UNIX for Dummies Questions & Answers

I have created and attached a shared memory segment. However I do not know how am I supposed to write and read data from it. I want to save various different data on this segment such as process IDs and other. Do you know how am I supposed to write these and be able to read such data in the correct manner, i.e. how can i read a single full pid without having to read the others. Thanks in advance and any answers given will be highly appreciated.

You read from and write to it the exact same way you read from and write to any other block of memory. As such, how you use it really depends on how you want to use it, which you haven't actually said.

You don't have to write ASCII text to it. That makes it a whole lot harder in fact since you have to scan through and hunt for things instead of doing random access. It may be simplest to use it as an array of native data types, which can be anything you want, including structures. You can build most other data structures out of arrays anyway.

Code:

int fd=shm_open("/asdf", O_RDWR|O_CREAT, 0777);
int *mem=mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
ftruncate(fd, 4096); // gotta make the file big enough to match the mapping!
// You now have a shared array of integers 4096/sizeof(int) in length, yay.
// Or a shared array of characters 4096 bytes in length.
// Or a shared array of structures 4096/sizeof(my_structure) in length.
// Or...  etc.