exec, execl, _privates, _nprivates, _tos – execute a file

#include <u.h>
#include <libc.h>

void* exec(char *name, char* argv[])

void* execl(char *name, ...)

void **_privates;

int    _nprivates;

#include <tos.h>

typedef struct Tos Tos;
struct Tos {
struct { ... } prof;      /* profiling data */
uvlong    cyclefreq;        /* cycle clock frequency */
vlong     kcycles;          /* kernel cycles */
vlong     pcycles;          /* process cycles (kernel + user) */
ulong     pid;             /* process id */
ulong     clock;           /* profiling clock */
/* top of stack is here */

extern Tos *_tos;

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, for example
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 original exec call. The result honors the two conventions of a program accepting as argument a file to be interpreted and argv[0] 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. However, a newly exec'ed process has no notification handler (see notify(2)).

The global cell _privates points to an array of _nprivates elements of per–process private data. This storage is private for each process, even if the processes share data segments.

When the new program begins, the global pointer _tos is set to the address of a structure that holds information allowing accurate time keeping and clock reading in user space. These data are updated by the kernel during of the life of the process, including across rforks and execs. If there is a user–space accessible fast clock (a processor cycle counter), cyclefreq will be set to its frequency in Hz. Kcycles (pcycles) counts the number of cycles this process has spent in kernel mode (kernel and user mode). Pid is the current process's id. Clock is the user–profiling clock (see prof(1)). Its 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 information.


prof(1), intro(2), stat(2)

If these functions fail, they return and set errstr. There can be no return to the calling process from a successful exec or execl; the calling image is lost.

There is a large but finite limit on the size of an argment list, typically around 409,600 bytes. The kernel constant TSTKSIZ controls this.

Both functions rely upon the system's demand paging, which in turn expects to be able to read full pages in a single read operation, no matter what the underlying file server is.

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