Slim Hmidi Slim Hmidi - 1 month ago 9
C Question

Empty file when using sprintf and system function on C

I want to save some information in a file text, I wrote this program:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>


int main(int argc,char *argv[])
{
FILE *fichier;
char buffer[20];
char command[200];
char command1[100];


system(" cat /etc/resolv.conf | grep nameserver | awk -F' ' '{print $2}' | cut -d'.' -f1-3 | awk '{print $1\".1\"}' > ethernet_dns.txt");

fichier=fopen("ethernet_dns.txt","r");
memset(&buffer,0,sizeof(buffer));
fread(buffer,20,1,fichier);
printf("buffer is: %s",buffer);

snprintf(command,sizeof(command),"ping -c 1 -W 1 %s > /tmp/ping_result",buffer);
printf("command is: %s",command);

system(command);


return 0;
}


Results:

buffer is: 127.0.1.1
command is : ping -c 1 -W 1 127.0.1.1


the system command returns this:

PING 127.0.1.1 (127.0.1.1) 56(84) bytes of data.
64 bytes from 127.0.1.1: icmp_seq=1 ttl=64 time=0.115 ms

--- 127.0.1.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.115/0.115/0.115/0.000 ms


But when I run : cat /tmp/ping_result.I got an empty file

Answer

Problem: Code is reading '\n' into buffer[] yet trying to that as part of the command. Need to trim the buffer. *** below

 // Insure file is open
 fichier=fopen("ethernet_dns.txt","r");
 assert(fichier);

 // Use fgets
 //memset(&buffer,0,sizeof(buffer));
 //fread(buffer,20,1,fichier);
 if (fgets(buffer, sizeof buffer, fichier)) {

   // lop off potential \n
   buffer[strcspn(buffer, "\n")] = '\0';  // ***

   printf("buffer is: <%s>\n",buffer);

   int n = snprintf(command, sizeof(command), "ping -c 1 -W 1  %s > /tmp/ping_result", 
     buffer);
   printf("command is: <%s>\n",command);

   // Only issue command if no problems occurred in snprintf()
   if (n > 0 && n < sizeof(command)) system(command);