execve() transforms the calling process into a new process. The new process is constructed from an ordinary file, whose name is pointed to by
path, called the
new process file. This file is either an executable object file, or a file of data for an interpreter. An executable object file consists of an identifying header, followed by pages of data representing the initial program (text) and initialized data pages. Additional pages may be specified by the header to be initialized with zero data; see
a.out(5).
An interpreter file begins with a line of the form:
#! interpreter [arg]
When an interpreter file is
execve()d the system runs the specified
interpreter. If the optional
arg is specified, it becomes the first argument to the
interpreter, and the name of the originally
execve()d file becomes the second argument; otherwise, the name of the originally
execve()d file becomes the first argument. The original arguments are shifted over to become the subsequent arguments. The zeroth argument, normally the name of the
execve()d file, is left unchanged. The interpreter named by
interpreter must not itself be an interpreter file. (See
script(7) for a detailed discussion of interpreter file execution.)
The argument
argv is a pointer to a null-terminated array of character pointers to null-terminated character strings. These strings construct the argument list to be made available to the new process. By custom, the first element should be the name of the executed program (for example, the last component of
path).
The argument
envp is also a pointer to a null-terminated array of character pointers to null-terminated strings. A pointer to this array is normally stored in the global variable
environ. These strings pass information to the new process that is not directly an argument to the command (see
environ(7)).
File descriptors open in the calling process image remain open in the new process image, except for those for which the close-on-exec flag is set (see
close(2) and
fcntl(2)). Descriptors that remain open are unaffected by
execve().
In the case of a new setuid or setgid executable being executed, if file descriptors 0, 1, or 2 (representing stdin, stdout, and stderr) are currently unallocated, these descriptors will be opened to point to some system file like
/dev/null. The intent is to ensure these descriptors are not unallocated, since many libraries make assumptions about the use of these 3 file descriptors.
Signals set to be ignored in the calling process are set to be ignored in the new process. Signals which are set to be caught in the calling process image are set to default action in the new process image. Blocked signals remain blocked regardless of changes to the signal action. The signal stack is reset to be undefined (see
sigaction(2) for more information).
If the set-user-ID mode bit of the new process image file is set (see
chmod(2)), the effective user ID of the new process image is set to the owner ID of the new process image file. If the set-group-ID mode bit of the new process image file is set, the effective group ID of the new process image is set to the group ID of the new process image file. (The effective group ID is the first element of the group list.) The real user ID, real group ID and other group IDs of the new process image remain the same as the calling process image. After any set-user-ID and set-group-ID processing, the effective user ID is recorded as the saved set-user-ID, and the effective group ID is recorded as the saved set-group-ID. These values may be used in changing the effective IDs later (see
setuid(2)).
The new process also inherits the following attributes from the calling process:
When a program is executed as a result of an
execve() call, it is entered as follows:
main(argc, argv, envp)
int argc;
char **argv, **envp;
where
argc is the number of elements in
argv (the “arg count”) and
argv points to the array of character pointers to the arguments themselves.