Tribblix: manual page: exec.2

EXEC(2) System Calls EXEC(2)

NAME

exec, execl, execle, execlp, execv, execve, execvp, execvpe, fexecve -
execute a file

SYNOPSIS

#include <unistd.h>

int
execl(const char *path, const char *arg0, ..., NULL);

int
execv(const char *path, char *const argv[]);

int
execle(const char *path, const char *arg0, ..., NULL,
char *const envp[]);

int
execve(const char *path, char *const argv[], char *const envp[]);

int
execlp(const char *file, const char *arg0, ..., NULL);

int
execvp(const char *file, char *const argv[]);

int
execvpe(const char *file, char *const argv[], char *const envp[]);

int
fexecve(int fd, char *const argv[], char *const envp[]);

DESCRIPTION

Each of the functions in the exec family replaces the current process
image with a new process image. The new image is constructed from a
regular, executable file called the new process image file. This file
is either an executable object file or a file of data for an
interpreter. There is no return from a successful call to one of these
functions because the calling process image is overlaid by the new
process image.

An interpreter file begins with a line of the form

#! pathname [arg]

where pathname is the path of the interpreter, and arg is an optional
argument. When an interpreter file is executed, the system invokes the
specified interpreter. The pathname specified in the interpreter file
is passed as arg0 to the interpreter. If arg was specified in the
interpreter file, it is passed as arg1 to the interpreter. The
remaining arguments to the interpreter are arg0 through argn of the
originally exec'd file. The interpreter named by pathname may also be
an interpreter file. There can be up to four nested interpreter files
before the final interpreter. The setid bits on nested interpreters
are silently ignored.

When a C-language program is executed as a result of this call, it is
entered as a C-language function call as follows:

int main(int argc, char *argv[]);

where argc is the argument count and argv is an array of character
pointers to the arguments themselves. In addition, the following
variable:

extern char **environ;

is initialized as a pointer to an array of character pointers to the
environment strings. The argv and environ arrays are each terminated
by a null pointer. The null pointer terminating the argv array is not
counted in argc.

The value of argc is non-negative, and if greater than 0, argv[0]
points to a string containing the name of the file. If argc is 0,
argv[0] is a null pointer, in which case there are no arguments.
Applications should verify that argc is greater than 0 or that argv[0]
is not a null pointer before dereferencing argv[0].

The arguments specified by a program with one of the exec functions are
passed on to the new process image in the main() arguments.

The path argument points to a path name that identifies the new process
image file.

The file argument is used to construct a pathname that identifies the
new process image file. If the file argument contains a slash
character, it is used as the pathname for this file. Otherwise, the
path prefix for this file is obtained by a search of the directories
passed in the PATH environment variable (see environ(7)). The
environment is supplied typically by the shell. If the process image
file is not a valid executable object file, execlp(), execvp(), and
execvpe() use the contents of that file as standard input to the shell.
In this case, the shell becomes the new process image. The standard to
which the caller conforms determines which shell is used. See
standards(7).

The fexecve() function is equivalent to execve(), except that instead
of using a named file, the file referenced by the file descriptor fd is
used. Note that this file descriptor must reference a regular file
which has typically been opened with O_EXEC. defined in <fcntl.h>.
The image is loaded from offset zero of the file, regardless of the
offset of fd.

The arguments represented by arg0... are pointers to null-terminated
character strings. These strings constitute the argument list
available to the new process image. The list is terminated by a null
pointer. The arg0 argument should point to a filename that is
associated with the process being started by one of the exec functions.

The argv argument is an array of character pointers to null-terminated
strings. The last member of this array must be a null pointer. These
strings constitute the argument list available to the new process
image. The value in argv[0] should point to a filename that is
associated with the process being started by one of the exec functions.

The envp argument is an array of character pointers to null-terminated
strings. These strings constitute the environment for the new process
image. The envp array is terminated by a null pointer. For execl(),
execv(), execvp(), and execlp(), the C-language run-time start-off
routine places a pointer to the environment of the calling process in
the global object extern char **environ, and it is used to pass the
environment of the calling process to the new process image.

The number of bytes available for the new process's combined argument
and environment lists is ARG_MAX. It is implementation-dependent
whether null terminators, pointers, and/or any alignment bytes are
included in this total.

