shm_open and ftruncate() in Shared Memory Programming

I want to create a Shared Memory Object and truncate it to a specific size.

SHMSIZE is defined with 512

MODE is set with S_IRUSR | S_IWUSR | S_IWGRP | S_IRGRP | S_IWOTH | S_IROTH

Here is my Code

char *shm_name = "SharedMemory";    
int fd; 

/* Open an Shared Memory Object for Read-/Write-Access */    
if((fd = shm_open(shm_name, O_RDWR | O_CREAT, MODE) < 0)) {     
     perror("nshm_open() in Caretaker failed");    
     exit(EXIT_FAILURE);
}

/* Truncate Shared Memory Object to specific size */
if((ftruncate(fd, SHMSIZE) < 0)) {
    perror("nftruncate() in Caretaker failed");
    exit(EXIT_FAILURE);
}

While Debugging i watched that the return value of shm_open() is 0 every time, but i can see this object in /dev/shm. And while executing ftruncate() it returns every time with error “invalid argument”.

Why fd is 0 every time and why ftruncate doesn’t work? What should i do?

Answer

The order of operations in this statement is wonky:

if((fd = shm_open(shm_name, O_RDWR | O_CREAT, MODE) < 0)) {

You’re assigning the result of shm_open(...) < 0 to fd, which is definitely not what you want.

Move the comparison outside the parentheses:

if((fd = shm_open(shm_name, O_RDWR | O_CREAT, MODE)) < 0) {
                                                   ^^^