UNIX for Dummies Questions & Answers

Guys, I have the following code

Code:

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>

void read2();
main(int argc, char** argv)
{
int pid,status;
    pid=fork();
    if ( pid == 0 )
    {
        read2(argv[1], argv[2]);
        exit(0);    
    }
    else
    wait(&status);
}

void read2(char * file, char * stringa)
{
    printf("%s, %s\n",file, stringa);
    execl("grep", "grep", "-c", stringa, file, (char *)0);
        
}

Why I don't see any outputs on the screen when I run my program ?
usying any .txt file I should get on stdout ( screen ) the number of lines that "stringa" shows up. But I don't.

What am I doing wrong ? A friend of mine tryied in his computer and it worked. Why it doesn't work in mine ?

Please post what Operating System and version you are running and what Operating System and version your friend is running.
Please post what commands were used to compile the program on each computer.

A few items that stand out:
* execl will not do path lookup for "grep".
* read2 prototype doesn't agree with its definition
* fork returns pid_t which you are assigning to an int.

Either your friend mistook printf output for success or they had grep in the current working directory.

Regards,
Alister

I'm using ubuntu 11.04 and compiling with: "gcc filename.c -o filename" or "gcc -g filename.c -o filename " when I have to debug using GDB.

Alister, can you explain me what you meant by
* execl will not do path lookup for "grep".
* read2 prototype doesn't agree with its definition
?

When I use perror I get an error "No such file or directory".
I don't understand why because when I launch the terminal and use "ls" to list all the files, of course I see the file I'm calling the program with. However even if I change the file name for another one or either the file source I still get the same error.

grep works like that, right ? GREP -C PATTERN FILE

so my command line is execl("usr/bin/grep", "grep", "-c", stringa, file, NULL); its supposed to execute GREP -C STRING FILE
but I still get the error.

does anyone can explain me why I'm so blind that I can't see where the problem is ?

When their first argument ("grep", for example) does not contain any slashes, execlp/execvp/execvpe (note the "p" in all of them) will search for the executable in directories listed in the PATH environment variable.

In your first post, you're using execl, which will not search PATH. So, if execl doesn't find "grep" in the current working directory, it fails.

In your most recent post, you're using a relative path, usr/bin/grep. You almost certainly intended to use an absolute path. Add a leading slash: /usr/bin/grep.

Regards,
Alister