NAME¶

SYNOPSIS¶

int memfd_create(const char *name, unsigned int flags);

DESCRIPTION¶

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():

MFD_CLOEXEC

Set the close-on-exec (FD_CLOEXEC) flag on the new file descriptor. See the description of the O_CLOEXEC flag in open(2) for reasons why this may be useful.

MFD_ALLOW_SEALING

Allow sealing operations on this file. See the discussion of the F_ADD_SEALS and F_GET_SEALS operations in fcntl(2), and also NOTES, below. The initial set of seals is empty. If this flag is not set, the initial set of seals will be F_SEAL_SEAL, meaning that no other seals can be set on the file.

MFD_HUGETLB (since Linux 4.14)

The anonymous file will be created in the hugetlbfs filesystem using huge pages. See the Linux kernel source file Documentation/vm/hugetlbpage.txt for more information about hugetlbfs. The hugetlbfs filesystem does not support file-sealing operations. Therefore, specifying both MFD_HUGETLB and MFD_ALLOW_SEALING in flags is disallowed.

MFD_HUGE_2MB, MFD_HUGE_1GB, ...

Used in conjunction with MFD_HUGETLB to select alternative hugetlb page sizes (respectively, 2 MB, 1 GB, ...) on systems that support multiple hugetlb page sizes. Definitions for known huge page sizes are included in the header file <sys/memfd.h>.

For details on encoding huge page sizes not included in the header file, see the discussion of the similarly named constants in mmap(2).

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¶

ERRORS¶

EFAULT

The address in name points to invalid memory.

EINVAL

flags included unknown bits.

EINVAL

name was too long. (The limit is 249 bytes, excluding the terminating null byte.)

EINVAL

Both MFD_HUGETLB and MFD_ALLOW_SEALING were specified in flags.

EMFILE

The per-process limit on the number of open file descriptors has been reached.

ENFILE

The system-wide limit on the total number of open files has been reached.

ENOMEM

There was insufficient memory to create a new anonymous file.

VERSIONS¶

CONFORMING TO¶

NOTES¶

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¶

The first program, t_memfd_create.c, creates a tmpfs(5) 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(5) file and set some seals on it:

$ ./t_memfd_create my_memfd_file 4096 sw &
[1] 11775
PID: 11775; fd: 3; /proc/11775/fd/3

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:

$ readlink /proc/11775/fd/3
/memfd:my_memfd_file (deleted)
$ ./t_get_seals /proc/11775/fd/3
Existing seals: WRITE SHRINK

#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¶

COLOPHON¶