ftruncate failed at the second time

问题

I'm trying to exceed the shared memory object after shm_open and ftruncate successfully at fisrt. Here is the code,

char *uuid = GenerateUUID();
int fd = shm_open(uuid, O_RDWR|O_CREAT|O_EXCL, S_IRUSR|S_IWUSR);
if(fd == -1) perror("shm_open");

size_t shmSize = sizeof(container);
int ret = ftruncate(fd, shmSize);
perror("ftruncate first");

ret = ftruncate(fd, shmSize * 2);
perror("ftruncate second");

It could pass the first ftruncate, but for the second ftruncate, it exceeds failed with errno=22, "Invalid argument".

I also tried to ftruncate the memory object after mmap, refer to the ftruncate's man page, the shared memory should be formatted as zero to the new length.

Besides, I also tried to ftruncate the memory object in the child process (This is an IPC topic among two processes), the ftruncate returns "Invalid fd, no such file or directory" but I could shm_open and mmap successfully in child process.

Any ideas? Thanks!

回答1:

I think this is a known "feature" of shm_open(), ftruncate(), mmap().

You have to ftruncate() the first time through to give the shared memory a length, but subsequent times ftruncate() gives that error number 22, which you can simply ignore.

回答2:

The used implementation seems to conform to an older specification where returning an error is an allowed behavior for ftruncate(fd, length) when length exceeds the previous length:

If the file previously was smaller than this size, ftruncate() shall either increase the size of the file or fail.

来源：https://stackoverflow.com/questions/20320742/ftruncate-failed-at-the-second-time