a new file descriptor. On error, −1 is returned and errno is set to indicate the error.

ERRORS

VERSIONS

The memfd_create() system call first appeared in Linux 3.17.

CONFORMING TO

The memfd_create() system call is Linux-specific.

NOTES

Glibc does not provide a wrapper for this system call; call it using syscall(2).

The memfd_create() system call provides a simple alternative to manually mounting a tmpfs filesystem and creating and opening a file in that filesystem. The primary purpose of memfd_create() is to create files and associated file descriptors that are used with the file-sealing APIs provided by fcntl(2).

The memfd_create() system call also has uses without file sealing (which is why file-sealing is disabled, unless explicitly requested with the MFD_ALLOW_SEALING flag). In particular, it can be used as an alternative to creating files in tmp or as an alternative to using the open(2) O_TMPFILE in cases where there is no intention to actually link the resulting file into the filesystem.

EXAMPLE

Below are shown two example programs that demonstrate the use of memfd_create() and the file sealing API.

The first program, t_memfd_create.c, creates a tmpfs file using memfd_create(), sets a size for the file, maps it into memory, and optionally places some seals on the file. The program accepts up to three command-line arguments, of which the first two are required. The first argument is the name to associate with the file, the second argument is the size to be set for the file, and the optional third argument is a string of characters that specify seals to be set on file.

The second program, t_get_seals.c, can be used to open an existing file that was created via memfd_create() and inspect the set of seals that have been applied to that file.

The following shell session demonstrates the use of these programs. First we create a tmpfs file and set some seals on it:

At this point, the t_memfd_create program continues to run in the background. From another program, we can obtain a file descriptor for the file created by memfd_create() by opening the /proc/PID/fd file that corresponds to the file descriptor opened by memfd_create(). Using that pathname, we inspect the content of the /proc/PID/fd symbolic link, and use our t_get_seals program to view the seals that have been placed on the file:

#include <sys/memfd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)

int
main(int argc, char *argv[])
{
    int fd;
    unsigned int seals;
    char *addr;
    char *name, *seals_arg;
    ssize_t len;

    if (argc < 3) {
        fprintf(stderr, "%s name size [seals]\n", argv[0]);
        fprintf(stderr, "\t'seals' can contain any of the "
                "following characters:\n");
        fprintf(stderr, "\t\tg − F_SEAL_GROW\n");
        fprintf(stderr, "\t\ts − F_SEAL_SHRINK\n");
        fprintf(stderr, "\t\tw − F_SEAL_WRITE\n");
        fprintf(stderr, "\t\tS − F_SEAL_SEAL\n");
        exit(EXIT_FAILURE);
    }

    name = argv[1];
    len = atoi(argv[2]);
    seals_arg = argv[3];

    /* Create an anonymous file in tmpfs; allow seals to be
       placed on the file */

    fd = memfd_create(name, MFD_ALLOW_SEALING);
    if (fd == −1)
        errExit("memfd_create");

    /* Size the file as specified on the command line */

    if (ftruncate(fd, len) == −1)
        errExit("truncate");

    printf("PID: %ld; fd: %d; /proc/%ld/fd/%d\n",
            (long) getpid(), fd, (long) getpid(), fd);

    /* Code to map the file and populate the mapping with data
       omitted */

    /* If a 'seals' command−line argument was supplied, set some
       seals on the file */

    if (seals_arg != NULL) {
        seals = 0;

        if (strchr(seals_arg, 'g') != NULL)
            seals |= F_SEAL_GROW;
        if (strchr(seals_arg, 's') != NULL)
            seals |= F_SEAL_SHRINK;
        if (strchr(seals_arg, 'w') != NULL)
            seals |= F_SEAL_WRITE;
        if (strchr(seals_arg, 'S') != NULL)
            seals |= F_SEAL_SEAL;

        if (fcntl(fd, F_ADD_SEALS, seals) == −1)
            errExit("fcntl");
    }

    /* Keep running, so that the file created by memfd_create()
       continues to exist */

    pause();

    exit(EXIT_SUCCESS);
}

#include <sys/memfd.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)

int
main(int argc, char *argv[])
{
    int fd;
    unsigned int seals;

    if (argc != 2) {
        fprintf(stderr, "%s /proc/PID/fd/FD\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    fd = open(argv[1], O_RDWR);
    if (fd == −1)
        errExit("open");

    seals = fcntl(fd, F_GET_SEALS);
    if (seals == −1)
        errExit("fcntl");

    printf("Existing seals:");
    if (seals & F_SEAL_SEAL)
        printf(" SEAL");
    if (seals & F_SEAL_GROW)
        printf(" GROW");
    if (seals & F_SEAL_WRITE)
        printf(" WRITE");
    if (seals & F_SEAL_SHRINK)
        printf(" SHRINK");
    printf("\n");

    /* Code to map the file and access the contents of the
       resulting mapping omitted */

    exit(EXIT_SUCCESS);
}

SEE ALSO

fcntl(2), ftruncate(2), mmap(2), shmget(2), shm_open(3)