File descriptors open in the calling process image remain open in the
new process image, except for those whose close-on-exec flag FD_CLOEXEC
is set. For those file descriptors that remain open, most attributes
of the open file descriptor, including file locks, remain unchanged;
however, the close-on-fork flag FD_CLOFORK is cleared from all file
descriptors. See fcntl(2) for more information on FD_CLOEXEC and
FD_CLOFORK.

The preferred hardware address translation size (see memcntl(2)) for
the stack and heap of the new process image are set to the default
system page size.

Directory streams open in the calling process image are closed in the
new process image.

The state of conversion descriptors and message catalogue descriptors
in the new process image is undefined. For the new process, the
equivalent of:

setlocale(LC_ALL, "C");

is executed at startup.

Signals set to the default action (SIG_DFL) in the calling process
image are set to the default action in the new process image (see
signal(3C)). Signals set to be ignored (SIG_IGN) by the calling
process image are set to be ignored by the new process image. Signals
set to be caught by the calling process image are set to the default
action in the new process image (see signal.h(3HEAD)). After a
successful call to any of the exec functions, alternate signal stacks
are not preserved and the SA_ONSTACK flag is cleared for all signals.

After a successful call to any of the exec functions, any functions
previously registered by atexit(3C) are no longer registered.

The saved resource limits in the new process image are set to be a copy
of the process's corresponding hard and soft resource limits.

If the ST_NOSUID bit is set for the file system containing the new
process image file, then the effective user ID and effective group ID
are unchanged in the new process image. If the set-user-ID mode bit of
the new process image file is set (see chmod(2)), the effective user ID
of the new process image is set to the owner ID of the new process
image file. Similarly, if the set-group-ID mode bit of the new process
image file is set, the effective group ID of the new process image is
set to the group ID of the new process image file. The real user ID
and real group ID of the new process image remain the same as those of
the calling process image. The effective user ID and effective group
ID of the new process image are saved (as the saved set-user-ID and the
saved set-group-ID) for use by setuid(2).

The privilege sets are changed according to the following rules:

1. The inheritable set, I, is intersected with the limit set,
L. This mechanism enforces the limit set for processes.

2. The effective set, E, and the permitted set, P, are made
equal to the new inheritable set.

The system attempts to set the privilege-aware state to non-PA both
before performing any modifications to the process IDs and privilege
sets as well as after completing the transition to new UIDs and
privilege sets, following the rules outlined in privileges(7).

If the {PRIV_PROC_OWNER} privilege is asserted in the effective set,
the set-user-ID and set-group-ID bits will be honored when the process
is being controlled by ptrace(3C). Additional restrictions can apply
when the traced process has an effective UID of 0. See privileges(7).

Any shared memory segments attached to the calling process image will
not be attached to the new process image (see shmop(2)). Any mappings
established through mmap() are not preserved across an exec. Memory
mappings created in the process are unmapped before the address space
is rebuilt for the new process image. See mmap(2).

Memory locks established by the calling process via calls to
mlockall(3C) or mlock(3C) are removed. If locked pages in the address
space of the calling process are also mapped into the address spaces
the locks established by the other processes will be unaffected by the
call by this process to the exec function. If the exec function fails,
the effect on memory locks is unspecified.

If _XOPEN_REALTIME is defined and has a value other than -1, any named
semaphores open in the calling process are closed as if by appropriate
calls to sem_close(3C).

Profiling is disabled for the new process; see profil(2).

Timers created by the calling process with timer_create(3C) are deleted
before replacing the current process image with the new process image.

For the SCHED_FIFO and SCHED_RR scheduling policies, the policy and
priority settings are not changed by a call to an exec function.

All open message queue descriptors in the calling process are closed,
as described in mq_close(3C).

Any outstanding asynchronous I/O operations may be cancelled. Those
asynchronous I/O operations that are not canceled will complete as if
the exec function had not yet occurred, but any associated signal
notifications are suppressed. It is unspecified whether the exec
function itself blocks awaiting such I/O completion. In no event,
however, will the new process image created by the exec function be
affected by the presence of outstanding asynchronous I/O operations at
the time the exec function is called.

All active contract templates are cleared (see contract(5)).

