Initial commit

.clang-format

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+---
+BasedOnStyle: WebKit
+TabWidth: 8
+IndentWidth: 8
+UseTab: ForIndentation
+AlwaysBreakAfterReturnType: None

.editorconfig

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+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+tab_width = 8
+
+[*.c]
+indent_size = 8
+indent_style = tab

.gitignore

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+*.o
+/test_shared

LICENSE

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+Copyright 2019 Lassi Kortela
+
+Permission to use, copy, modify, and distribute this software for any
+purpose with or without fee is hereby granted, provided that the above
+copyright notice and this permission notice appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

build-macos.sh

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+#!/bin/sh
+set -eux
+${CC:-clang} -Wall -Wextra -pedantic -std=c99 -c shm_open_anon.c
+${CC:-clang} -Wall -Wextra -pedantic -std=c99 -c test_shared.c
+${CC:-clang} -Wall -Wextra -pedantic -std=c99 \
+             -o test_shared shm_open_anon.c test_shared.c

shm_open_anon.c

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+// Copyright 2019 Lassi Kortela
+// SPDX-License-Identifier: ISC
+
+#include <sys/types.h>
+
+#include <sys/mman.h>
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+//
+
+#undef IMPL_CLASSIC
+#undef IMPL_DEV_SHM
+#undef IMPL_SHM_ANON
+#undef IMPL_SHM_MKSTEMP
+
+#ifdef __APPLE__
+#ifdef __MACH__
+#define IMPL_CLASSIC
+#endif
+#endif
+
+#ifdef __NetBSD__
+#define IMPL_CLASSIC
+#endif
+
+#ifdef __DragonFly__
+#define IMPL_CLASSIC
+#endif
+
+#ifdef __linux__
+#define IMPL_DEV_SHM
+#endif
+
+#ifdef __FreeBSD__
+#define IMPL_SHM_ANON
+#endif
+
+#ifdef __OpenBSD__
+#define IMPL_SHM_MKSTEMP
+#endif
+
+//
+
+static int save_errno_and_close(int fd)
+{
+	int save;
+
+	save = errno;
+	close(fd);
+	errno = save;
+	return -1;
+}
+
+static int fd_without_close_on_exec(int fd)
+{
+	int flags;
+
+	if ((flags = fcntl(fd, F_GETFD)) == -1)
+		return save_errno_and_close(fd);
+	flags &= ~FD_CLOEXEC;
+	if (fcntl(fd, F_SETFD, flags) == -1)
+		return save_errno_and_close(fd);
+	return fd;
+}
+
+//
+
+#ifdef IMPL_CLASSIC
+int shm_open_anon_private(void)
+{
+	char name[16];
+	int fd;
+
+	snprintf(name, sizeof(name), "/tmp/XXXXXXXX");
+	if (mktemp(name) == NULL)
+		return -1;
+	if ((fd = shm_open(name, O_RDWR | O_CREAT | O_EXCL, 0600)) == -1)
+		return -1;
+	if (shm_unlink(name) == -1)
+		return save_errno_and_close(fd);
+	return fd;
+}
+#endif
+
+//
+
+#ifdef IMPL_DEV_SHM
+int shm_open_anon_private(void)
+{
+	char name[16];
+	int fd;
+
+	snprintf(name, sizeof(name), "/dev/shm/XXXXXXXX");
+	if ((fd = mkostemp(name, O_CLOEXEC)) == -1)
+		return -1;
+	if (shm_unlink(name) == -1)
+		return save_errno_and_close(fd);
+	return fd;
+}
+#endif
+
+//
+
+#ifdef IMPL_SHM_MKSTEMP
+int shm_open_anon_private(void)
+{
+	char name[16];
+	int fd;
+
+	snprintf(name, sizeof(name), "/tmp/XXXXXXXX");
+	if ((fd = shm_mkstemp(name)) == -1)
+		return -1;
+	if (shm_unlink(name) == -1)
+		return save_errno_and_close(fd);
+	return fd;
+}
+#endif
+
+//
+
+#ifdef IMPL_SHM_ANON
+int shm_open_anon_private(void)
+{
+	return shm_open(SHM_ANON, 0, 0);
+}
+#endif
+
+//
+
+int shm_open_anon_shared(void)
+{
+	return fd_without_close_on_exec(shm_open_anon_private());
+}

shm_open_anon.h

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+int shm_open_anon_private(void);
+int shm_open_anon_shared(void);

test_shared.c

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+#include <sys/types.h>
+
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "shm_open_anon.h"
+
+#define PROGNAME "test_shared"
+
+static void
+diesys(const char *msg)
+{
+    fprintf(stderr, "%s: %s\n", msg, strerror(errno));
+    exit(1);
+}
+
+static void *
+map_shared_memory_from_fd(int fd, size_t *out_size)
+{
+    struct stat st;
+    void *buf;
+    size_t size;
+
+    if (fstat(fd, &st) == -1) {
+        diesys("fstat");
+    }
+    *out_size = size = (size_t)st.st_size;
+    buf = mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+    if (buf == MAP_FAILED) {
+        diesys("mmap");
+    }
+    return buf;
+}
+
+static void
+parent(void)
+{
+    char *child_args[3] = {"./" PROGNAME, "child", 0};
+    char *buf;
+    size_t bufsize;
+    pid_t child;
+    int fd, status;
+
+    bufsize = 10 * 1024 * 1024;
+    if ((fd = shm_open_anon_shared()) == -1)
+        diesys("shm_open_anon_shared");
+    if (ftruncate(fd, (off_t)bufsize) == -1)
+        diesys("ftruncate");
+    buf = map_shared_memory_from_fd(fd, &bufsize);
+    memset(buf, 0, bufsize);
+    snprintf(buf, bufsize, "hello from parent");
+    if ((child = fork()) == -1)
+        diesys("fork");
+    if (!child) {
+        if (dup2(fd, 3) == -1)
+            diesys("dup2");
+        execv(child_args[0], child_args);
+        diesys("execv");
+    }
+    if (waitpid(child, &status, 0) == -1)
+        diesys("cannot wait for child");
+    printf("Parent: %s\n", buf);
+}
+
+static void
+child(void)
+{
+    char *buf;
+    size_t bufsize;
+
+    buf = map_shared_memory_from_fd(3, &bufsize);
+    printf("Child: %s\n", buf);
+    snprintf(buf, bufsize, "hello from child");
+}
+
+int
+main(int argc, char **argv)
+{
+    (void)argv;
+    if (argc == 1) {
+        parent();
+    } else if ((argc == 2) && !strcmp(argv[1], "child")) {
+        child();
+    } else {
+        fprintf(stderr, "usage\n");
+        exit(1);
+    }
+    return 0;
+}