在C中调用execvp后如何将错误消息传递到文件

Question

When I invoke execvp with a command and its arguments, sometimes the command is not legitimate.

For example, if I do this in my shell (bash shell) using my forked SON process:

$ ls ffdfdfd

then the output is:

$ ls: cannot access ffdfdfd: No such file or directory

Now, I want to pass that exact message to a file. I've tried with perror , this way:

void directErrors(char * arg)
{
    perror(arg);  // execute the problem to screen

    // now execute the problem to file

    FILE* myFile = fopen("errors.log", "a");

     if(myFile == NULL)
     {
         perror("fopen");
         exit(-1);
     }

     fprintf(myFile, "%s: %s\n", arg, strerror(errno));
     fclose(myFile);
}

but all it does is write that command X failed.

How can I direct the exact output that execvp got after its invocation?

In my code I invoke execvp like this:

executeCurrentCommand = execvp(*(arg)[0], *arg);

Answer 1

Remember that exec works by completely replacing your program with the program you are exec'ing. So, no code you write after an exec() call will ever run:

execvp(cmd, args);
/* This code is never reached.  */

The only way exec calls ever return is if the command you are trying to invoke cannot be run: for example if you try to exec a non-existent file or a file without execute privileges.

In your case, the command ( ls ) does exist and so the exec does succeed... which means that no code after it will ever be run. The ls command will generate the error message to stderr just as it always does.

If you want to capture the output of ls you'll have to be more fancy. For example you can open a file for append to the log file, then use dup() to duplicate that as the stderr file descriptor, then do your exec call. The new process will inherit that setting and all output that ls sends to stderr will get appended to the log. If you want to capture both stdout and stderr, dup them both. That's how the shell, etc. does it.

BTW, using *(arg)[0] looks very bizarre to me: why do you use that? More reasonable would be (*arg)[0] (I would think).