EXECL(3)
NAME
execl, execv, execle, execlp, execvp, exect, environ − execute a file
USAGE
execl(name, arg0, arg1, ..., argn, 0) char *name, *arg0, *arg1, ..., *argn;
execv(name, argv) char *name, *argv[];
execle(name, arg0, arg1, ..., argn, 0, envp) char *name, *arg0, *arg1, ..., *argn, *envp[];
exect(name, argv, envp) char *name, *argv[], envp[];
extern char **environ;
DESCRIPTION
These routines provide various interfaces to the execve system call. Refer to execve(2) for a full description of their properties; only brief descriptions are provided here.
Exec in all its forms overlays the calling process with the named file, then transfers to the entry point of the core image of the file. There can be no return from a successful exec; the calling core image is lost.
The name argument is a pointer to the name of the file to be executed. The pointers arg[0], arg[1], ..., address null-terminated strings. In most cases, arg[0] is the name of the file.
Two interfaces are available. Execl is useful when a known name with known arguments is being called; the arguments to execl are the character strings that comprise the file (name) and the arguments. The first argument is usually the same as the filename (or its last component). A zero argument ends the argument list.
The execv version is useful when the number of arguments is not known in advance; the arguments to execv include 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 zero pointer.
The exect version is used when the executed file is to be manipulated with ptrace(2). It forces the child to stop after executing its first instruction. The parent (which must expect to trace the child) may then adjust the child’s state.
When a C program is executed, it is called as follows: main(argc, argv, envp) int argc; char **argv, **envp; where argc is the argument count and argv is an array of character pointers to the arguments themselves. The first member of the array points to a string containing the name of the file.
Argv is directly usable in another execv because argv[argc] is zero.
Envp is a pointer to an array of strings that constitute the environment of the process. Each string consists of a name, an equals sign (=), and a null-terminated value. The array of pointers is terminated by a null pointer. The shell passes an environment entry for each global shell variable that is defined when the program is called. The C run-time start-off routine places a copy of envp in the global cell environ, which execv and execl use to pass the environment to any subprograms executed by the current program.
Execlp and execvp are called with the same arguments as execl and execv, but 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/sh shell, invoked if command file found by execlp or execvp
DIAGNOSTICS
A return constitutes the diagnostic if any of the following hold true: name cannot be found name is not executable name is not an object module maximum memory was exceeded the arguments require too much space
The return value is -1. Even if the caller is the super-user, at least one of the execute-permission bits must be set for a file to be executed.