exec(2)
NAME
execl, execv, execle, execve, execlp, execvp, exec, exece − execute a file
SYNTAX
int execl(name, arg0, arg1,..., argn, 0)
char *name, *arg0, *arg1,..., *argn;
int execv(name, argv)
char *name, *argv[ ];
int execle(name, arg0, arg1,..., argn, 0, envp)
char *name, *arg0, *arg1,..., *argn, *envp[ ];
int execve(name, argv, envp)
char *name, *argv[ ], *envp[ ];
int execlp(file, arg0fB, arg1,..., argn, 0)
char *file, *arg0, *arg1,..., *argn;
int execvp(file, argv)
char *file, *argv[ ];
DESCRIPTION
The exec system call, in all its forms, overlays the calling process with file name and then transfers to the entry point of the core image of this file. There can be no return from a successful exec call because the calling core image is lost.
File descriptors open in the calling process remain open in the new process, except for those whose close-on-exec flag is set. For further information, see fcntl(2). For those file descriptors that remain open, the file pointer is unchanged. Ignored signals remain ignored across these calls, but signals that are caught are reset to their default values. For further information, see signal(2).
Each user has a real user ID and group ID and an effective user ID and group ID. The real ID identifies the person using the system. The effective ID determines his access privileges. The exec call changes the effective user and group IDs to the owner of the executed file if the file has the “set-user-ID” or “set-group-ID” modes. The real user ID is not affected.
The new process also inherits the following attributes from the calling process:
nice value see nice(2)
process ID
parent process ID
process group ID
semadj values see semop(2)
tty group ID see exit(2) and signal(2)
trace flag see ptrace(2) request 0
time left until an alarm clock signal see alarm(2)
current working directory
root directory
file mode creation mask see umask(2)
file size limit see ulimit(2)
utime, stime, cutime, and cstime see times(2)
The specified name is a pointer to the name of the file to be executed. The pointers arg[0], arg[1]... address null-terminated strings. Conventionally, arg[0] is the name of the file.
From C, two interfaces are available. The execl call is useful when a known file with known arguments is being called. The arguments to execl are the character strings constituting the file and the arguments. The first argument is conventionally the same as the file name (or its last component). A 0 argument must end the argument list.
The execv call is useful when the number of arguments is unknown in advance. The arguments to execv are the name of the file to be executed and a vector of strings containing the arguments. The last argument string must be followed by a 0 pointer.
When a C program is executed, it is called as follows:
main(argc, argv, envp)
int argc;
char **argv, **envp;
The argc is the argument count, and argv is an array of character pointers to the arguments themselves. As indicated, argc is conventionally at least one and the first member of the array points to a string containing the name of the file.
The argv is directly usable in another execv because argv[argc] is 0.
The envp is a pointer to an array of strings that constitute the environment of the process. Each string consists of a name, an equal sign (=), and a null-terminated value. The array of pointers is terminated by a null pointer. The sh(1) shell passes an environment entry for each global shell variable defined when the program is called. For a list of conventionally used names, see environ(5). The C run-time start-off routine places a copy of envp in the global cell environ, which then is used by execv and execl to pass the environment to any subprograms executed by the current program. The exec calls use lower-level routines, as follows, to pass an environment explicitly:
execle(file, arg0, arg1, . . . , argn, 0, environ);
execve(file, argv, environ);
The execlp and execvp calls are called with the same arguments as execl and execv, but they duplicate the shell’s actions in searching for an executable file in a list of directories. The directory list is obtained from the environment.
FILES
/bin/shshell, invoked if command file found by execlp or execvp calls.
RESTRICTIONS
If the execvp call is called to execute a file that turns out to be a shell command file, and if it is impossible to execute the shell, the values of argv[0] and argv[−1] will be modified before return.
RETURN VALUE
If unsuccessful, the exec call returns a −1, and the global variable errno indicates the error code.
DIAGNOSTICS
The exec call will fail if:
[E2BIG] The number of bytes in the new process’s argument list is greater than the system-imposed limit of 5120 bytes.
[EACCES] Search permission is denied for a directory listed in the new process file’s path prefix.
[EACCES] The new process file mode denies execute permission.
[EACCES] The new process file is not a regular file.
[EFAULT] The specified name points outside the process’s allocated address space.
[EFAULT] The name, argv, or envp point to an illegal address (outside of the user’s image).
[ENFILE] There is insufficient system space to contain an inode.
[ENOENT] One or more components of the new process file’s path name does not exist.
[ENOENT] The target file is a directory.
[ENOEXEC] The new process file has the appropriate access permission, but the file header contains an invalid magic number.
[ENOEXEC] The text or data segment is null.
[ENOEXEC] An overlay is larger than the overlay max size.
[ENOEXEC] Unable to read file header.
[ENOEXEC] Found a bad magic number in the file header.
[ENOMEM] The new process requires more memory than is allowed.
[ENOTDIR] A component of the new process path, the name is not a directory.
[ETXTBSY] The new process file is a pure procedure (shared text) file that is currently open for reading or writing by some process.
ASSEMBLER
(exec = 11.)
sys exec; name; argv
(exece = 59.)
sys exece; name; argv; envp
The plain exec call is obsoleted by the exece, but, for historical reasons, it remains.
When the called file starts execution on the PDP11, the stack pointer points to a word containing the number of arguments. Just above this number is a list of pointers to the argument strings, followed by a null pointer, followed by the pointers to the environment strings and then another null pointer. The strings themselves follow; a 0 word is left at the very top of memory.
sp→ nargs
arg0
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argn
0
env0
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envm
0
arg0: <arg0\0>
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env0: <env0\0>
0