Question

Receiving the message “Invalid argument” when using shmget with the second parameter not being NULL.

It compiles ok, but when executing, I get this error message.

I’ve been stuck on this all day long. Hope you can help me! 🙂

#include <sys/ipc.h>        
#include <sys/shm.h>        
#include <stdlib.h>     
#include <stdio.h>          


int main()
{
    int idSharedMem;
    int *varSharedMem1;
    int *varSharedMem2;

    /* Create the shared memory */
    idSharedMem = shmget((key_t) 0001, sizeof(int), IPC_CREAT | 0666);

    if (idSharedMem == -1)
    {
        perror("shmget");
    }

    /* Allocate a memory address and attached it to a variable */
    varSharedMem1 = shmat(idSharedMem, NULL, 0);

    if (varSharedMem1 == (int *) -1)
    {
        perror("shmat1");
    }

    /* Sign a value to the variable */
    *varSharedMem1 = 5;

    /* Attach an existing allocated memory to another variable */
    varSharedMem2 = shmat(idSharedMem, varSharedMem1, 0);

    if (varSharedMem2 == (int *) -1)
    {
        /* PRINTS "shmat2: Invalid argument" */
        perror("shmat2");
    }

    /* Wanted it to print 5 */
    printf("Recovered value %d\n", *varSharedMem2);

    return(0);
}

Answer 1

With shmat(idSharedMem, varSharedMem1, 0); you’re trying to attach the segment at the location of varSharedMem1. However you have previously attached a segment at that location which will result in EINVAL. Linux provides a SHM_REMAP flag you can use to replace previously mapped segments.

shmat manpage:

The (Linux-specific) SHM_REMAP flag may be specified in shmflg to
indicate that the mapping of the segment should replace any existing
mapping in the range starting at shmaddr and continuing for
the size of the segment. (Normally an EINVAL error would result if a mapping already exists in this address range.) In this
case, shmaddr must not be NULL.