Jan Kończak

In order to run an executable file, an existing process has to 'exec' into it – that is, the process has to ask the kernel to replace its memory with the code (and data) of the executable file.
So, typically to start a new process, one has to fork and then exec… in the child:

prog1              ,------.
-------------------| fork |--------------------------------------
pid: x (ppid: y)   `------\
                           \ prog1              ,-------. prog2
                            `-------------------| exec… |--------
                             pid: z (ppid: x)   `-------'

To this end, a family of functions starting with exec is provided:
Needs header:
unistd.h int execlp(const char *file, const char *arg0, ... /*, (char *)0 */)
int execl (const char *path, const char *arg0, ... /*, (char *)0 */)
int execle(const char *path, const char *arg0, ... /*, (char *)0,*/ char *const envp[])
int execvp(const char *file, char *const argv[])
int execv (const char *path, char *const argv[])
int execve(const char *path, char *const argv[], char *const envp[])
(Those functions are documented more cleanly in the Linux manual).

After a successful execution of the exec… function the next instructions of the process are those of the new executable file. Hence, it is pointless to check the return value of exec… – it may only return -1, and if the instructions following exec… do execute, then exec… must have failed.

Importantly, upon executing exec… the list of open files is retained.
Almost all resources are released. See the documentation for other exceptions.

execl… vs execv…
The former takes a list of arguments (terminated by a NULL sentinel), the latter takes a pointer to the argument vector (the last element must be a NULL sentinel as well).

char arg0[] = "ls";     char arg2[] = "-a";
char arg1[] = "-l";     char arg3[] = "/tmp";
# Argument list:
execlp("ls",    arg0, arg1, arg2, arg3, NULL);
# Argument vector:
char *argv[] = {arg0, arg1, arg2, arg3, NULL};
execvp("ls", argv);

Notice that the 0th argument is the program name.

exec…p vs exec… (without p):
The latter requires a path to the executable, and the former searches for the executable within directories specified by the PATH environmental variable if a name rather than a path was provided (ls is a name, /bin/ls is a path, ./ls is a path).

# succeeds iff there is an executable file named 'ls' in the current working directory
execl ("ls", "ls", "-la", "/tmp",  NULL);
# succeeds iff there is an executable file named 'ls' in one of the directories in the PATH list
execlp("ls", "ls", "-la", "/tmp",  NULL);
# succeeds iff '/bin/ls' is an executable file
execl ("/bin/ls", "ls", "-la", "/tmp",  NULL);
# as above - searching the PATH is abandoned if the argument is a path
execlp("/bin/ls", "ls", "-la", "/tmp",  NULL);

UNIX-like and/or POSIX-compliant operating systems use environment variables. By default the values of all such variables are inherited upon exec….
exec…e functions have an extra argument that should point to an array of the environment variables, allowing thereby to override them for newly exec'ed process.
To access an unprocessed array of environment variables for the current process, one must have in the source code the following lines:

#include <unistd.h>
extern char **environ;

The environ external variable and the envp argument of exec…e functions use a NULL sentinel.
(Normally, to access such variables of the running process one shall use the getenv and setenv functions.
The environ variable is useful when one wants to pass a slightly modified set of environment variables.)

Exercise 1 Write a program that executes ps with the argument -F. (Remember that the arguments include the command name.)

Exercise 2 Write a program prog that, when executed as prog [arg]..., executes the ls -l -t -r [arg]... command.

Exercise 3 Write a program prog that, when executed as prog cmd [arg]..., executes and measures the runtime (as wall clock time) of cmd [arg]....
To measure time, you can use the following C11 code:

#include <time.h>
...
    struct timespec start, end;
    timespec_get(&start, TIME_UTC);
    ...
    timespec_get(&end, TIME_UTC);
    double elapsedSec = (end.tv_sec - start.tv_sec) + (end.tv_nsec - start.tv_nsec) / 1e9;

Exercise 4 Modify the code of the previous exercise:
    • close the standard input and output of the parent process right after fork
    • write output from parent process to the standard error
    • output whether the child process executed normally and output its return value

POSIX defines the following functions:
Needs header:
unistd.h int dup(int fildes)
int dup2(int fildes, int target)
to duplicate file descriptors.

Duplicating a file descriptor is something different than opening the same file twice. When opening the same file twice, one can choose different set of flags (such as O_RDONLY, O_RDWR, O_APPEND), and the descriptors have a different position in file (byte that will be read/written upon next read/write). Duplicated file descriptors refer to the same state of the file (flags, position etc.).

The dup2 function atomically closes the descriptor target, and then duplicates the descriptor fildes to the descriptor target.
This is commonly used to replace standard streams, as in the following example (that shows redirection of the standard output):

Each file descriptor has to be closed separately.

Exercise 5 Write a program that executes ps with the argument -F and writes its output to a file output.

Exercise 6 Write a program that, when executed as prog fname [arg]..., will act as tr [arg]... < fname.

Exercise 7 Write a program that:
    • assigns ELOOP to errno
    • executes perror function so that the error is output on the terminal
    • executes perror function so that the error is written to a file
    • assigns EMFILE to errno
    • executes perror function so that the error is output on the terminal
    • executes perror function so that the error is written to a file
Notice that perror always writes to standard error, so to make it write to a file, one has to replace the file descriptor 2.