Linux下面exec函数

一、简介：

　　exec系列的函数，在Linux系统中起到很重要的作用，如shell进程的创建就是其中一种应用，一般而言先fork()出来一个子进程，然后exec()执行这个子进程。有6种exec函数可供使用，包括：exxecl、execv、execle、execve、execlp、execvp，但其中只有execve是内核的系统调用

关系如图：



下面写了两个程序来尝试下这些函数：(参考自：apue)

apue.h

/* Our own header, to be included before all standard system headers */

#ifndef    _APUE_H
#define    _APUE_H

#if defined(SOLARIS)
#define _XOPEN_SOURCE    500    /* Single UNIX Specification, Version 2  for Solaris 9 */
#define CMSG_LEN(x)    _CMSG_DATA_ALIGN(sizeof(struct cmsghdr)+(x))
#elif !defined(BSD)
#define _XOPEN_SOURCE    600    /* Single UNIX Specification, Version 3 */
#endif

#include <sys/types.h>        /* some systems still require this */
#include <sys/stat.h>
#include <sys/termios.h>    /* for winsize */
#ifndef TIOCGWINSZ
#include <sys/ioctl.h>
#endif
#include <stdio.h>        /* for convenience */
#include <stdlib.h>        /* for convenience */
#include <stddef.h>        /* for offsetof */
#include <string.h>        /* for convenience */
#include <unistd.h>        /* for convenience */
#include <signal.h>        /* for SIG_ERR */

#define    MAXLINE    4096            /* max line length */

/*
* Default file access permissions for new files.
*/
#define    FILE_MODE    (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)

/*
* Default permissions for new directories.
*/
#define    DIR_MODE    (FILE_MODE | S_IXUSR | S_IXGRP | S_IXOTH)

typedef    void    Sigfunc(int);    /* for signal handlers */

#if    defined(SIG_IGN) && !defined(SIG_ERR)
#define    SIG_ERR    ((Sigfunc *)-1)
#endif

#define    min(a,b)    ((a) < (b) ? (a) : (b))
#define    max(a,b)    ((a) > (b) ? (a) : (b))

/*
* Prototypes for our own functions.
*/
char    *path_alloc(int *);                /* {Prog pathalloc} */
long     open_max(void);                /* {Prog openmax} */
void     clr_fl(int, int);                /* {Prog setfl} */
void     set_fl(int, int);                /* {Prog setfl} */
void     pr_exit(int);                    /* {Prog prexit} */
void     pr_mask(const char *);            /* {Prog prmask} */
Sigfunc    *signal_intr(int, Sigfunc *);    /* {Prog signal_intr_function} */

int         tty_cbreak(int);                /* {Prog raw} */
int         tty_raw(int);                    /* {Prog raw} */
int         tty_reset(int);                /* {Prog raw} */
void     tty_atexit(void);                /* {Prog raw} */
#ifdef    ECHO    /* only if <termios.h> has been included */
struct termios    *tty_termios(void);        /* {Prog raw} */
#endif

void     sleep_us(unsigned int);            /* {Ex sleepus} */
ssize_t     readn(int, void *, size_t);        /* {Prog readn_writen} */
ssize_t     writen(int, const void *, size_t);    /* {Prog readn_writen} */
void     daemonize(const char *);            /* {Prog daemoninit} */

int         s_pipe(int *);                    /* {Progs streams_spipe sock_spipe} */
int         recv_fd(int, ssize_t (*func)(int,
const void *, size_t));/* {Progs recvfd_streams recvfd_sockets} */
int         send_fd(int, int);                /* {Progs sendfd_streams sendfd_sockets} */
int         send_err(int, int,
const char *);        /* {Prog senderr} */
int         serv_listen(const char *);        /* {Progs servlisten_streams servlisten_sockets} */
int         serv_accept(int, uid_t *);        /* {Progs servaccept_streams servaccept_sockets} */
int         cli_conn(const char *);        /* {Progs cliconn_streams cliconn_sockets} */
int         buf_args(char *, int (*func)(int,
char **));            /* {Prog bufargs} */

int         ptym_open(char *, int);    /* {Progs3 ptyopen_streams ptyopen_bsd ptyopen_linux} */
int         ptys_open(char *);            /* {Progs3 ptyopen_streams ptyopen_bsd ptyopen_linux} */
#ifdef    TIOCGWINSZ
pid_t     pty_fork(int *, char *, int, const struct termios *,
const struct winsize *);        /* {Prog ptyfork} */
#endif

int        lock_reg(int, int, int, off_t, int, off_t); /* {Prog lockreg} */
#define    read_lock(fd, offset, whence, len) \
lock_reg((fd), F_SETLK, F_RDLCK, (offset), (whence), (len))
#define    readw_lock(fd, offset, whence, len) \
lock_reg((fd), F_SETLKW, F_RDLCK, (offset), (whence), (len))
#define    write_lock(fd, offset, whence, len) \
lock_reg((fd), F_SETLK, F_WRLCK, (offset), (whence), (len))
#define    writew_lock(fd, offset, whence, len) \
lock_reg((fd), F_SETLKW, F_WRLCK, (offset), (whence), (len))
#define    un_lock(fd, offset, whence, len) \
lock_reg((fd), F_SETLK, F_UNLCK, (offset), (whence), (len))

pid_t    lock_test(int, int, off_t, int, off_t);        /* {Prog locktest} */

#define    is_read_lockable(fd, offset, whence, len) \
(lock_test((fd), F_RDLCK, (offset), (whence), (len)) == 0)
#define    is_write_lockable(fd, offset, whence, len) \
(lock_test((fd), F_WRLCK, (offset), (whence), (len)) == 0)

void    err_dump(const char *, ...);        /* {App misc_source} */
void    err_msg(const char *, ...);
void    err_quit(const char *, ...);
void    err_exit(int, const char *, ...);
void    err_ret(const char *, ...);
void    err_sys(const char *, ...);

void    log_msg(const char *, ...);            /* {App misc_source} */
void    log_open(const char *, int, int);
void    log_quit(const char *, ...);
void    log_ret(const char *, ...);
void    log_sys(const char *, ...);

void    TELL_WAIT(void);        /* parent/child from {Sec race_conditions} */
void    TELL_PARENT(pid_t);
void    TELL_CHILD(pid_t);
void    WAIT_PARENT(void);
void    WAIT_CHILD(void);

#endif    /* _APUE_H */

echoall.c

#include "apue.h"

int main(int argc,char *argv[])
{
int i;
char **ptr;
extern char **environ;

for(i = 0 ; i < argc; i++)
printf("argv[%d]:%s\n",i,argv[i]);

for(ptr = environ; *ptr != 0; ptr++)
printf("%s\n",*ptr);

exit(0);
}

exec_test1.c

#include "apue.h"
#include <sys/wait.h>

char *env_init[] = { "USER=unkown" , "PATH=/tmp" , NULL};

int main(void)
{
pid_t pid;

if((pid = fork()) < 0){
perror("fork error");
//err_sys("fork error");
}else if(pid == 0){
if(execle("/home/hadoop/exec_test/echoall","echoall","myargl","MY ARG2",(char *)0, env_init) < 0)
perror("execle error");
//err_sys("execle error");
}

if(waitpid(pid,NULL,0) < 0)
perror("wait error");
//err_sys("wait error");

if((pid = fork()) < 0){
perror("wait error");
//err_sys("wait error");
}else if(pid == 0){
if(execlp("echoall","echoall","only 1 arg",(char *)0) < 0)
perror("execlp error");
//err_sys("execlp error");
}

exit(0);
}