FEXECVE(3) Linux Programmer's Manual FEXECVE(3)
fexecve - execute program specified via file descriptor
int fexecve(int fd, char *const argv, char *const envp);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Since glibc 2.10:
_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
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,
and the caller must have permission to execute the file that it refers
A successful call to fexecve() never returns. On error, the function
does return, with a result value of -1, and errno is set appropriately.
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
ENOSYS The /proc filesystem could not be accessed.
fexecve() is implemented since glibc 2.3.2.
For an explanation of the terms used in this section, see
|Interface | Attribute | Value |
|fexecve() | Thread safety | MT-Safe |
POSIX.1-2008. This function is not specified in POSIX.1-2001, and is
not widely available on other systems. It is specified in
On Linux, fexecve() is implemented using the proc(5) filesystem, so
/proc needs to be mounted and available at the time of the call.
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
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 char-
acters #!) and the close-on-exec flag has been set for fd, then fex-
ecve() 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.
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Linux 2015-07-23 FEXECVE(3)
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