This section describes the exec family of functions, for executing
a file as a process image. You can use these functions to make a child
process execute a new program after it has been forked.
To see the effects of exec from the point of view of the called
program, See Program Basics.
The functions in this family differ in how you specify the arguments,
but otherwise they all do the same thing. They are declared in the
header file unistd.h.
— Function: int execv (const char *filename, char *const argv)
The execv function executes the file named by filename as a
new process image.
The argv argument is an array of null-terminated strings that is
used to provide a value for the argv argument to the main
function of the program to be executed. The last element of this array
must be a null pointer. By convention, the first element of this array
is the file name of the program sans directory names. See Program Arguments, for full details on how programs can access these arguments.
The environment for the new process image is taken from the
environ variable of the current process image; see
Environment Variables, for information about environments.
This is similar to execv, but permits you to specify the environment
for the new program explicitly as the env argument. This should
be an array of strings in the same format as for the environ
variable; see Environment Access.
This is similar to execl, but permits you to specify the
environment for the new program explicitly. The environment argument is
passed following the null pointer that marks the last argv
argument, and should be an array of strings in the same format as for
the environ variable.
— Function: int execvp (const char *filename, char *const argv)
The execvp function is similar to execv, except that it
searches the directories listed in the PATH environment variable
(see Standard Environment) to find the full file name of a
file from filename if filename does not contain a slash.
This function is useful for executing system utility programs, because
it looks for them in the places that the user has chosen. Shells use it
to run the commands that users type.
This function is like execl, except that it performs the same
file name searching as the execvp function.
The size of the argument list and environment list taken together must
not be greater than ARG_MAX bytes. See General Limits. In
the GNU system, the size (which compares against ARG_MAX)
includes, for each string, the number of characters in the string, plus
the size of a char *, plus one, rounded up to a multiple of the
size of a char *. Other systems may have somewhat different
rules for counting.
These functions normally don't return, since execution of a new program
causes the currently executing program to go away completely. A value
of -1 is returned in the event of a failure. In addition to the
usual file name errors (see File Name Errors), the following
errno error conditions are defined for these functions:
The combined size of the new program's argument list and environment
list is larger than ARG_MAX bytes. The GNU system has no
specific limit on the argument list size, so this error code cannot
result, but you may get ENOMEM instead if the arguments are too
big for available memory.
The specified file can't be executed because it isn't in the right format.
Executing the specified file requires more storage than is available.
If execution of the new file succeeds, it updates the access time field
of the file as if the file had been read. See File Times, for more
details about access times of files.
The point at which the file is closed again is not specified, but
is at some point before the process exits or before another process
image is executed.
Executing a new process image completely changes the contents of memory,
copying only the argument and environment strings to new locations. But
many other attributes of the process are unchanged:
Current working directory and root directory. See Working Directory. In the GNU system, the root directory is not copied when
executing a setuid program; instead the system default root directory
is used for the new program.
Elapsed processor time associated with the process; see Processor Time.
If the set-user-ID and set-group-ID mode bits of the process image file
are set, this affects the effective user ID and effective group ID
(respectively) of the process. These concepts are discussed in detail
in Process Persona.
Signals that are set to be ignored in the existing process image are
also set to be ignored in the new process image. All other signals are
set to the default action in the new process image. For more
information about signals, see Signal Handling.
File descriptors open in the existing process image remain open in the
new process image, unless they have the FD_CLOEXEC
(close-on-exec) flag set. The files that remain open inherit all
attributes of the open file description from the existing process image,
including file locks. File descriptors are discussed in Low-Level I/O.
Streams, by contrast, cannot survive through exec functions,
because they are located in the memory of the process itself. The new
process image has no streams except those it creates afresh. Each of
the streams in the pre-exec process image has a descriptor inside
it, and these descriptors do survive through exec (provided that
they do not have FD_CLOEXEC set). The new process image can
reconnect these to new streams using fdopen (see Descriptors and Streams).
Published under the terms of the GNU General Public License