parsing ssid with iwconfig in c - c

I am about building a bar for DWM (ubuntu linux), showing wifi details such as the ssid.
Thats my code:
#include <stdio.h>
#include <stdlib.h>
int main( int argc, char *argv[] )
{
FILE *fp;
int status;
char path[1035];
/* Open the command for reading. */
fp = popen("iwconfig", "r");
if (fp == NULL) {
printf("Failed to run command\n" );
exit;
}
char s[500];
/* Read the output a line at a time - output it. */
while (fgets(path, sizeof(path)-1, fp) != NULL) {
sprintf(s,"%s%s",s, path);
}
//printf("%s",s);
/* close */
pclose(fp);
char delimiter[1] = "s";
char *ptr;
ptr = strtok(s, delimiter);
printf("SSID: %s\n", ptr);
return 0;
}
i am getting overflowerrors and dont know what to do.
I dont think, thats a good way to get the ssid either... :/
Suggestions?

I would rather use direct information from the kernel (such as netdevice(7)) rather than calling a sub-process.
Maybe this header can help: http://lxr.free-electrons.com/source/include/linux/wireless.h
Edit: if you still want to use popen, which don't you just add a | grep Essid: ?
$ /sbin/ifconfig 2>/dev/null | grep ESSID | cut -d: -f2
"pink-panter"

Related

Redirect output of command line to a variable in C

Is there a way to redirect output of a command line which returns integer as an output to a variable in C?
for example, if the command is "cmd", then is there a way to redirect its output (an integer) and store it in variable in C?
I tried using popen and fgets but it seems to be working only with characters. Any suggestions?
It works perfectly fine with popen and fgets:
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
const char *cmd = argc > 1 ? argv[1] : "echo 42";
char buf[32];
FILE *fp = popen(cmd, "r");
if( fp == NULL ){
perror("popen");
return 1;
}
if( fgets(buf, sizeof buf, fp) == buf ){
int v = strtol(buf, NULL, 10);
printf("read: %d\n", v);
}
return 0;
}
If you want to convert a character string from the standard input, you could use fgets and then use atoi to convert the input to an integer.
If you want to convert the output of a command, let's say ls and store the output of the command to a variable, you could learn about fork, dup2, pipe, and exec function family.
More about this topic on this tutorial : Capture the output of a child in C. This tutorial also provide an example with popen if you want to keep things "high level".
Here is an even simpler example using popen() and fscanf():
#include <errno.h>
#include <stdio.h>
#include <string.h>
int main(int argc, char *argv[]) {
FILE *fp = popen("date '+%s'", "r");
long seconds;
if (fp == NULL) {
fprintf(stderr, "popen failed: %s\n", strerror(errno));
return 1;
}
if (fscanf(fp, "%ld", &seconds) == 1) {
printf("epoch seconds: %ld\n", seconds);
pclose(fp);
return 0;
} else {
fprintf(stderr, "invalid program output\n");
pclose(fp);
return 1;
}
}

Using popen to read and write in C using two executables

I understand that popen doesn't allow simultaneous read and write.
To get around this, I created two files, 1.c for writing, and 2.c for reading. The files are included below.
When I run 1.out, I get the expected output on stdout:
bodhi#bodhipc:~/Downloads$ ./1.out
Stockfish 11 64 BMI2 by T. Romstad, M. Costalba, J. Kiiski, G. Linscott
bodhi#bodhipc:~/Downloads$
However, 2.out doesn't give any output on stdout:
bodhi#bodhipc:~/Downloads$ ./2.out
bodhi#bodhipc:~/Downloads$
Where am I going wrong?
1.c:
#include <stdio.h>
#include <stdlib.h>
int main( int argc, char *argv[] )
{
FILE *fp;
char path[1035];
/* Open the command for writing. */
fp = popen("./stockfish", "w");
if (fp == NULL) {
printf("Failed to run command\n" );
exit(1);
}
fprintf(fp,"uci\n");
/* close */
pclose(fp);
return 0;
}
2.c:
#include <stdio.h>
#include <stdlib.h>
int main( int argc, char *argv[] )
{
FILE *fp;
char path[1035];
/* Open the command for reading. */
fp = popen("./1.out", "r");
if (fp == NULL) {
printf("Failed to run command\n" );
exit(1);
}
/* Read the output a line at a time - output it.*/
while (fgets(path, sizeof(path), stdout) != NULL) {
printf("%s", path);
printf("Done!\n");
}
/* close */
pclose(fp);
return 0;
}
while (fgets(path, sizeof(path), stdout) != NULL) {
you don't want to read from stdout, instead:
while (fgets(path, sizeof(path), fp) != NULL) {

How to get device name on which a file is located from its path in c?

