CODEWITHSUNDEEP
clinuxkernelubuntu-12.04
Krozark
Krozark

Reputation: 860

live update on file /proc/<pid>/status when open

I am trying to read information from the /proc/<pid>/status file (to get the memory used).

To do this, I open the file in read mode:

file = fopen("/proc/self/status", "r");

After this step, to get the memory, I read the line that starts with "VmRSS".

My problem is this:

Each time I read this line, it's the same value, even if the file has changed.

I am doing this to get real-time memory usage of my program. So I call fopen() 1 time, and then I call fseek() to go to the beginning of my file when I need updated information.

char line[128];
fseek(file, 0, SEEK_SET);
while (fgets(line, 128, file) != NULL)
{
  //...
}

But, the file is not updated, unless I reopen it. I don't want to reopen it for performance reasons.

I tried to change "r" to "r+" (to have a "Open a file for update", according to the documentation of fopen()), but fopen returns NULL in this case.

So my question:

Do you have any idea on how my program can open a file and see changes from made by another programme (here the kernel) using only one call to fopen()?

Note :

I use Ubuntu 12.04

Upvotes: 2

Views: 2006

Answers (2)

Guoqing Ma
Guoqing Ma

Reputation: 41

Maybe you can open the /proc/<pid>/status file with open but not fopen.

int fd = open('/proc/<pid>/status', O_RDONLY, MYF(0));

// read

seek(fd, 0L, MY_SEEK_SET, MYF(0));

// read

Upvotes: 0

Saddam Abu Ghaida
Saddam Abu Ghaida

Reputation: 6739

You need to reopen the file To avoid race conditions, proc is a file system in memeory and most /proc content are being fixed on open.

Upvotes: 4

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