CODEWITHSUNDEEP
c
nandan
nandan

Reputation: 133

system call not parsing output to a file in c, file created but no data inside the file

I have written a small code which takes input from the user and passes it to the system ("command>/tmp/j"), here the file j is being created but there is no information inside it related to the input present in the command. For eg: If I have given ps -f as input to the string command, the system() should execute it and store it inside file /tmp/j, but here the file j is being created but there is no output data inside it.

I have seen many questions, but all those are using popen() and the input is predefined in them.

#include<stdio.h>
#include<sys/syscall.h>
main()
{
  enum msgtype {PROCESS_LIST_REQUEST=1, PROCESS_LIST_RESPONSE, DIRECTORY_LIST_REQUEST, DIRECTORY_LIST_RESPONSE, ERROR_REQUEST};
  struct head 
  {
      int version;
      int   msg_length;
      int   header_length;
      enum msgtype msg_type;
      char data;
      char *reqtype;
  };

  struct head *buf;
  buf = malloc((sizeof(struct head)));
  buf->reqtype=malloc(40);
  char req[10];
  char *command;
  command = malloc((sizeof(char) * 10));
  fgets(req, sizeof(req),stdin);
  buf->reqtype = req;
  printf("%s" , buf->reqtype); //just to make sure correct thing is present
  command = buf->reqtype;
  printf("%s",command);//just to make sure correct thing is present
  system ("command>/tmp/j");
  {
      FILE *fp;
      char c;
      fp = fopen("/tmp/j", "r");
      if (fp == NULL)
           printf("File doesn't exist\n"); 
      else {
       do {
            c = getc(fp); 
            putchar(c); 
    } while (c != EOF); 
   }
  fclose(fp);
}
}

Upvotes: 0

Views: 63

Answers (2)

pm100
pm100

Reputation: 50120

As somebody else says

 system ("command>/tmp/j");

tries to run a program called 'command'. YOu need to do

char buff[256];
snprintf(buff, sizeof(buff), "%s>/tmp/j", command);
system(buff);

Upvotes: 0

Andrew Henle
Andrew Henle

Reputation: 1

This line

  system ("command>/tmp/j");

Tries to run an executable literally called command, and redirects its output to /tmp/j. The redirection happens, thus creating the file /tmp/j, but then command (whatever it may be) produces no output.

Also, this

  command = malloc((sizeof(char) * 10));

followed by this

  command = buf->reqtype;

causes the memory from the malloc() call to leak.

Upvotes: 1

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