term% cat index.txt EXEC(2) System Calls Manual EXEC(2)
NAME
execl, execv, execle, execve, execlp, execvp, exec, exece, environ -
execute a file
SYNOPSIS
execl(name, arg0, arg1, ..., argn, 0)
char *name, *arg0, *arg1, ..., *argn;
execv(name, argv)
char *name, *argv[];
execle(name, arg0, arg1, ..., argn, 0, envp)
char *name, *arg0, *arg1, ..., *argn, *envp[];
execve(name, argv, envp)
char *name, *argv[], *envp[];
extern char **environ;
DESCRIPTION
Exec in all its forms overlays the calling process with the named file,
then transfers to the entry point of the core image of the file. There
can be no return from a successful exec; the calling core image is
lost.
Files remain open across exec unless explicit arrangement has been
made; see ioctl(2). Ignored/held signals remain ignored/held across
these calls, but signals that are caught (see signal(2)) are reset to
their default values.
Each user has a real user ID and group ID and an effective user ID and
group ID. The real ID identifies the person using the system; the ef‐
fective ID determines his access privileges. Exec changes the effec‐
tive user and group ID to the owner of the executed file if the file
has the ‘set-user-ID' or ‘set-group-ID' modes. The real user ID is not
affected.
The name argument is a pointer to the name of the file to be executed.
If the first two bytes of that file are the ASCII string ‘#!', then the
first line of the file is taken to be ASCII and determines the name of
the program to execute. The first nonblank string following ‘#!' in
that line is substituted for name. Any second string, separated from
the first by blanks or tabs, is inserted between the first two argu‐
ments (arguments 0 and 1) passed to the invoked file.
The argument pointers arg0, arg1, ... or the pointers in argv address
null-terminated strings. Conventionally argument 0 is the name of the
file.
Execl is useful when a known file with known arguments is being called;
the arguments to execl are the character strings constituting the file
and the arguments. A 0 argument must end the argument list.
Execv is useful when the number of arguments is unknown in advance; the
arguments to execv are the name of the file to be executed and a vector
of strings containing the arguments. The last argument string must be
followed by a 0 pointer.
When a C program is executed, it is called as follows:
main(argc, argv, envp)
int argc;
char **argv, **envp;
where argc is the argument count and argv is an array of character
pointers to the arguments themselves. As indicated, argc is conven‐
tionally at least one and the first member of the array points to a
string containing the name of the file.
Argv is directly usable in another execv because argv[argc] is 0.
Envp is a pointer to an array of strings that constitute the environ‐
ment of the process. Each string consists of a name, a ‘=', and a
null-terminated value; or a name, a pair of parentheses (), a value
bracketed by { and }, and by a null. The array of pointers is termi‐
nated by a null pointer. The shell sh(1) passes an environment entry
for each global shell variable defined when the program is called. See
environ(5) for some conventionally used names.
The C run-time start-off routine places a copy of envp in the global
cell environ, which is used by execv and execl to pass the environment
to any subprograms executed by the current program. The exec routines
use lower-level routines as follows to pass an environment explicitly:
execve(file, argv, environ);
execle(file, arg0, arg1, . . . , argn, 0, environ);
Execlp and execvp are called with the same arguments as execl and ex‐
ecv, but duplicate the shell's actions in searching for an executable
file in a list of directories. The directory list is obtained from the
environment. Exect is the same as execve, except it arranges for a
stop to occur on the first instruction of the new core image for the
benefit of tracers, see proc(4).
FILES
/bin/sh shell, invoked if command file found by execlp or execvp
SEE ALSO
fork(2), environ(5)
DIAGNOSTICS
If the file cannot be found, if it is not executable, if it does not
start with a valid magic number (see a.out(5)), if maximum memory is
exceeded, or if the arguments require too much space, a return consti‐
tutes the diagnostic; the return value is -1. Even for the super-user,
at least one of the execute-permission bits must be set for a file to
be executed.
BUGS
If execvp is called to execute a file that turns out to be a shell com‐
mand file, and if it is impossible to execute the shell, the values of
argv[0] and argv[-1] will be modified before return.
The path search of execlp and execvp does not extend to names substi‐
tuted by ‘#!'.
EXEC(2)