How to call a proc file from *.c program? - Linux

pat_and_cami · #1 02-12-2006

Hi,

I am new to Linux programming. As part of learning, I need to create a *.c program where we call certain /proc files (i.e. such as meminfo, version, uptime, etc...) from our program. Can anyone point me to a simple program on how one would do this (i.e. can you directly call uptime() or ... straight from my program - I would be very surprised if one could)?

Thanks,

Pat//

vino · #2 02-12-2006

Since you are on a learning curve, read through this - The /proc File System

It has almost everything that you have asked for.

cbkihong · #3 02-13-2006

I'm pretty sure you just read() it.

One way to see how other programs does it.

Run strace on a program which is known to access /proc. The system calls involved will be logged. Inside you will find calls to read from the proc filesystem.

pat_and_cami · #4 02-14-2006

Hi cbkihong,

You mention that I can run a program 'known' to access a file in the '/proc' subsystem. Is there a good example program you can recommend I run (and how do you run with strace?).

Thanks,

Patrick//

cbkihong · #5 02-14-2006

On Linux, a program which reads /proc I can currently think of is "lsmod".

If I run

Code:

strace lsmod 2>&1 >/dev/null | vim -

In the captured output, you will find lines like

Quote:

open("/proc/modules", O_RDONLY) = 3
fstat64(1, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 0
ioctl(1, SNDCTL_TMR_TIMEBASE or TCGETS, 0xbff5e5c4) = -1 ENOTTY (Inappropriate ioctl for device)
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7fff000
fstat64(3, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7ffe000
read(3, "radeon 125637 2 - Live 0xe0f2400"..., 1024) = 1024
read(3, "e 0xe0caf000\nsnd_pcm_oss 49017 0"..., 1024) = 1024
read(3, "ata_piix, Live 0xe0b81000\nsd_mod"..., 1024) = 108
read(3, "", 1024) = 0
close(3) = 0

So open() /proc/modules gives a file descriptor 3, so you just look for lines whose file descriptors are 3 and you can then trace the system calls involved. Here you see that apparently the lsmod program tries to read() from the open()ed file descriptor in 1024-byte chunks until EOF is reached, and finally close() it.

If you know of a program which reads /proc in a similar fashion you can try to strace it to find out what is involved.

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