Calling system with a string variable

This is a discussion on Calling system with a string variable within the C Programming forums, part of the General Programming Boards category; Can a call to system be string variable or actual string in double quoted form? Code: sprintf(cmd, "cp %d.txt %d.csv",i,i); ...

  1. #1
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    Calling system with a string variable

    Can a call to system be string variable or actual string in double quoted form?
    Code:
        	sprintf(cmd, "cp %d.txt %d.csv",i,i);
    	pid_err=system(cmd); //Problem here: it's printing garbage on the screen

  2. #2
    Registered User ssharish2005's Avatar
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    Quote Originally Posted by 911help View Post
    Can a call to system be string variable or actual string in double quoted form?
    Code:
            sprintf(cmd, "cp %d.txt %d.csv",i,i);
        pid_err=system(cmd); //Problem here: it's printing garbage on the screen
    >Can a call to system be string variable or actual string in double quoted form?
    Either of them is acceptable. Fo instance

    Code:
    char str[] = "clear";
    system(str);
    or
    system("clear");
    >Problem here: it's printing garbage on the screen
    If the return value was other than 0 then the system function failed.

    ssharish
    Last edited by ssharish2005; 12-27-2007 at 08:44 AM.

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    I am using gdb to walk through the break points.

    printf("print before system(cmd) call\n");
    pid_err=system(cmd); //Problem here: it's printing garbage on the screen
    printf("print after system(cmd) call\n");

    Soon as I call system(cmd), garbage is printed out on the screen. why?

    sample output:
    ��ႇ
    .x��ႇ
    .x��ႇ
    .x��ႇ
    �<\�p��<\�p�

  4. #4
    Registered User ssharish2005's Avatar
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    Check the system() return value and see if your system call was successful.

    Code:
    if( system("clear") )
    {
        printf( " Error " );
        return 1;
    }
    else
       printf( " < Message > " );
    ssharish

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    I don't know what the problem here. I tried to gzip the file using system(). Text file is there, but never get gzipped. I get garbage printed out on the console soon as I call system();.

    Do I have to have root access on the linux shell to be able to call system? Does
    sprintf(cmd, "gzip -c ./data_id_%d.txt",i); put a '\0' in cmd, after placing the string "gzip -c ./data_id_%d.txt"?

    server.conf:
    Code:
    num_of_files=3;
    file_temp.c:
    Code:
    #include <string.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <fcntl.h>
    #include <unistd.h>
    
    #define SIZE 256
    
    FILE *configfile;
    int  num_files;
    char conf[100];
    char *buffer;
    
    char cmd[SIZE];
    
    int main (int argc, char *argv[])
    {
    int i=0;
    	configfile= fopen("./server.conf", "r");
    	fgets(conf,100, configfile);
    	fclose(configfile);
        buffer=strtok(conf,";");
        strtok(buffer,"=");
        num_files=atoi(strtok(NULL,"=")); 
        sprintf(cmd, "gzip -c ./data_id_%d.txt",i);
        printf("system(%s)\n", cmd);
        if( system(cmd) )
        {
        	printf( " Error \n" );
        	return 1;
        }
        else
        	printf( " < System call success > \n" );
        printf("After the system call\n");
        
        exit (0);
    }
    Here is my gdb output.
    Code:
    ****printted whole bunch of garbage here.********* and then stuff below*********
    System call has success > 
    
    Breakpoint 2, main () at file_temp.c:34
    34          printf("After the system call\n");
    (gdb) continue
    Continuing.
    After the system call
    
    Program exited normally.

  6. #6
    and the hat of int overfl Salem's Avatar
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    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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    Salem, I tried fork as you mentioned on Daniweb. It gave a problem. I wanted to try system call. This is also giving me problems.

    Can someone help with this? No one had done zipping stuff from system call?

    Anything else I need to do, i.e setuid?
    Last edited by 911help; 12-27-2007 at 01:07 PM. Reason: this is driving me crazy. ;(

  8. #8
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    Quote Originally Posted by 911help View Post
    ****printted whole bunch of garbage here.********* and then stuff below*********
    The "garbage" is undoubtedly the output of gzip -c. (With the -c flag, gzip sends output to stdout.)

    D
    Last edited by Dave Evans; 12-27-2007 at 05:01 PM.

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    million thanks for everyone.

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