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Manual page for exec(2)

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

SYNOPSIS

#include <unistd.h>

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

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

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

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

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

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

DESCRIPTION

Each of the functions in the exec family overlays a new process image on an old process. The new process image is constructed from an ordinary, executable file. This file is either an executable object file, or a file of data for an interpreter. There can be 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 must not be an interpreter file.

When a C program is executed, it is called as follows:

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

where argc is the argument count, argv is an array of character pointers to the arguments themselves, and envp is an array of character pointers to the environment strings. As indicated, argc is at least one, and the first member of the array points to a string containing the name of the file.

The arguments arg0, ..., argn point to null-terminated character strings. These strings constitute the argument list available to the new process image. Conventionally at least arg0 should be present. It will become the name of the process, as displayed by the ps.1 command. The arg0 argument points to a string that is the same as path (or the last component of path). The list of argument strings is terminated by a (char *)0 argument.

The argv argument is an array of character pointers to null-terminated strings. These strings constitute the argument list available to the new process image. By convention, argv must have at least one member, and it should point to a string that is the same as path (or its last component). The argv argument is terminated by a null pointer.

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 argument is terminated by a null pointer. For execl(), execv(), execvp(), and execlp(), the C 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.

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

The file argument points to the new process file. If file does not contain a slash character, the path prefix for this file is obtained by a search of the directories passed in the PATH environment variable (see environ.5 The environment is supplied typically by the shell. If the new process file is not an executable object file, execlp() and execvp() use the contents of that file as standard input to the shell. In a standard-conforming application (see standards.5 the exec family of functions use /usr/bin/ksh (see ksh.1 otherwise, they use /usr/bin/sh (see sh.1

File descriptors open in the calling process remain open in the new process, except for those whose close-on-exec flag is set; (see fcntl.2 For those file descriptors that remain open, the file pointer is unchanged.

Signals that are being caught by the calling process are set to the default disposition in the new process image (see signal.3c Otherwise, the new process image inherits the signal dispositions of the calling process.

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

If the set-user-ID mode bit of the new process file is set (see chmod.2 the effective user ID of the new process is set to the owner ID of the new process file. Similarly, if the set-group-ID mode bit of the new process file is set, the effective group ID of the new process is set to the group ID of the new process file. The real user ID and real group ID of the new process remain the same as those of the calling process.

If the effective user-ID is root or super-user, the set-user-ID and set-group-ID bits will be honored when the process is being controlled by ptrace.

The shared memory segments attached to the calling process will not be attached to the new process (see shmop.2 Memory mappings in the calling process are unmapped before the new process begins execution (see mmap.2

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

Timers created by timer_create.3r are deleted before the new process begins execution.

Any outstanding asynchronous I/O operations may be cancelled.

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

nice value (see nice.2

scheduler class and priority (see priocntl.2

process ID

parent process ID

process group ID

supplementary group IDs

semadj values (see semop.2

session ID (see exit.2 and signal.3c

trace flag (see ptrace.2 request 0)

time left until an alarm (see alarm.2

current working directory

root directory

file mode creation mask (see umask.2

resource limits (see getrlimit.2

utime, stime, cutime, and cstime (see times.2

file-locks (see fcntl.2 and lockf.3c

controlling terminal

process signal mask (see sigprocmask.2

pending signals (see sigpending.2

Upon successful completion, each of the functions in the exec family marks for update the st_atime field of the file, unless the file is on a read-only file system. Should the function succeed, the process image file is considered to have been opened by the open.2 system called. The corresponding close() 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 functions in the exec family.

RETURN VALUES

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

ERRORS

Each of the functions in the exec family will fail and return to the calling process if one or more of the following are true:

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.
EACCES: The new process file is not an ordinary file.
EACCES: The new process file mode denies 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.
EINTR: A signal was caught during the execution of one of the functions in the exec family.
ELOOP: Too many symbolic links were encountered in translating path or file.
EMULTIHOP: Components of path require hopping to multiple remote machines and the file system type does not allow it.
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.
ENOEXEC: The function call is not an execlp() or execvp(), and the new process file has the appropriate access permission but an invalid magic number in its header.
ENOLINK: path points to a remote machine and the link to that machine is no longer active.
ENOMEM: The new process requires more memory than is allowed by the limit imposed by getrlimit(), see brk.2
ENOTDIR: A component of the new process path of the file prefix is not a directory.

ATTRIBUTES

See attributes.5 for descriptions of the following attributes:

+---------------+---------------------------------------------+
|ATTRIBUTE TYPE |               ATTRIBUTE VALUE               |
+---------------+---------------------------------------------+
|MT-Level       | execle() and execve() are Async-Signal-Safe |
+---------------+---------------------------------------------+

WARNINGS

If a program is setuid to a user ID other than the super-user, 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.

Last modified 07/October/97