# Multiple clients calling a server IPC
**URL:** https://forum.legato.io/t/multiple-clients-calling-a-server-ipc/4532
**Category:** Uncategorized
**Created:** [July 5, 2019, 2:43am UTC](https://forum.legato.io/t/multiple-clients-calling-a-server-ipc/4532 "2019-07-05T02:43:00Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@sast5](https://forum.legato.io/u/sast5)
#### Post date: [July 5, 2019, 2:43am UTC](https://forum.legato.io/t/multiple-clients-calling-a-server-ipc/4532/1 "2019-07-05T02:43:00Z")
Is this safe? I have a server that writes to a file and I want multiple apps to be able to call the server to write new files. If they call while the last function is still executing while this be a problem or is the state saved?
for example
void FileManager\_StoreFile(const char\* writeData)
{
FILE\* fp = fopen(filename,“w+”);
//\<— Another client calls api
//write to file etc
}
will write Data be lost if another client calls the function before it is written? if so how can I fix this?
---
### Author:  [@jyijyi](https://forum.legato.io/u/jyijyi)
#### Post date: [July 5, 2019, 3:13am UTC](https://forum.legato.io/t/multiple-clients-calling-a-server-ipc/4532/2 "2019-07-05T03:13:07Z")
did you try to use semaphore API to lock this critical section?
> **[Semaphore API - Legato Docs](https://docs.legato.io/latest/c_semaphore.html)**
>
> legato™ is an open source Linux-based embedded platform designed to simplify connected IoT application development
---
### Author:  [@sast5](https://forum.legato.io/u/sast5)
#### Post date: [July 5, 2019, 3:15am UTC](https://forum.legato.io/t/multiple-clients-calling-a-server-ipc/4532/3 "2019-07-05T03:15:46Z")
No Sorry I’m not super familiar with thread concepts but that’s what I was looking for I think thank you
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### Author:  [@sast5](https://forum.legato.io/u/sast5)
#### Post date: [July 7, 2019, 9:11pm UTC](https://forum.legato.io/t/multiple-clients-calling-a-server-ipc/4532/4 "2019-07-07T21:11:57Z")
Actually I don’t think that will work it says for threads in the same process theres not really a way for me to get a reference to the semaphore from outside of the executable?
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### Author:  [@jyijyi](https://forum.legato.io/u/jyijyi)
#### Post date: [July 8, 2019, 1:19am UTC](https://forum.legato.io/t/multiple-clients-calling-a-server-ipc/4532/5 "2019-07-08T01:19:36Z")
did you write a sample code to try that?
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### Author:  [@raf](https://forum.legato.io/u/raf)
#### Post date: [July 8, 2019, 1:21am UTC](https://forum.legato.io/t/multiple-clients-calling-a-server-ipc/4532/6 "2019-07-08T01:21:20Z")
Hi @sast5.
You really only need the reference to the semaphore _within_ the executable providing the API.
Basically, you could create a semaphore in the `COMPONENT_INIT` section with an initial count of `1` then use that reference in your API function(s).
For example:
```auto
COMPONENT_INIT
{
/*
* Create a semaphore with an initial count of 1
* This signals that 1 resource (a single file) is available
*/
}
```
```auto
void FileManager_StoreFile(const char* writeData)
{
/*
* Pend (wait) on the semaphore using either:
* le_sem_WaitWithTimeOut()
* le_sem_Wait()
* le_sem_TryWait()
* Note: the first client calling the API will `consume` the semaphore
*/
// If le_sem above returned LE_OK, it is safe to `use` the resource.
FILE* fp = fopen(filename,“w+”);
// ... do stuff with the file, etc.
/*
* ***<— Another client calls api***
* Other clients won't be able to acquire the resource until
* the `consuming` client releases (posts) it.
* The behavior will depend on which `waiting` method you chose to implement.
*/
// Finally, Post the semaphore so other clients may `use` it.
le_sem_Post(MySemRef);
}
```
Raf
---
### Author:  [@sast5](https://forum.legato.io/u/sast5)
#### Post date: [July 8, 2019, 2:48am UTC](https://forum.legato.io/t/multiple-clients-calling-a-server-ipc/4532/7 "2019-07-08T02:48:50Z")
Oh ok thanks makes more sense with an example (Couldn’t see any other posts about semaphores on the forums/Doc samples)