exec(2) exec(2)
NAME [Toc] [Back]
execl(), execle(), execlp(), execv(), execve(), execvp() - execute a
file
SYNOPSIS [Toc] [Back]
#include <unistd.h>
extern char **environ;
int execl(const char *path,
const char *arg0, ...
/*
* [const char *arg1, ..., const char *argn,]
*/
(char *)0
);
int execle(const char *path,
const char *arg0, ...
/*
* [const char *arg1, ..., const char *argn,]
*/
(char *)0,
char * const envp[]
);
int execlp(const char *file,
const char *arg0, ...
/*
* [const char *arg1, ..., const char *argn,]
*/
(char *)0
);
int execv(const char *path, char * const argv[]);
int execve(const char *path, char * const argv[], char * const envp[]);
int execvp(const char *file, char * const argv[]);
Remarks [Toc] [Back]
The ANSI C ", ..." construct denotes a variable length argument list
whose optional and required members are given in the associated comment
(/* */).
DESCRIPTION [Toc] [Back]
The exec*() system calls, in all their forms, load a program from an
ordinary, executable file into the current process, replacing the
current program. The path or file argument refers to either an
executable object file or a file of data for an interpreter. In the
latter case, the file of data is also called a script file.
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exec(2) exec(2)
If the calling process is multi-threaded, a call to any of the exec
functions will cause all threads and light weight processes in the
calling process to be terminated and the new executable image to be
loaded and executed. No thread specific data destructor functions are
called. If the exec function fails and returns to the caller, threads
and light weight processes (LWPs) in the calling process will not be
terminated.
An executable object file consists of a header (see a.out(4)), text
segment, and data segment. The data segment contains an initialized
portion and an uninitialized portion (bss). For execlp() and execvp()
the POSIX shell (see sh-posix(1)) can be loaded to interpret a script
instead. A successful call to exec*() does not return because the new
program overwrites the calling program.
When a C program is executed, it is called as follows:
main (int argc, char **argv, char **envp)
where argc is the argument count and argv is the address of an array
of character pointers to the arguments themselves. As indicated, argc
usually has a value of at least one, and the first member of the array
points to a string containing the name of the file. Exit conditions
from main are discussed in exit(2).
path points to a path name that identifies the executable file
containing the new program.
file (in execlp() or execvp()) points to a file name identifying the
executable file containing the new program. The path prefix for this
file is obtained by searching the directories passed in the
environment variable PATH (see environ(5)). The environment is
supplied by the shell (see sh(1)). If file does not have an
executable magic number (see magic(4)), it is passed to the POSIX
shell as a shell script.
arg0, ..., argn are one or more pointers to null-terminated character
strings. These strings constitute the argument list available to the
new program. By convention, at least arg0 must be present and point
to a string identical to path or to path's last component.
argv is an array of character pointers to null-terminated strings.
These strings constitute the argument list available to the new
program. By convention, argv must have at least one member, and must
point to a string that is identical to path or path's last component.
argv is terminated by a null pointer.
envp is an array of character pointers to null-terminated strings.
These strings constitute the environment in which the new program
runs. envp is terminated by a null pointer. For execle() and
execve(), the C run-time start-off routine places a pointer to the
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exec(2) exec(2)
environment of the calling program in the global cell:
extern char **environ;
and it is used to pass the environment of the calling program to the
new program.
Multi-threaded applications should not use the environ variable to
access or modify any environment variable while another thread is
concurrently modifying any environment variable. Calling any function
which is dependent upon any environment variable is considered a use
of the environ variable to access that environment variable.
Open file descriptors remain open, except for those whose close-onexec
flag is set (see fcntl(2)). The file offset, access mode, and
status flags of open file descriptors are unchanged.
Note that normal executable files are open only briefly when they
start execution. Other executable file types can be kept open for a
long time, or even indefinitely under some circumstances.
The processing of signals by the process is unchanged by exec*(),
except that signals caught by the process are set to their default
values (see signal(2)).
If the set-user-ID mode bit of the executable file pointed to by path
or file is set (see chmod(2)), exec*() sets the effective user ID of
the new process to the user ID of the executable file. Similarly, if
the set-group-ID mode bit of the executable file is set, the effective
group ID of the process is set to the group ID of the executable file.
The real user ID and real group ID of the process are unchanged.
Normally, the set-user-ID and set-group-ID functions do not apply to
scripts; thus, if execlp() or execvp() executes a script, the setuser-ID
and set-group-ID bits are ignored, even if they are set. This
behavior can be overridden by setting the kernel tunable
secure_sid_scripts to zero.
The saved user ID and saved group ID of the process are always set to
the effective user ID and effective group ID, respectively, of the
process at the end of the exec*(), whether or not set-user-ID or setgroup-ID
is in effect.
The shared memory segments attached to the calling program are not
attached to the new program (see shmop(2)).
Text and data segment memory locks are not passed on to the new
program (see plock(2)).
Profiling is disabled for the new process (see profil(2)).
