exec(3) Library Functions Manual exec(3)
NAME
execl, execlp, execle, execv, execvp, execvpe - execute a file
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
extern char **environ;
int execl(const char *pathname, const char *arg, ...
/*, (char *) NULL */);
int execlp(const char *file, const char *arg, ...
/*, (char *) NULL */);
int execle(const char *pathname, const char *arg, ...
/*, (char *) NULL, char *const envp[] */);
int execv(const char *pathname, char *const argv[]);
int execvp(const char *file, char *const argv[]);
int execvpe(const char *file, char *const argv[], char *const envp[]);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
execvpe():
_GNU_SOURCE
DESCRIPTION
The exec() family of functions replaces the current process image with a
new process image. The functions described in this manual page are lay-
ered on top of execve(2). (See the manual page for execve(2) for fur-
ther details about the replacement of the current process image.)
The initial argument for these functions is the name of a file that is
to be executed.
The functions can be grouped based on the letters following the "exec"
prefix.
l - execl(), execlp(), execle()
The const char *arg and subsequent ellipses can be thought of as arg0,
arg1, ..., argn. Together they describe a list of one or more pointers
to null-terminated strings that represent the argument list available to
the executed program. The first argument, by convention, should point
to the filename associated with the file being executed. The list of
arguments must be terminated by a null pointer, and, since these are
variadic functions, this pointer must be cast (char *) NULL.
By contrast with the 'l' functions, the 'v' functions (below) specify
the command-line arguments of the executed program as a vector.
v - execv(), execvp(), execvpe()
The char *const argv[] argument is an array of pointers to null-termi-
nated strings that represent the argument list available to the new pro-
gram. The first argument, by convention, should point to the filename
associated with the file being executed. The array of pointers must be
terminated by a null pointer.
e - execle(), execvpe()
The environment of the new process image is specified via the argument
envp. The envp argument is an array of pointers to null-terminated
strings and must be terminated by a null pointer.
All other exec() functions (which do not include 'e' in the suffix) take
the environment for the new process image from the external variable en-
viron in the calling process.
p - execlp(), execvp(), execvpe()
These functions duplicate the actions of the shell in searching for an
executable file if the specified filename does not contain a slash (/)
character. The file is sought in the colon-separated list of directory
pathnames specified in the PATH environment variable. If this variable
isn't defined, the path list defaults to a list that includes the direc-
tories returned by confstr(_CS_PATH) (which typically returns the value
"/bin:/usr/bin") and possibly also the current working directory; see
VERSIONS for further details.
execvpe() searches for the program using the value of PATH from the
caller's environment, not from the envp argument.
If the specified filename includes a slash character, then PATH is ig-
nored, and the file at the specified pathname is executed.
In addition, certain errors are treated specially.
If permission is denied for a file (the attempted execve(2) failed with
the error EACCES), these functions will continue searching the rest of
the search path. If no other file is found, however, they will return
with errno set to EACCES.
If the header of a file isn't recognized (the attempted execve(2) failed
with the error ENOEXEC), these functions will execute the shell
(/bin/sh) with the path of the file as its first argument. (If this at-
tempt fails, no further searching is done.)
All other exec() functions (which do not include 'p' in the suffix) take
as their first argument a (relative or absolute) pathname that identi-
fies the program to be executed.
RETURN VALUE
The exec() functions return only if an error has occurred. The return
value is -1, and errno is set to indicate the error.
ERRORS
All of these functions may fail and set errno for any of the errors
specified for execve(2).
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌────────────────────────────────────────┬───────────────┬─────────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────────────────┼───────────────┼─────────────┤
│ execl(), execle(), execv() │ Thread safety │ MT-Safe │
├────────────────────────────────────────┼───────────────┼─────────────┤
│ execlp(), execvp(), execvpe() │ Thread safety │ MT-Safe env │
└────────────────────────────────────────┴───────────────┴─────────────┘
VERSIONS
The default search path (used when the environment does not contain the
variable PATH) shows some variation across systems. It generally in-
cludes /bin and /usr/bin (in that order) and may also include the cur-
rent working directory. On some other systems, the current working is
included after /bin and /usr/bin, as an anti-Trojan-horse measure. The
glibc implementation long followed the traditional default where the
current working directory is included at the start of the search path.
However, some code refactoring during the development of glibc 2.24
caused the current working directory to be dropped altogether from the
default search path. This accidental behavior change is considered
mildly beneficial, and won't be reverted.
The behavior of execlp() and execvp() when errors occur while attempting
to execute the file is historic practice, but has not traditionally been
documented and is not specified by the POSIX standard. BSD (and possi-
bly other systems) do an automatic sleep and retry if ETXTBSY is encoun-
tered. Linux treats it as a hard error and returns immediately.
Traditionally, the functions execlp() and execvp() ignored all errors
except for the ones described above and ENOMEM and E2BIG, upon which
they returned. They now return if any error other than the ones de-
scribed above occurs.
STANDARDS
environ
execl()
execlp()
execle()
execv()
execvp()
POSIX.1-2008.
execvpe()
GNU.
HISTORY
environ
execl()
execlp()
execle()
execv()
execvp()
POSIX.1-2001.
execvpe()
glibc 2.11.
BUGS
Before glibc 2.24, execl() and execle() employed realloc(3) internally
and were consequently not async-signal-safe, in violation of the re-
quirements of POSIX.1. This was fixed in glibc 2.24.
Architecture-specific details
On sparc and sparc64, execv() is provided as a system call by the kernel
(with the prototype shown above) for compatibility with SunOS. This
function is not employed by the execv() wrapper function on those archi-
tectures.
SEE ALSO
sh(1), execve(2), execveat(2), fork(2), ptrace(2), fexecve(3), sys-
tem(3), environ(7)
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