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?
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) {
^^^
You can also try by first declaring fd then comparing it to 0.
int fd = shm_open(shm_name, O_RDWR | O_CREAT, MODE);
if (fd < 0){
perror("\nshm_open() in Caretaker failed");
}
It brings about less confusion after all.