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exec(2) exec(2)
The process also retains the following attributes:
+ current working directory
+ file creation mode mask (see umask(2))
+ file locks (see fcntl(2)), except for files closed-onexecution
+ file size limit (see ulimit(2))
+ interval timers (see getitimer(2))
+ nice value (see nice(2))
+ nice value (see parent process ID)
+ pending signals
+ process ID
+ process group ID
+ real user ID
+ real group ID
+ process start time
+ real-time priority (see rtprio(2))
+ root directory (see chroot(2))
+ semadj values (see semop(2))
+ session membership
+ signal mask (see sigvector(2))
+ supplementary group IDs
+ time left until an alarm clock signal (see alarm(2))
+ trace flag (see the PT_SETTRC request of ptrace(2))
+ tms_utime, tms_stime, tms_cutime, and tms_cstime (see
times(2))
For a script file, the initial line of a script file must begin with
#! as the first two bytes, followed by zero or more spaces, followed
by interpreter or interpreter argument, as in:
#! interpreter [argument]
One or more spaces or tabs must separate interpreter and argument.
The first line should end with either a newline or a null character.
When the script file is executed, the system executes the specified
interpreter as an executable object file. Even in the case of
execlp() or execvp(), no path searching is done of the interpreter
name.
The argument is anything that follows interpreter and tabs or spaces.
If an argument is given, it is passed to the interpreter as argv[1],
and the name of the script file is passed as argv[2]. Otherwise, the
name of the script file is passed as argv[1]. argv[0] is passed as
specified in the exec*() call. All other arguments specified in the
exec*() call are passed following the name of the script file (that
is, beginning at argv[3] if there is an argument; otherwise, at
argv[2]).
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exec(2) exec(2)
Some interpreters process the interpreter and the argument internally,
and do not provide the interpreter and the argument to the users
script.
If the initial line of the script file exceeds a system-defined
maximum number of characters, exec*() fails. The minimum value for
this limit is 32.
The set-user-ID and set-group-ID bits are honored for the script but
not for the interpreter.
For a executable object file, the arguments are passed as argv[1],
..., argv[n]. argv[0] is passed as specified in the exec*() call,
unless either argv or argv[0] is null as specified, in which case a
pointer to a null string is passed as argv[0].
RETURN VALUE [Toc] [Back]
If exec*() returns to the calling program, an error has occurred; the
return value is -1 and errno is set to indicate the error.
ERRORS [Toc] [Back]
If exec*() fails and returns to the calling program, errno is set to
one of the following values:
[E2BIG] The number of bytes in the new program's
argument list plus environment is greater
than the system-imposed limit. This limit is
at least 5120 bytes on HP-UX systems.
[EACCES] Read permission is denied for the executable
file or interpreter, and the trace flag (see
ptrace(2) request PT_SETTRC) of the process
is set.
[EACCES] Search permission is denied for a directory
listed in the executable file's or the
interpreter's path prefix.
[EACCES] The executable file or the interpreter is not
an ordinary file.
[EACCES] The file described by path or file is not
executable. The superuser cannot execute a
file unless at least one access permission
bit or entry in its access control list has
an execute bit set.
[EFAULT] path, argv, or envp point to an illegal
address. The reliable detection of this
error is implementation dependent.
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exec(2) exec(2)
[EINTR] A signal was caught during the exec*() system
call.
[EINVAL] The executable file is incompatible with the
architecture on which the exec*() has been
performed, and is presumed to be for a
different architecture. It is not guaranteed
that every architecture's executable files
will be recognized.
[EINVAL] argv points to NULL and null pointer
dereferencing is allowed. In this case, NULL
is a valid address, but is considered an
invalid argument.
[ELOOP] Too many symbolic links were encountered in
translating the path name.
[ENAMETOOLONG] The executable file's path name or the
interpreter's path name exceeds PATH_MAX
bytes, or the length of a component of the
path name exceeds NAME_MAX bytes while
_POSIX_NO_TRUNC is in effect.
[ENOENT] path points to an empty string.
[ENOENT] One or more components of the executable
file's path name or the interpreter's path
name does not exist.
[ENOEXEC] The executable file is shorter than indicated
by the size values in its header, or is
otherwise inconsistent. The reliable
detection of this error is implementation
dependent.
[ENOEXEC] The function call is not execlp() or
execvp(), and the executable file has the
appropriate access permission, but there is
neither a valid magic number nor the
characters #! as the first two bytes of the
file's initial line.
[ENOEXEC] The number of bytes in the initial line of a
script file exceeds the system's maximum.
[ENOMEM] The new process requires more memory than is
available or allowed by the system-imposed
maximum.
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exec(2) exec(2)
[ENOTDIR] A component of the executable file's path
prefix or the interpreter's path prefix is
not a directory.
[ETXTBSY] The executable file is currently open for
writing.
WARNINGS [Toc] [Back]
Unsharable executable files are not supported. These are files whose
EXEC_MAGIC magic number was produced with the -N option of ld (see
ld(1)).
It is recommended to use the execve() call in multi-threaded
applications to avoid possible deadlocks.
DEPENDENCIES [Toc] [Back]
HP Process Resource Manager
If the optional HP Process Resource Manager (PRM) software is
installed and configured, the process's process resource group ID is
not changed by exec*(). See prmconfig(1) for a description of how to
configure HP PRM, and prmconf(4) for the definition of process
resource group.
SEE ALSO [Toc] [Back]
sh(1), sh-posix(1), kctune(1M), alarm(2), exit(2), fork(2), nice(2),
ptrace(2), semop(2), signal(2), times(2), ulimit(2), umask(2),
a.out(4), acl(5), environ(5), signal(5), thread_safety(5).
HP Process Resource Manager: prmconfig(1), prmconf(4) in HP Process
Resource Manager User's Guide.
STANDARDS CONFORMANCE [Toc] [Back]
environ: AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1
execl(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1
execle(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1
execlp(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1
execv(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1
execve(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1
execvp(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1
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