shm_open.c

#define _POSIX_C_SOURCE 200112L
#include <errno.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <time.h>
#include <unistd.h>

static void
randname(char *buf)
{
    struct timespec ts;
    long r;
    clock_gettime(CLOCK_REALTIME, &ts);
    r = ts.tv_nsec;
    for (int i = 0; i < 6; ++i) {
        buf[i] = 'A' + (r & 15) + (r & 16) * 2;
        r >>= 5;
    }
}

static int
create_shm_file(void)
{
    int retries = 100;
    int fd;
    do {
        char name[] = "/wl_shm-XXXXXX";
        randname(name + sizeof(name) - 7);
        --retries;
        fd = shm_open(name, O_RDWR | O_CREAT | O_EXCL, 0600);
        if (fd >= 0) {
            shm_unlink(name);
            return fd;
        }
    } while (retries > 0 && errno == EEXIST);
    return -1;
}

int
allocate_shm_file(size_t size)
{
    int fd = create_shm_file();
    int ret;
    if (fd < 0)
        return -1;
    do {
        ret = ftruncate(fd, size);
    } while (ret < 0 && errno == EINTR);
    if (ret < 0) {
        close(fd);
        return -1;
    }
    return fd;
}