CreateFileMapping/MapViewOfFile and fork

[Sean Daley]

As part of an application we're developing, we use
CreateFileMapping/MapViewOfFile to be able to share information among
processes.  I noticed today that if I try to reference the map
immediately after calling fork() but before calling exec, that my
application will crash.

I should be able to resolve this in my app but I was just curious if
this was unexpected behavior or not.

I've attached my cygcheck.out and will include some test code at the end.

######################
sdaley@sdaley2 ~
$ gcc -Wall parent_test.cpp -o parent_test

sdaley@sdaley2 ~
$ ./parent_test.exe
Dumping map in parent
 --> length = 11
 --> name = test-string
Dumping map in child
    17 [main] parent_test 2460 _cygtls::handle_exceptions: Error while dumping
state (probably corrupted stack)

sdaley@sdaley2 ~
$
######################

Sean

##############################
parent.cpp
##############################

#include <stdio.h>
#include <unistd.h>
#include <windows.h>

#define BUF_SIZE 1024

void *view;
HANDLE hFileMapping;

typedef struct usrInfo {
    int usrlen;
    unsigned char data[1];
} usrInfo;

static void* create_file_map();
static void dump_file_map();

int main(int argc, char **argv)
{
    void *shmp = create_file_map();

    if (shmp == NULL)
    {
        fprintf(stderr, "shmp == NULL\n");
        exit(1);
    }

    usrInfo *usr = (usrInfo*)shmp;
    usr->usrlen = strlen("test-string");
    unsigned char *p = usr->data;
    memcpy(p, "test-string", usr->usrlen);
    p[usr->usrlen] = '\0';

    printf("Dumping map in parent\n");
    dump_file_map();

    if (fork() == 0)
    {
        //const char *args[2];
        //args[0] = "child_test";
        //args[1] = NULL;
        //execvp("./child_test", (char **)args);
        //printf("execvp failed\n");
        printf("Dumping map in child\n");
        dump_file_map();
        exit(1);
    }

    sleep(5);

    UnmapViewOfFile(view);
    CloseHandle(hFileMapping);

    return 0;
}

static void
dump_file_map()
{
    usrInfo *usr = (usrInfo*)view;
    if (usr != NULL)
    {
        printf(" --> length = %d\n", usr->usrlen);
        printf(" --> name = %s\n", usr->data);
    }
}

static void *
create_file_map()
{
    SECURITY_ATTRIBUTES sa;

    sa.nLength = sizeof(sa);
    sa.lpSecurityDescriptor = NULL;
    sa.bInheritHandle = TRUE;

    hFileMapping = CreateFileMapping(INVALID_HANDLE_VALUE,
                                     &sa, PAGE_READWRITE, 0, BUF_SIZE,
                                     "TEST-PAGE");

    if (hFileMapping == 0)
    {
        fprintf(stderr, "CreateFileMapping failed\n");
        return NULL;
    }

    view = MapViewOfFile(hFileMapping, FILE_MAP_ALL_ACCESS, 0, 0, BUF_SIZE);

    return view;
}
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