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Manual page for EXECVE(2)

execve, execv - execute a file

SYNOPSIS

int execve(const char*path, char *const argv[], char *const envp);

int execv(const char*path, char *const argv[]);

extern char **environ;

DESCRIPTION

The execve and execv functions transform the calling process into a new process. The new process is constructed from an ordinary file 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''. When an interpreter file is exec'd, the system runs the specified interpreter,giving it the name of the originally exec'dfileasanargumentand shifting over the rest of the original arguments.

There can be no return from a successful or execv, since the calling core image is lost. This is the mechanism whereby different process images become active.

The argument argv is a null-terminated array of character pointers to null-terminated character strings. These strings constitute the argument list to be made available to the new process. By convention, at least one argument must be present in this array, and the first element of this array should be the name of the executed program (that is, the last component of path).

The execve function takes an additional argument envp which is a null-terminated array of character pointers to null-terminated strings. These strings pass information to the new process that is not directly an argument to the command (see environ.7

Descriptors open in the calling process remain open in the new process, except for those for which the close-on-exec flag is set (see fcntl.2 Descriptors that remain open are unaffected by execve or execv.

Directory streams open in the calling process image are closed in the new process image.

Ignored signals remain ignored across an execve or execv, but signals that are caught are reset to their default values. Blocked signals remain blocked regardless of changes to the signal action. The signal stack is reset to be undefined (see sigvec.2 for more information).

Each process has real user and group IDs and effective user and group IDs. The real ID identifies the person using the system; the effective ID determines his access privileges. The execve and execv functions change the effective user and group ID 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 effective user ID and effective group ID of the new process image will be saved (as the saved set-user-ID and the saved set-group-ID) for use by the setuid.2

The new process also inherits the following attributes from the calling process:

process ID	see getpid(2)
parent process ID	see getppid(2)
process group ID	see getpgrp(2)
session membership	see setsid(2)
real user ID	see getuid(2)
real group ID	see getgid(2)
supplementary group IDs	see getgroups(2)
time left until an alarm clock signal	see getitimer(2)
current working directory	see getwd(3) or getcwd(3P)
root directory	see chroot(2)
file mode mask	see umask(2)
signal mask	see sigprocmask(2)
pending signals	see sigpending(2)
tms_utime, tms_stime, tms_cutime, tms_cstime	see times(2)
controlling terminal	see tty(4)
resource usages	see getrusage(2)
resource limits	see getrlimit(2)

When the executed program begins, it is called as follows:

	main(int argc, char *argv);

where argc is the number of elements in argv (the ``arg count'') and argv is the array of character pointers to the arguments themselves.

Envp is a pointer to an array of strings that constitute the environment of the process. A pointer to this array is also stored in the global variable ``environ''. Each string consists of a name, an ``='', and a null-terminated value. The array of pointers is terminated by a null pointer. The shell sh.1 passes an environment entry for each global shell variable defined when the program is called. See environ.7 for some conventionally used names.

RETURN VALUE

If execve or execv returns to the calling process an error has occurred; the return value will be -1 and the global variable errno will contain an error code.

ERRORS

The execve and execv functions will fail and return to the calling process if one or more of the following are true:

[ENOTDIR]: A component of the path prefix is not a directory.
[EINVAL]: The pathname contains a character with the high-order bit set.
[ENAMETOOLONG]: A component of path exceeds 255 characters, or the entire pathname exceeds 1023 characters. For POSIX applications these values are given by the constants {NAME_MAX} and {PATH_MAX}, respectively.
[ENOENT]: The new process file does not exist or path points to an empty string.
[ELOOP]: Too many symbolic links were encountered in translating the pathname.
[EACCES]: Search permission is denied for a component of the path prefix.
[EACCES]: The new process file is not an ordinary file.
[EACCES]: The new process file mode denies execute permission.
[ENOEXEC]: The new process file has the appropriate access permission, but has an invalid magic number in its header.
[ETXTBSY]: The new process file is a pure procedure (shared text) file that is currently open for writing or reading by some process.
[E2BIG]: The number of bytes in the new process's argument list is larger than the system-imposed limit. The limit in the system as released is 20480 bytes (NCARGS in <sys/param.h>.
[EFAULT]: The new process file is not as long as indicated by the size values in its header.
[EFAULT]: path, argv, or envp point to an illegal address.
[EIO]: An I/O error occurred while reading from the file system.

CAVEATS