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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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fexecve(3) Library Functions Manual fexecve(3)
fexecve - execute program specified via file descriptor
Standard C library (libc, -lc)
#include <unistd.h>
int fexecve(int fd, char *const argv[], char *const envp[]);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
fexecve():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
fexecve() performs the same task as execve(2), with the
difference that the file to be executed is specified via a file
descriptor, fd, rather than via a pathname. The file descriptor
fd must be opened read-only (O_RDONLY) or with the O_PATH flag
and the caller must have permission to execute the file that it
refers to.
A successful call to fexecve() never returns. On error, the
function does return, with a result value of -1, and errno is set
to indicate the error.
Errors are as for execve(2), with the following additions:
EINVAL fd is not a valid file descriptor, or argv is NULL, or
envp is NULL.
ENOENT The close-on-exec flag is set on fd, and fd refers to a
script. See BUGS.
ENOSYS The kernel does not provide the execveat(2) system call,
and the /proc filesystem could not be accessed.
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼─────────┤
│ fexecve() │ Thread safety │ MT-Safe │
└─────────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2008.
glibc 2.3.2.
On Linux with glibc versions 2.26 and earlier, fexecve() is
implemented using the proc(5) filesystem, so /proc needs to be
mounted and available at the time of the call. Since glibc 2.27,
if the underlying kernel supports the execveat(2) system call,
then fexecve() is implemented using that system call, with the
benefit that /proc does not need to be mounted.
The idea behind fexecve() is to allow the caller to verify
(checksum) the contents of an executable before executing it.
Simply opening the file, checksumming the contents, and then
doing an execve(2) would not suffice, since, between the two
steps, the filename, or a directory prefix of the pathname, could
have been exchanged (by, for example, modifying the target of a
symbolic link). fexecve() does not mitigate the problem that the
contents of a file could be changed between the checksumming and
the call to fexecve(); for that, the solution is to ensure that
the permissions on the file prevent it from being modified by
malicious users.
The natural idiom when using fexecve() is to set the close-on-
exec flag on fd, so that the file descriptor does not leak
through to the program that is executed. This approach is
natural for two reasons. First, it prevents file descriptors
being consumed unnecessarily. (The executed program normally has
no need of a file descriptor that refers to the program itself.)
Second, if fexecve() is used recursively, employing the close-on-
exec flag prevents the file descriptor exhaustion that would
result from the fact that each step in the recursion would cause
one more file descriptor to be passed to the new program. (But
see BUGS.)
If fd refers to a script (i.e., it is an executable text file
that names a script interpreter with a first line that begins
with the characters #!) and the close-on-exec flag has been set
for fd, then fexecve() fails with the error ENOENT. This error
occurs because, by the time the script interpreter is executed,
fd has already been closed because of the close-on-exec flag.
Thus, the close-on-exec flag can't be set on fd if it refers to a
script, leading to the problems described in NOTES.
execve(2), execveat(2)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-05-02 fexecve(3)
Pages that refer to this page: execve(2), execveat(2), open(2), exec(3), posix_spawn(3), signal-safety(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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