Passing argument to system() in C

Question

I want to find the number of context switches of a program. I know there is a file /proc/PID/status which reports the number of context switches continuously while the program is running. However, after the process is finished that file is deleted, so I am not able to check the content.

For this reason, in the following C code, I am trying to copying that file before and after the region of interest.

int pid_num = getpid();
system("sudo cp /proc/$pid_num/status start.txt");
// do
system("sudo cp /proc/$pid_num/status finish.txt");

As I run the program, I get this message

 cp: cannot stat '/proc//status': No such file or directory

It seems that $pid_num in the system function is not correct. What is the correct form then?

Answer 1

C uses format specifiers equivalent to more generic $var shell counterpart. You need to prepare the command-string before invoking system() call.

#define START_FILE "start.txt"
#define END_FILE "finish.txt"
#define MAX_CMD_BUFFSIZE 256 // adjust as necessary

char cmdbuf[MAX_CMD_BUFFSIZE];

pid_t pid_num = getpid();
snprintf(cmdbuf, sizeof(cmdbuf), "sudo cp -f /proc/%d/status %s", pid_num, START_FILE);
int cmd_status = system(cmdbuf);
// verify 

snprintf(cmdbuf, sizeof(cmdbuf), "sudo cp -f /proc/%d/status %s", pid_num, END_FILE);
cmd_status = system(cmdbuf);
// verify

• use -f command option to replace target file if it already exists.

• Also, you need to run this program with an user with sudo privileges; which is inviting trouble. If process status file is available for every user for reading, check if you can drop sudo from command.