Let's say I have a file in Linux with this path:
/path/to/file/test.mp3
I want to know the path to its device. For example I want to get something like:
/dev/sdb1
How do I do this with the C programming language?
I know the terminal command to do it, but I need C functions that will do the job.
EDIT:
I have read this question before asking mine. It doesn't concretly mention code in C, it's more related to bash than to the C language.
Thanks.
You need to use stat on the file path, and get the device ID st_dev and match that to a device in /proc/partitions
Read this for how to interpret st_dev: https://web.archive.org/web/20171013194110/http://www.makelinux.net:80/ldd3/chp-3-sect-2
I just needed that inside a program I am writing...
So instead of running "df" and parsing the output, I wrote it from scratch.
Feel free to contribute!
To answer the question:
You first find the device inode using stat() then iterate and parse /proc/self/mountinfo to find the inode and get the device name.
/*
Get physical device from file or directory name.
By Zibri <zibri AT zibri DOT org>
https://github.com/Zibri/get_device
*/
#include <stdio.h>
#include <unistd.h>
#include <stdint.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <stdlib.h>
#include <libgen.h>
int get_device(char *name)
{
struct stat fs;
if (stat(name, &fs) < 0) {
fprintf(stderr, "%s: No such file or directory\n", name);
return -1;
}
FILE *f;
char sline[256];
char minmaj[128];
sprintf(minmaj, "%d:%d ", (int) fs.st_dev >> 8, (int) fs.st_dev & 0xff);
f = fopen("/proc/self/mountinfo", "r");
if (f == NULL) {
fprintf(stderr, "Failed to open /proc/self/mountinfo\n");
exit(-1);
}
while (fgets(sline, 256, f)) {
char *token;
char *where;
token = strtok(sline, "-");
where = strstr(token, minmaj);
if (where) {
token = strtok(NULL, " -:");
token = strtok(NULL, " -:");
printf("%s\n", token);
break;
}
}
fclose(f);
return -1;
}
int main(int argc, char **argv)
{
if (argc != 2) {
fprintf(stderr, "Usage:\n%s FILE OR DIRECTORY...\n", basename(argv[0]));
return -1;
}
get_device(argv[1]);
return 0;
}
output is just the device name.
Example:
$ gcc -O3 getdevice.c -o gd -Wall
$ ./gd .
/dev/sda4
$ ./gd /mnt/C
/dev/sda3
$ ./gd /mnt/D
/dev/sdb1
$
Use this command to print the partition path:
df -P <pathname> | awk 'END{print $1}'

Can't understand why this causes an error

I have a large directory of music which is listed in a file called op. I have been able to build a command which will randomly pick a song from the op file using some creative math with the nanosecond output from the date command. It works fine from the command line:
sed -n $((10#$(date +%N)%$(wc -l /shared/southpark/music/op|cut -d ' ' -f 1)))p /shared/southpark/music/op
I want to include this command in a c program and read the line in with popen.
#include <stdio.h>
#include <string.h>
int main (int argc, char *argv[])
{
char command[201];
char buf[501];
FILE *fp;
strcpy(command, "sed -n $((10#$(date +%N)%$(wc -l /shared/southpark/music/op|cut -d ' ' -f 1)))p /shared/southpark/music/op");
if((fp = popen(command, "r")) == NULL)
{
fprintf(stderr, "music_player: popen failed\n");
return(1);
}
if(fgets(buf, sizeof(buf), fp) == NULL)
{
fprintf(stderr, "music_player: fgets failed\n");
return(1);
}
printf("%s\n", buf);
pclose(fp);
return(0);
}
But when I run it, I get the following error:
sh: 1: arithmetic expression: expecting EOF: "10#271445839%2278"
music_player: fgets failed
How can I do this? I'm not understanding the error message.
popen executes your command using
/bin/sh -c "command"
and your sh doesn't understand the 10# base-conversion prefix. You've been running the command in bash previously.
To fix, you have two options:
Discard the unnecessary 10# prefix (it is the default) for sh compatibility
Use bash:
popen("bash -c 'command'", ...)
After trying and failing with both of nneonneo's options, I had to resort to placing the command in a bash script file and then I popen'ed the script. It gives me the desired results.
#include <stdio.h>
#include <string.h>
int main (int argc, char *argv[])
{
char command[201];
char buf[501];
FILE *fp;
strcpy(command, "/dea/testing/popen/get_file");
if((fp = popen(command, "r")) == NULL)
{
fprintf(stderr, "music_player: popen failed\n");
return(1);
}
if(fgets(buf, sizeof(buf), fp) == NULL)
{
fprintf(stderr, "music_player: fgets failed\n");
return(1);
}
printf("%s", buf);
pclose(fp);
return(0);
}

C: Run a System Command and Get Output? [duplicate]

This question already has answers here:
How can I run an external program from C and parse its output?
(8 answers)
Closed last month.
I want to run a command in linux and get the text returned of what it outputs, but I do not want this text printed to screen. Is there a more elegant way than making a temporary file?
You want the "popen" function. Here's an example of running the command "ls /etc" and outputing to the console.
#include <stdio.h>
#include <stdlib.h>
int main( int argc, char *argv[] )
{
FILE *fp;
char path[1035];
/* Open the command for reading. */
fp = popen("/bin/ls /etc/", "r");
if (fp == NULL) {
printf("Failed to run command\n" );
exit(1);
}
/* Read the output a line at a time - output it. */
while (fgets(path, sizeof(path), fp) != NULL) {
printf("%s", path);
}
/* close */
pclose(fp);
return 0;
}
You need some sort of Inter Process Communication. Use a pipe or a shared buffer.

Resources