Michele Mondelli wrote:
> A simple example of using pipe in Linux.
>
> /*
> * Run the 'ls' command and print the output to stdout.
> *
> * Compile with
> * gcc -Wall pipe.c -o pipe
> *
> *
> * Mithenks <mithenks@xxxxxxxxxxxxxx>
> *
> */
> #include <stdio.h>
> #include <stdlib.h>
>
> int main() {
>
> FILE *pipe;
> char buffer[1024];
>
> if ( (pipe = popen("ls","r") ) < 0 ) {
This should be:
if ( (pipe = popen("ls","r") ) == NULL ) {
> perror("popen(): ");
> exit(-1);
> }
>
>
Don't forget to clear the buffer, printf requires NULL terminated strings.
memset(buffer, 0, sizeof(buffer));
> fread(buffer,sizeof(buffer),1,pipe);
In a real implementation, you'll be looping on fread(), and you'll want
to check the return value, like so:
int rlen=0;
if ( (rlen = fread(buffer,sizeof(buffer),1,pipe) ) != 1 ) {
/*we have a short read or error*/
if( feof(pipe) != 0 ) {
printf("Finished\n");
exit(0);
}
else if ( ferror(pipe) != 0 ) {
printf("Error reading from pipe.\n");
exit(-1);
}
}
> printf("Output:\n%s\n",buffer);
>
> pclose(pipe);
>
> return 0;
> }
So, that got a little messy, I've attached a re-write. It's not the
most efficient thing in the world, but it should get you started.
When in doubt, 'man function_name'. For example, 'man popen' told me I
should be checking for a NULL return value, and 'man fread' told me I
should be checking the number of items returned, _not_ the number of
bytes. 'man brain' returned NULL for me, so any errors are solely my
responsibility. ;-)
hth,
Jason.
/*
* Run the 'ls' command and print the output to stdout.
*
* Compile with
* gcc -Wall pipe.c -o pipe
*
*
* Mithenks <mithenks@xxxxxxxxxxxxxx>
* Changes added by Jason <lcprog@xxxxxxxxxxxxxx>
*
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
FILE *pipe=NULL;
char buffer[1024];
int rlen=0;
int blen=0;
if ( (pipe = popen("ls","r") ) == NULL ) {
perror("popen(): ");
exit(-1);
}
/*we subtract 1 so there is always a NULL terminator*/
blen = sizeof(buffer) - 1;
while (1) {
memset(buffer,0,sizeof(buffer));
if ( (rlen = fread(buffer,blen,1,pipe) ) != 1 ) {
/*we have a short read or an error*/
if ( feof(pipe) != 0 ) {
fprintf(stderr,"Finished");
printf("%s",buffer);
break;
}
else if ( ferror(pipe) != 0 ) {
fprintf(stderr,"Error reading from pipe.\n");
break;
}
else {
/*short read*/
printf("%s",buffer);
}
}
else {
/*we got a full buffer*/
printf("%s",buffer);
}
}
if(pipe != NULL) {
pclose(pipe);
}
exit(0);
}
