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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-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. 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
[email protected]
Linux man-pages 6.15 2025-05-17 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 2025-09-06 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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