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       fexecve - execute program specified via file descriptor


       #include <unistd.h>

       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 to.


       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 NULL.

       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 attributes(7).

       │InterfaceAttributeValue   │
       │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 POSIX.1-2008.


       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 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)


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