Exec Functions

The exec functions execute programs. We typically proved the binary/program name to exec along with any arguments we may need to pass to the specific exec function.

Note that these functions take over the process. Anything that we place in the program to happen after calling them does not happen, unless an error occurs. The solution to execute a program and continue doing other stuff is to fork, run exec in the child process, and then make the parent process wait.

We divid the exec functions into two categories, the execl functions and the execv functions.

The execl functions

The l in execl stands for list.

The execl() Function

// generalized format
execl(char *pathToExecutable, char arg0, char *arg1, ... , NULL)

Some Details

  • loads and runs an executable path with args

  • arg0 is the name of the executable

  • arg1 is the first space separated real argument

  • NULL follows all the args

  • returns -1 if there is an error

// example with ls
execl("/bin/ls", "ls", "-al", "-h", NULL);

The execlp() Function

// generalized format
execlp(char *file, char *arg0, char *arg1, ..., NULL)

Some Details

  • Main difference between execl and execlp is the file is sought for using the PATH environment variable.

  • To view you PATH environment variable run $PATH in a bash terminal. The directories listed indicate where your machine searches for executable files.

// example with ls
execlp("ls", "ls", "-al", "-h", NULL)

The execle() Function

// Generalized Format
execle(char *executable, char *arg0, char *arg1, ..., NULL, char *envp[])

Some Details

  • Allows inclusion of an array of environment variables (envp in the sample above)

  • Environment variables are in the format “NAME=VALUE”

// example with ls

// providing our own PATH var and a var named LANG
char *envp[] = {
    "PATH=/bin:/usr/bin",
    "LANG=C",
    NULL    // envp must be NULL-terminated
};

execlp("ls", "ls", "-al", "-h", NULL, envp)

The execv functions

The v in execv stands for vector (an array). That is, we provide an array instead of listing out every argument.

The execv() Function

// General Format
execv(char* pathToExecutable, char *args[])

Some Details

  • The first element of args is the name of the executable

  • The args array should have atleast one NULL in it.

// Example with ls
char *args[] = {
    "ls",
    "-l",
    NULL,  // NULL terminated, 
    NULL   // could have more than 1 NULL here (useful for project)
};

execv("/bin/ls", args)

The execvp() Function

// General Format
execvp(char* executable, char *args[])

Some Details

  • Similar to execv, but uses PATH variable to find the file to execute

  • First element of args is also the filename

  • args is NULL terminated

// Example with ls
char *args[] = {
    "ls",
    "-l",
    NULL,  // NULL terminated, 
    NULL   // could have more than 1 NULL here (useful for project)
};

execvp(args[0], args)

The execvpe() Function

// General Format
execvpe(char* executable, char *args[], char *env[])

Some Details

  • Allows for the passing in of environment variables like with execle()
// An example with ls and envp

char *args[] = {
    "ls",
    "-l",
    NULL,  // NULL terminated, 
    NULL   // could have more than 1 NULL here (useful for project)
};

char *envp[] = {
    "PATH=/bin:/usr/bin",
    "LANG=C",
    NULL         // envp must be NULL-terminated
};

execvpe(args[0], args, envp)