A Data Scientist's blog

08:29 AM

I'm new to C and Linux and I am confused on how things work. I tried to search but I don't really know if I'm searching for the right thing because lots of other things come up.

Currently, I figured out that I can use shell commands inside C by using the method system(). They display out on the screen fine, but my problem is that I need to assign them to a variable, and not have them display on the screen (yet).

I'm planning to put the output of "ls -l" to a variable. I'm thinking of an array of strings (I think there are no strings, but an array char, so I guess this is an array of array of chars?) which will contain every line of the "ls -l" command except for the first line. (I think I need it in an array because I'm supposed to scroll through it in a menu-like style where I'm following example 18 on this link:
http://www.tldp.org/HOWTO/NCURSES-Pr...tml#MENUBASICS
but I think it's another question for another time, hopefully I can figure it out and not have to though, hahah)

What I have is this, but it doesn't work.

Code:

#include 
int main() {
   char output[] = system( "ls -l" );
   printf( "%s", output);
   return 0;
}

which gives this error.

Code:

system.c: In function 'main':
system.c:5: error: invalid initializer

I believe it is wrong, but I'm completely clueless. Perhaps you can help, thanks.

Hivemind · 01-28-2005, 08:27 AM

Two things. If a function returns a pointer-to-char (dynamically allocated or static) and you want to assign the return value to a variable you do:

Code:

char *ptr = some_func_returning_pointer_to_char();

Second, system() doesn't return char* it returns int. Do a:

Code:

$ man 3 system

for more details on the return value.

The system() call is part of standard C (and C++ of course) but how it operates and what you can do with the processes it spawns is platform-dependent.

deiussum · 01-28-2005, 09:29 AM

man popen

I think this does what you are looking for.

jim mcnamara · 01-28-2005, 02:39 PM

popen example:

Code:

#include                
#include 
#include 
#include 

void process(const char *filespec)
{
	char cmd[296]={0x0}; /* 40 + 256 */  
	char tmp[256]={0x0};
	long total=0;
	int retval=0;
	FILE *in=NULL;
	
    snprintf(cmd,sizeof(cmd)-40,"ll %s | /usr/bin/cut -c 35-45 2>/dev/null",filespec);
    in=popen(cmd, "r");
    if(in==NULL)
    {
         perror("Shell execution error");
         exit(EXIT_FAILURE);
    }           
    while(fgets(tmp,4095,in)!=NULL) 
    {
         total+=atol(tmp);
    }        
    if(!feof(in))
    {
        perror("Input stream error");
        exit(EXIT_FAILURE);
    }    
    retval=pclose(in);
    if(retval==EOF || retval==127)    
    {
        perror("Shell invocation error");
        exit(EXIT_FAILURE); 
    }    
    fprintf(stdout,"%-64s total: %d\n",filespec,total);
}

int main(int argc, char *argv[])
{
    if(argc<2)
    {
    	char filespec[256]={0x0};
    	while(fgets(filespec,sizeof(filespec),stdin)!=NULL)
    	{
    	    char *p=memchr(filespec,'\n', sizeof(filespec));
    		if(p!=NULL) *p=0x0;
    	    process(filespec);
    	}      	  
    }	
    else
    {      	
        int i=1;
        while(i

Miaire · 01-30-2005, 01:40 PM

Thanks for the answers, I have gotten what I needed. Figured out char arrays (strings) and casting char to int in the process.

This is what I came up with:

Code:

#include 
#include 

int main()
{
   int size=0;
   FILE *in=NULL;
   char temp[256];
   
   in=popen("ls -a -l | wc -l", "r");
   fgets(temp, 255, in);
   size = atoi(temp) - 1;

   char list[size][256];

   in=popen("ls -a -l", "r"); //for reading
   fgets(list[0], 255, in); // discard total: xxx, first line
  
   int i;
   for( i = 0; i < size; i++) {
      fgets(list[i], 255, in);   
   }

   for(i=0; i < size; i++) {
      printf("%s", list[i]);
   }
   return 0;
}

It does what I expect it to do (though it probably have hidden problems somewhere like memory things I don't really understand yet. Any comments on it?) , now to format and continue on with ncurses. Thanks for help!

Field	Description
int fildes	The file descriptor of where to write the output. You can either use a file descriptor obtained from the open system call, or you can use 0, 1, or 2, to refer to standard input, standard output, or standard error, respectively.
const void *buf	A null terminated character string of the content to write.
size_t nbytes	The number of bytes to write. If smaller than the provided buffer, the output is truncated.
return value	Returns the number of bytes that were written. If value is negative, then the system call returned an error.

01-28-2005, 08:27 AM	#2
Hivemind Member Registered: Sep 2004 Posts: 273 Rep:	Two things. If a function returns a pointer-to-char (dynamically allocated or static) and you want to assign the return value to a variable you do: Code: char ptr = some_func_returning_pointer_to_char(); Second, system() doesn't return char it returns int. Do a: Code: $ man 3 system for more details on the return value. The system() call is part of standard C (and C++ of course) but how it operates and what you can do with the processes it spawns is platform-dependent.

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01-28-2005, 09:29 AM	#3
deiussum Member Registered: Aug 2003 Location: Santa Clara, CA Distribution: Slackware Posts: 895 Rep:	man popen I think this does what you are looking for.