Question

The php code is supposed to read the shared memory I create in a C program below. However, I am reading all 0s no matter what I set the values in the shared memory to.
php code:

<?php
    $shm_id = shmop_open(9875, "a", 0, 0);
    echo "shmid=".$shm_id."    ";
    echo "size=".shmop_size($shm_id). " ";
    $shm_data = shmop_read($shm_id, 0, 8);
    if($shm_data == FALSE) 
        echo "failed to read"; 
    else
    {
        echo "data=";
        for($i=0;$i < 8;$i++)
        {
            echo $shm_data[$i];
            if($shm_data[$i] == 1)
                echo "1";
            else if($shm_data[$i] == 0)
                echo "0";
            else echo "v";
        }
    }
    shmop_close($shm_id);
    ?> 

The C code creates the shared memory block of size 8 bytes.

I have this code to create the shared memory block. I pass in a key of 9875, which I use in the php code.
C Code:

buf1 = allocArray_shared(8, sizeof(unsigned char), &shmid, 9875);
for(i = 0; i < 8; i++)
{
      buf1[i] = 0xFF;
}

void* allocArray_shared(int elementCount, int elementByteSize, int* shmid, key_t key)
{
    //size of entire array(cols*rows* byte size + row pointers)
    int array_size = elementByteSize * elementCount;


    //Allocate enough space for all elements + row pointers
    *shmid = shmget(key, array_size, 00666 | IPC_CREAT);

    char * arr = (char*)shmat(*shmid, NULL, 0);
    if(!arr) return NULL;



    //Return the pointer to the first row pointer
    return (void*)arr;
}

I checked that the C program is creating the shared memory region with ipcs command and it is there with the correct key value and the shmid value that gets returned from shmget. I also initialize the data to all 0xFF.

The php program successfully opens the shared memory region, and the shmid is always printed as 2, which does not match the shmid generated by shmget(does this matter, or does php map that to the correct shmid?).

The data in $shm_data is printed as all 0’s, while it should be 0xFF!!

Any tips on getting this working?

Answer 1

I give credit to Marc B and Joshua for answering my question.

The main issue was that 0xFF was not a valid value and also I was not comparing values with the correct operator.