c++ linux system command

I have the following problem:

I use in my program this function:

  system("echo -n 60  > /file.txt"); 

it works 开发者_如何转开发fine.

But I don't want to have constant value. I do so:

   curr_val=60;
   char curr_val_str[4];
   sprintf(curr_val_str,"%d",curr_val);
   system("echo -n  curr_val_str > /file.txt");

I check my string:

   printf("\n%s\n",curr_val_str);

Yes,it is right. but system in this case doesn't work and doesn't return -1. I just print string!

How can I transfer variable like integer that will be printed in file like integer, but don't string?

So I want to have variable int a and I want to print value of a with system function in file. A real path to my file.txt is /proc/acpi/video/NVID/LCD/brightness. I can't write with fprintf. I don't know why.

you cannot concatenate strings like you are trying to do. Try this:

curr_val=60;
char command[256];
snprintf(command, 256, "echo -n %d > /file.txt", curr_val);
system(command);

The system function takes a string. In your case it's using the text *curr_val_str* rather than the contents of that variable. Rather than using sprintf to just generate the number, use it to generate the entire system command that you require, i.e.

sprintf(command, "echo -n %d > /file.txt", curr_val);

first ensuring that command is large enough.

The command that is actually (erroneously) executed in your case is:

 "echo -n curr_val_str  > /file.txt"

Instead, you should do:

char full_command[256];
sprintf(full_command,"echo -n  %d  > /file.txt",curr_val);
system(full_command);

#define MAX_CALL_SIZE 256
char system_call[MAX_CALL_SIZE];
snprintf( system_call, MAX_CALL_SIZE, "echo -n %d > /file.txt", curr_val );
system( system_call );

man snprintf

The correct way would be similar to this:

curr_val=60;
char curr_val_str[256];
sprintf(curr_val_str,"echo -n  %d> /file.txt",curr_val);
system(curr_val_str);

Just DON'T. :)

Why resort to system() for such a simple operation?

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>

int write_n(int n, char * fname) {

    char n_str[16];
    sprintf(n_str, "%d", n);

    int fd;
    fd = open(fname, O_RDWR | O_CREAT);

    if (-1 == fd)
        return -1; //perror(), etc etc

    write(fd, n_str, strlen(n_str)); // pls check return value and do err checking
    close(fd);

}

Have you considered using C++'s iostreams facility instead of shelling out to echo? For example (not compiled):

std::ostream str("/file.txt");
str << curr_val << std::flush;

Alternately, the command you pass to system must be fully formatted. Something like this:

curr_val=60;
std::ostringstream curr_val_str;
curr_val_str << "echo -n " << curr_val << " /file.txt";
system(curr_val_str.str().c_str());

Use snprintf to avoid security issues.

What about using std::string & std::to_string...

std::string cmd("echo -n " + std::to_string(curr_val) + " > /file.txt");
std::system(cmd.data());