Using popen("ls -la") produces strange result - c

Question

I wrote some C code for it to get the result of an "ls -la" command using popen and write the result into an C. The code looks like this:
unsigned int ls(char *destination, const char *username, const char *relative_path)
{
printf("LS IMP\n");
//if(!username || !relative_path) return -1;
FILE *ls_pipe = NULL;
unsigned long ls_pipe_size = -1;
const char ls_command[] = "ls -la ";
char ls_path[255] = "/home/";
char ls_full_command[255];
char buffer[255];
bzero(buffer, 255);
char *entries = NULL;
bzero(ls_full_command, 255);
strcat(ls_path, username);
strcat(ls_path, relative_path);
strcat(ls_full_command, ls_command);
strcat(ls_full_command, ls_path);
printf("AFTER CATS\n");
ls_pipe = popen(ls_full_command, "r");
if(ls_pipe == NULL) return -1;
printf("Pipe ok!");
fseek(ls_pipe, 0, SEEK_END);
ls_pipe_size = ftell(ls_pipe);
rewind(ls_pipe);
printf("Filesize: %lu\n", ls_pipe_size);
int i;
for(i = 0; i < 100; i++)
{
fread(buffer, 1, 255, ls_pipe);
printf("%s", buffer);
}
//entries = (char*) malloc(sizeof(char) * ls_pipe_size);
//if(entries == NULL) return -1;
printf("Entries ok!\n");
//if(ls_pipe_size != fread(destination, sizeof(char), ls_pipe_size, ls_pipe)) return -1;
fclose(ls_pipe);
return strlen(destination);
}
The problem is the size of the pipe is huge (?) and in the result after the proper result three entries start to appear non-stop for like infinity.
Is there any way of reading from it without knowing the exact number of lines of the result using something like another popen with wc -l?
Thanks
P.S there are some modifications in the code when i was trying to test what's going wrong and the malloc didn't work because of the insane size of the pipe.

Answer 1

You can't seek on a pipe — period. Any value you get back from ftell() is immaterial or erroneous. You can't rewind a pipe because you can't seek on a pipe. You can only read data once from a pipe.
So, you need to redesign the code to read an indefinite amount of data.
Here's some reasonably working code — but I needed to adapt it to Mac OS X and my machine, so instead of /home/ it uses /Users/, and the call to ls() uses my user name. The code properly handles buffers full of data that do not end with a null (listing about 570 lines of output for my bin directory). I've left the interface to ls unchanged although it almost doesn't use destination and returning the length of destination is otherwise unrelated to what it is doing. It also uses pclose() to close the pipe. Using pclose() avoids leaving zombies around and returns the exit status of the executed program where fclose() will not.
#include <assert.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
static unsigned int ls(char *destination, const char *username, const char *relative_path)
{
printf("LS IMP\n");
assert(destination != 0 && username != 0 && relative_path != 0);
const char ls_command[] = "ls -la ";
char ls_path[255] = "/Users/";
char ls_full_command[255];
snprintf(ls_full_command, sizeof(ls_full_command), "%s %s%s/%s",
ls_command, ls_path, username, relative_path);
FILE *ls_pipe = popen(ls_full_command, "r");
if (ls_pipe == NULL)
return -1;
printf("Pipe ok!\n");
char buffer[255];
int nbytes;
while ((nbytes = fread(buffer, 1, 255, ls_pipe)) > 0)
printf("%.*s", nbytes, buffer);
putchar('\n');
printf("Entries ok!\n");
pclose(ls_pipe);
return strlen(destination);
}
int main(void)
{
unsigned int length = ls("/", "jleffler", "bin");
printf("ls() returned %u\n", length);
return(0);
}