The new process also inherits the following attributes from the calling
process:

+o controlling terminal

+o current working directory

+o file-locks (see fcntl(2) and lockf(3C))

+o file mode creation mask (see umask(2))

+o file size limit (see ulimit(2))

+o limit privilege set

+o nice value (see nice(2))

+o parent process ID

+o pending signals (see sigpending(2))

+o privilege debugging flag (see privileges(7) and getpflags(2))

+o process ID

+o process contract (see contract(5) and process(5))

+o process group ID

+o process signal mask (see sigprocmask(2))

+o processor bindings (see processor_bind(2))

+o processor set bindings (see pset_bind(2))

+o project ID

+o real group ID

+o real user ID

+o resource limits (see getrlimit(2))

+o root directory

+o scheduler class and priority (see priocntl(2))

+o semadj values (see semop(2))

+o session membership (see exit(2) and signal(3C))

+o supplementary group IDs

+o task ID

+o time left until an alarm clock signal (see alarm(2))

+o tms_utime, tms_stime, tms_cutime, and tms_cstime (see
times(2))

+o trace flag (see ptrace(3C) request 0)

A call to any exec function from a process with more than one thread
results in all threads being terminated and the new executable image
being loaded and executed. No destructor functions will be called.

Upon successful completion, each of the functions in the exec family
marks for update the st_atime field of the file. If an exec function
failed but was able to locate the process image file, whether the
st_atime field is marked for update is unspecified. Should the
function succeed, the process image file is considered to have been
opened with open(2). The corresponding close(2) is considered to occur
at a time after this open, but before process termination or successful
completion of a subsequent call to one of the exec functions. The
argv[] and envp[] arrays of pointers and the strings to which those
arrays point will not be modified by a call to one of the exec
functions, except as a consequence of replacing the process image.

The saved resource limits in the new process image are set to be a copy
of the process's corresponding hard and soft limits.

RETURN VALUES

If a function in the exec family returns to the calling process image,
an error has occurred; the return value is -1 and errno is set to
indicate the error.

ERRORS

The exec functions will fail if:

E2BIG The number of bytes in the new process's argument
list is greater than the system-imposed limit of
ARG_MAX bytes. The argument list limit is sum of
the size of the argument list plus the size of the
environment's exported shell variables.

EACCES Search permission is denied for a directory listed
in the new process file's path prefix.

The new process file is not an ordinary file.

The new process file mode denies execute permission.

The {FILE_DAC_SEARCH} privilege overrides the
restriction on directory searches.

The {FILE_DAC_EXECUTE} privilege overrides the lack
of execute permission.

EAGAIN Total amount of system memory available when reading
using raw I/O is temporarily insufficient.

EFAULT An argument points to an illegal address.

EINVAL The new process image file has the appropriate
permission and has a recognized executable binary
format, but the system does not support execution of
a file with this format.

EINTR A signal was caught during the execution of one of
the functions in the exec family.

The exec functions except for fexecve() will fail if:

ELOOP Too many symbolic links were encountered in translating path or
file, or too many nested interpreter files.

ENAMETOOLONG
The length of the file or path argument exceeds PATH_MAX, or
the length of a file or path component exceeds NAME_MAX while
_POSIX_NO_TRUNC is in effect.

ENOENT One or more components of the new process path name of the file
do not exist or is a null pathname.

ENOLINK
The path argument points to a remote machine and the link to
that machine is no longer active.

ENOTDIR
A component of the new process path of the file prefix is not a
directory.

The exec functions, except for execlp(), execvp(), and execvpe(), will
fail if:

ENOEXEC
The new process image file has the appropriate access
permission but is not in the proper format.

The fexecve() function will fail if:

EBADF The fd argument is not a valid file descriptor opened for
execution.

ENOMEM There was insufficient memory for constructing the path
corresponding to the file descriptor.

The exec functions except for fexecve() may fail if:

ENAMETOOLONG
Pathname resolution of a symbolic link produced an intermediate
result whose length exceeds PATH_MAX.

The exec functions may fail if:

ENOMEM The new process image requires more memory than is allowed by
the hardware or system-imposed by memory management
constraints. See brk(2).

ETXTBSY
The new process image file is a pure procedure (shared text)
file that is currently open for writing by some process.

USAGE

As the state of conversion descriptors and message catalogue
descriptors in the new process image is undefined, portable
applications should not rely on their use and should close them prior
to calling one of the exec functions.

Applications that require other than the default POSIX locale should
call setlocale(3C) with the appropriate parameters to establish the
locale of the new process.

The environ array should not be accessed directly by the application.

INTERFACE STABILITY

Committed

MT-LEVEL
The execle(), execve() and fexecve() functions are Async-Signal-Safe

WARNINGS

If a program is setuid to a user ID other than the superuser, and the
program is executed when the real user ID is super-user, then the
program has some of the powers of a super-user as well.

illumos June 25, 2025 illumos