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System Calls using C w/BASH

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# 1  
Old 03-23-2012
System Calls using C w/BASH
Hello all,

I'm attempting to write a basic application that appends an arbitrary list of txt files to one txt file.

So, for example, if the application is run like so:

appendFileToFile file1.txt file2.txt file3.txt the system command should, hopefully be, cat [theFiles] >> testfile.txt

I'm using C, then making a call to the system in BASH. However, I keep getting segmentation faults.

My code below:

Code:
#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[])
{
    
    int i;
    char theCommand[] = "cat ";
  
    for (i = 1; i < argc; i++)
    {
        strcat(theCommand, argv[i]);
    }
    strcat(theCommand, " >> testfile.txt");
    system(theCommand);

    return 0;
}

Thanks for you time.
# 2  
Old 03-23-2012
Your buffer only has five bytes of space.

Writing beyond that is rampaging through stack space, overwriting every local variable behind it until it finds and corrupts your stack frame itself, crashing your program the next time it tries to do anything to the stack.

Code:
#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[])
{
    
    int i;
    char theCommand[16384] = "cat ";
  
    for (i = 1; i < argc; i++)
    {
        strcat(theCommand, argv[i]);
    }
    strcat(theCommand, " >> testfile.txt");
    system(theCommand);

    return 0;
}

This User Gave Thanks to Corona688 For This Post:
whyte_rhyno
# 3  
Old 03-23-2012
Quote:
Originally Posted by Corona688
Your buffer only has five bytes of space.

Writing beyond that is rampaging through stack space, overwriting every local variable behind it until it finds and corrupts your stack frame itself, crashing your program the next time it tries to do anything to the stack.

Code:
#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[])
{
    
    int i;
    char theCommand[16384] = "cat ";
  
    for (i = 1; i < argc; i++)
    {
        strcat(theCommand, argv[i]);
    }
    strcat(theCommand, " >> testfile.txt");
    system(theCommand);

    return 0;
}


D'oh! That would explain the segmentation fault, then.

Thank you very much for your help!

I had to put some white space in between the files because it was concatenating them together like: file.txtfile2.txtfile3.txt, is this the acceptable way of doing it (below)?

Code:
#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[])
{
    
    int i;
    char theCommand[16384] = "cat ";
  
    for (i = 1; i < argc; i++)
    {
        strcat(theCommand, argv[i]);
        strcat(theCommand, " ");
    }
    strcat(theCommand, " >> testfile.txt");
    system(theCommand);

    return 0;
 }

# 4  
Old 03-23-2012
It should work fine.

Try putting it before the command instead of after, though. Then you can just make your command "cat" instead of "cat ".
# 5  
Old 03-23-2012
I would suggest you prefer strncat to strcat. strncat is the secure sister of strcat:

Quote:
char * strncat ( char * destination, char * source, size_t num ); Append characters from string
Appends the first num characters of source to destination, plus a terminating null-character. If the length of the C string in source is less than num, only the content up to the terminating null-character is copied.
strncat - C++ Reference

usage of strncat is the same as strcat. The only difference is that strncat has an aditional third argument, that you use to tell the function the maximum amount of characters that should be copied to the destination buffer. This is very important to prevent your programs being vulnerable to buffer overflow errors.

Code:
for (i = 1; i < argc; i++)
    {
        strncat(theCommand, argv[i], sizeof(theCommand) - 1);
    }

Last edited by Corona688; 03-23-2012 at 03:44 PM.. Reason: horizontally stretching the window
These 2 Users Gave Thanks to pflynn For This Post:
Corona688 whyte_rhyno
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