exec, execl, _clock - execute a file
int exec(char *name, char* argv)
int execl(char *name, ...)
Exec and execl overlay the calling process with the named file, then transfer to the entry
point of the image of the file.
Name points to the name of the file to be executed; it must not be a directory, and the
permissions must allow the current user to execute it (see stat(2)). It should also be a
valid binary image, as defined in the a.out(6) for the current machine architecture, or a
shell script (see rc(1)). The first line of a shell script must begin with followed by
the name of the program to interpret the file and any initial arguments to that program,
ls | mc
When a C program is executed, it is called as follows:
void main(int argc, char *argv)
Argv is a copy of the array of argument pointers passed to exec; that array must end in a
null pointer, and argc is the number of elements before the null pointer. By convention,
the first argument should be the name of the program to be executed. Execl is like exec
except that argv will be an array of the parameters that follow name in the call. The
last argument to execl must be a null pointer.
For a file beginning #!, the arguments passed to the program (/bin/rc in the example
above) will be the name of the file being executed, any arguments on the #! line, the
name of the file again, and finally the second and subsequent arguments given to the orig-
inal exec call. The result honors the two conventions of a program accepting as argument
a file to be interpreted and argv naming the file being executed.
Most attributes of the calling process are carried into the result; in particular, files
remain open across exec (except those opened with OCEXEC OR'd into the open mode; see
open(2)); and the working directory and environment (see env(3)) remain the same. How-
ever, a newly exec'ed process has no notification handler (see notify(2)).
When the new program begins, the global cell _clock is set to the address of a cell that
keeps approximate time expended by the process at user level. The time is measured in
milliseconds but is updated at a system-dependent lower rate. This clock is typically
used by the profiler but is available to all programs.
The above conventions apply to C programs; the raw system interface to the new image is as
follows: the word pointed to by the stack pointer is argc; the words beyond that are the
zeroth and subsequent elements of argv, followed by a terminating null pointer; and the
return register (e.g. R0 on the 68020) contains the address of the clock.
In Alef, the intent and syntax are the same but the implementation is different. Exec (or
execl; this description applies to either) may be called only by a proc holding a single
task, typically the implicit main task of the proc. First, access(2) is called to see if
the named file exists and has execute permission. If not, exec returns -1 immediately.
Otherwise, it will never return: it frees resources private to the invoking proc and calls
the exec system call. If this fails, it calls the bare _exits system call (see exits(2))
with the error string as argument. Therefore, if the file looks executable, the calling
process is lost, whether the exec succeeds or not.
If these functions fail, they return and set errstr. There can be no return from a suc-
cessful exec or execl; the calling image is lost.