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01-28-2005, 08:21 AM
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#1
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LQ Newbie
Registered: Jan 2005
Location: Atlanta, GA
Distribution: Fedora Core 3
Posts: 11
Rep:
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in C, Assigning output of system() to a variable
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 <stdlib.h>
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.
Last edited by Miaire; 01-28-2005 at 08:29 AM.
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01-28-2005, 08:27 AM
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#2
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Member
Registered: Sep 2004
Posts: 273
Rep:
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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:
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
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#3
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Member
Registered: Aug 2003
Location: Santa Clara, CA
Distribution: Slackware
Posts: 895
Rep:
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man popen
I think this does what you are looking for.
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01-28-2005, 02:39 PM
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#4
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Member
Registered: May 2002
Posts: 964
Rep:
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popen example:
Code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
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<argc)
{
process(argv[i]);
i++;
}
}
return 0;
}
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01-30-2005, 01:40 PM
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#5
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LQ Newbie
Registered: Jan 2005
Location: Atlanta, GA
Distribution: Fedora Core 3
Posts: 11
Original Poster
Rep:
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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 <ncurses.h>
#include <stdio.h>
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!
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