Working with the Unix OS

Processes I
Executed in child process to overlay itself with a specified binary program file.
So:
Cost of fork:
Exec:
produces a child process different from parent.
copies all instructions and data of parent, only to be overlaid by exec. Some VM versions
of UNIX use "copy-on-write" with parent and child processes sharing pages till
overwritten (e.g. by exec) does NOT change semantics of fork.
all executions on UNIX (apart from booting) are achieved by exec.
Parent and child
- are clones (except for pid)
- share wd (working directory) (and 1 or 2 other things)
- share open files
The child typically does: exec
e.g.
if ((child-id = fork()) ! = 0)
{ /*parent * /
/* assume> 0 */
foo = wait(&status); /* and returns status */
}
else
{ /* child: execute 'ls' */
execl("/bin/ls", "ls", "-l", NULL);
exit(1); /* could not exec */
}
int execl(path, arg0, arg1, ..., argn, NULL);
| | |
binary prog file name local utility child may process these via argc, argv of its “main”
/* argv[argc] may not necessarily = NULL;
use argc to count arg's rather than look for NULL */
/* environment pointed to by environ is also accessible by child */
The exec family:
Argument Environment Path
Format Passing Search?
execl list auto no
execv array* auto no
execle list manual# no
execve array* manual no
execlp list auto yes+
execvp array* auto yes
* if no. of arg is unknown at compile time (c.f. "argv")
# manually passing an environment pointer instead of automatically using environ
+ e.g., /bin:/usr/bin:/usr/me/bin::
execv
will execute the file if it is a binary OR a shell command file.
- testenv program
main(argc,argv,envp)
int argc; char * argv[]; char * envp[];
{
int cntr;
printf("%d\n",argc);
for (cntr = 0; cntr < argc; cntr++)
{
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