EXECVE(2) COMMAND REFERENCE EXECVE(2) NAME execve - execute a file SYNOPSIS execve(path, argv, envp) char *path, *argv[], *envp[]; DESCRIPTION Execve transforms the calling process into a new process. The new process is constructed from path, 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 initialize with zero data. See a.out(5). An interpreter file begins with a line of the form ``#! interpreter''. The length of this line cannot exceed SHSIZE, defined in <sys/user.h> (currently 32). When an interpreter file is execve'd, the system execve's the specified interpreter. The original arguments are passed to the interpreter as one argument (arg 1) and path , the name of the originally execve'd file, is passed as an additional argument (arg 2). There can be no return from a successful execve because the calling core image is lost. This is the mechanism whereby different process images become active. The argument argv is an 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 (i.e. the last component of path). The argument envp is also an array of character pointers to null-terminated strings. These strings pass information to the new process which are not directly arguments 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 close(2). Descriptors which remain open are unaffected by execve. Ignored signals remain ignored across an execve, but signals that are caught are reset to their default values. The signal stack is reset to be undefined; see sigvec(2) for Printed 5/12/88 1
EXECVE(2) COMMAND REFERENCE EXECVE(2) 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. Execve changes 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 new process also inherits the following attributes from the calling process: process ID see getpid(2) parent process IDsee getppid(2) process group ID see getpgrp(2) access groups see getgroups(2) working directorysee chdir(2) root directory see chroot(2) control terminal see tty(2) resource usages see getrusage(2) interval timers see getitimer(2) resource limits see getrlimit(2) file mode mask see umask(2) signal mask see sigvec(2) When a "C" program is executed as a result of the call, it is called 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 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(1sh) passes an environment entry for each global shell variable defined when the program is called. See environ(7) for some conventionally used names. DIAGNOSTICS Execve will fail and return to the calling process if one or more of the following are true: [ENAMETOOLONG] The new process file's pathname is too long. Printed 5/12/88 2
EXECVE(2) COMMAND REFERENCE EXECVE(2) [ENOENT] One or more components of the new process file's pathname do not exist, or the interpreter to be used to execute the new process file does not exist. [ENOTDIR] A component of the new process file's or the interpreter's pathname is not a directory. [EACCES] Search permission is denied for a directory listed in the new process file's or the interpreter's path prefix. [EACCES] The new process file or the interpreter is not an ordinary file. [EACCES] The new process file mode or the interpreter mode denies execute permission. [ENOEXEC] The new process file or the interpreter has the appropriate access permission, but has an invalid magic number in its header (see a.out(5)). [ETXTBSY] The new process file or the interpreter is a pure procedure (shared text) file that is currently open for writing or reading by some process. [ENOMEM] The new process requires more virtual memory than is allowed by the imposed maximum (getrlimit(2)). [E2BIG] The number of bytes in the new process's argument list is larger than the system-imposed limit of NCARGS, defined in <sys/param.h>. [ENOEXEC] The new process file is not as long as indicated by the size values in its header. [ENOEXEC] The interpreter name is longer than SHSIZE, defined in <sys/user.h>. [EIO] An I/O error occurred while reading from or writing to the file system. [EFAULT] Path, argv, or envp point to an illegal address. [ENOMEM] Swap space is not available for the new process, or the new process file's textsize, datasize or stacksize exceed the system-imposed limits MAXTSIZ, MAXDSIZ or MAXSSIZ, defined in <machine/vmparam.h>. RETURN VALUE If execve returns to the calling process an error has Printed 5/12/88 3
EXECVE(2) COMMAND REFERENCE EXECVE(2) occurred; the return value will be -1 and the global variable errno will contain an error code. CAVEATS If a program is setuid to a non-super-user, but is executed when the real uid is ``root'', then the program has the powers of a super-user as well. SEE ALSO close(2), exit(2), fork(2), getrlimit(2), sigvec(2), execl(3c), a.out(5), and environ(7). Printed 5/12/88 4
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