shm_open_anon.c

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// Copyright 2019 Lassi Kortela
// SPDX-License-Identifier: ISC
 
#ifdef __linux__
#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif
#include <linux/memfd.h>
#include <linux/unistd.h>
#endif
 
#include <sys/types.h>
 
#include <sys/mman.h>
 
#include <errno.h>
#include <fcntl.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
 
#undef IMPL_MEMFD
#undef IMPL_POSIX
#undef IMPL_SHM_ANON
#undef IMPL_SHM_MKSTEMP
#undef IMPL_UNLINK_OR_CLOSE
 
#ifdef __linux__
#ifdef __NR_memfd_create
#define IMPL_MEMFD
#endif
#endif
 
#ifdef __FreeBSD__
#define IMPL_SHM_ANON
#endif
 
#ifdef __OpenBSD__
#define IMPL_SHM_MKSTEMP
#endif
 
#if !defined(IMPL_MEMFD) && !defined(IMPL_SHM_ANON) && \
    !defined(IMPL_SHM_MKSTEMP)
#define IMPL_POSIX
#endif
 
#if defined(IMPL_POSIX) || defined(IMPL_SHM_MKSTEMP)
#define IMPL_UNLINK_OR_CLOSE
#endif
 
#ifdef IMPL_UNLINK_OR_CLOSE
static int
shm_unlink_or_close(const char *name, int fd)
{
        int save;
 
        if (shm_unlink(name) == -1) {
                save = errno;
                close(fd);
                errno = save;
                return -1;
        }
        return fd;
}
#endif
 
#ifdef IMPL_POSIX
int
shm_open_anon(void)
{
        char name[16] = "/shm-";
        struct timespec tv;
        unsigned long r;
        char *const limit = name + sizeof(name) - 1;
        char *start;
        char *fill;
        int fd, tries;
 
        *limit = 0;
        start = name + strlen(name);
        for (tries = 0; tries < 4; tries++) {
                clock_gettime(CLOCK_REALTIME, &tv);
                r = (unsigned long)tv.tv_sec + (unsigned long)tv.tv_nsec;
                for (fill = start; fill < limit; r /= 8)
                        *fill++ = '0' + (r % 8);
#ifdef O_NOFOLLOW
                fd = shm_open(
                  name, O_RDWR | O_CREAT | O_EXCL | O_NOFOLLOW, 0600);
#else
                fd = shm_open(
                  name, O_RDWR | O_CREAT | O_EXCL, 0600);
#endif
                if (fd != -1)
                        return shm_unlink_or_close(name, fd);
                if (errno != EEXIST)
                        break;
        }
        return -1;
}
#endif
 
#ifdef IMPL_SHM_MKSTEMP
int
shm_open_anon(void)
{
        char name[16] = "/shm-XXXXXXXXXX";
        int fd;
 
        if ((fd = shm_mkstemp(name)) == -1)
                return -1;
        return shm_unlink_or_close(name, fd);
}
#endif
 
#ifdef IMPL_SHM_ANON
int
shm_open_anon(void)
{
        return shm_open(SHM_ANON, O_RDWR, 0);
}
#endif
 
#ifdef IMPL_MEMFD
int
shm_open_anon(void)
{
        return syscall(
          __NR_memfd_create, "shm_anon", (unsigned int)(MFD_CLOEXEC));
}
#endif