exec (system call)

The POSIX standard declares exec functions in the unistd.h header file, in the C language. The same functions are declared in process.h for DOS (see below), OS/2, and Microsoft Windows.

int execl(char const *path, char const *arg0, ...);

int execle(char const *path, char const *arg0, ..., char const *envp[]);

int execlp(char const *file, char const *arg0, ...);

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

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

int execvp(char const *file, char const *argv[]);

Some implementations provide these functions named with a leading underscore (e.g. _execl).

The base of each is exec (execute), followed by one or more letters:

e – An array of pointers to environment variables is explicitly passed to the new process image.

l – Command-line arguments are passed individually (a list) to the function.

p – Uses the PATH environment variable to find the file named in the file argument to be executed.

v – Command-line arguments are passed to the function as an array (vector) of pointers.

path

The argument specifies the path name of the file to execute as the new process image. Arguments beginning at arg0 are pointers to arguments to be passed to the new process image. The argv value is an array of pointers to arguments.

arg0

The first argument arg0 should be the name of the executable file. Usually it is the same value as the path argument. Some programs may incorrectly rely on this argument providing the location of the executable, but there is no guarantee of this nor is it standardized across platforms.

envp

Argument envp is an array of pointers to environment settings. The exec calls named ending with an e alter the environment for the new process image by passing a list of environment settings through the envp argument. This argument is an array of character pointers; each element (except for the final element) points to a null-terminated string defining an environment variable.

Each null-terminated string has the form:

name=value

where name is the environment variable name, and value is the value of that variable. The final element of the envp array must be null.

In the execl, execlp, execv, and execvp calls, the new process image inherits the current environment variables.

A file descriptor open when an exec call is made remains open in the new process image, unless was fcntled with FD_CLOEXEC or opened with O_CLOEXEC (the latter was introduced in POSIX.1-2001). This aspect is used to specify the standard streams (stdin, stdout and stderr) of the new program.

A successful overlay destroys the previous memory address space of the process, and all its memory areas, that were not shared, are reclaimed by the operating system. Consequently, all its data that were not passed to the new program, or otherwise saved, become lost.

A successful exec replaces the current process image, so it cannot return anything to the program that made the call. Processes do have an exit status, but that value is collected by the parent process.

If an exec function does return to the calling program, an error occurs, the return value is −1, and errno is set to one of the following values: