程序A创建子进程，并调用进程B，根据不调用的不同情况，最后显示结果不同。
程序A：
#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <errno.h>
int main() {
pid_t pid, rpid;
int stat;
if ((pid = fork()) < 0) {
perror("fork failue.");
}
printf("pid = %d\n", pid); // 打印子父进程的获得的返回值
if (pid == 0) {
int x = execl("./test123", "./test", NULL);
printf("x = %d\n", x); // execl调用失败返回-1
perror("execl failue"); // 直接使用perror函数输出错误字符串（相比数字，字符串更容易理解）
printf("%s\n", strerror(errno)); // 间接使用strerror输出错误字符串
} else {
rpid = waitpid(-1, &stat, 0); // 等待任意子进程结束
if (WIFEXITED(stat)) { // 状态字拥有24位，一些宏函数对状态字进行解释
printf("rpid = %d, stat = %d\n", rpid, WEXITSTATUS(stat));
}
}
return 0;
}
程序B：
#include <stdio.h>
int main(int argc, char **argv) {
printf("hello world.\n");
exit(0x3244); // return与exit均能正常返回，返回值为 int & 0xff
}
调用成功后：
liuxu@kylin:~/projects/webserver$ ./ptest
pid = 4723
pid = 0
hello world.
rpid = 4723, stat = 68
调用失败后：
liuxu@kylin:~/projects/webserver$ ./ptest
pid = 4738
pid = 0
x = -1
execl failue: No such file or directory
No such file or directory
rpid = 4738, stat = 0
PS：error码含义
errno.00 is: Success 成功
errno.01 is: Operation not permitted 不允许此类操作
errno.02 is: No such file or directory 没有此文件或目录
errno.03 is: No such process 没有此进程
errno.04 is: Interrupted system call 中断系统调用
errno.05 is: Input/output error 输入输出错误
errno.06 is: No such device or address 没有此设备或地址
errno.07 is: Argument list too long 参数过长
errno.08 is: Exec format error 执行格式错误
errno.09 is: Bad file descriptor 无效的文件描述符
errno.10 is: No child processes 不存在子进程
errno.11 is: Resource temporarily unavailable 资源暂时不可用
errno.12 is: Cannot allocate memory 分配内存失败
errno.13 is: Permission denied 权限错误
errno.14 is: Bad address 地址错误
errno.15 is: Block device required 块设备请求
errno.16 is: Device or resource busy 设备或资源忙
errno.17 is: File exists 文件已经存在
errno.18 is: Invalid cross-device link 无效的交叉连接设备
errno.19 is: No such device 没有此设备
errno.20 is: Not a directory 不是一个目录
errno.21 is: Is a directory 是一个目录
errno.22 is: Invalid argument 参数非法
errno.23 is: Too many open files in system 系统打开文件太多
errno.24 is: Too many open files 打开文件太多
errno.25 is: Inappropriate ioctl for device 设备部支持该操作
errno.26 is: Text file busy 文本文件忙
errno.27 is: File too large 文件太大
errno.28 is: No space left on device 设备没有空间，一般为存储设备
errno.29 is: Illegal seek 非法的seek操作
errno.30 is: Read-only file system 只读文件系统
errno.31 is: Too many links 太多连接
errno.32 is: Broken pipe 管道破裂
errno.33 is: Numerical argument out of domain 数值参数超出取值范围
errno.34 is: Numerical result out of range 数值结果超出取值范围
errno.35 is: Resource deadlock avoided 资源死锁
errno.36 is: File name too long 文件名太长
errno.37 is: No locks available 没有可用的锁
errno.38 is: Function not implemented 函数没有执行
errno.39 is: Directory not empty 目录非空
errno.40 is: Too many levels of symbolic links 太多级符号连接
errno.41 is: Unknown error 41 41未知错误
errno.42 is: No message of desired type 不被接受的消息类型
errno.43 is: Identifier removed 标识符已被删除
errno.44 is: Channel number out of range 。。。
errno.45 is: Level 2 not synchronized
errno.46 is: Level 3 halted
errno.47 is: Level 3 reset
errno.48 is: Link number out of range
errno.49 is: Protocol driver not attached
errno.50 is: No CSI structure available
errno.51 is: Level 2 halted
errno.52 is: Invalid exchange
errno.53 is: Invalid request descriptor
errno.54 is: Exchange full
errno.55 is: No anode
errno.56 is: Invalid request code
errno.57 is: Invalid slot
errno.58 is: Unknown error 58
errno.59 is: Bad font file format
errno.60 is: Device not a stream
errno.61 is: No data available
errno.62 is: Timer expired
errno.63 is: Out of streams resources
errno.64 is: Machine is not on the network
errno.65 is: Package not installed
errno.66 is: Object is remote
errno.67 is: Link has been severed
errno.68 is: Advertise error
errno.69 is: Srmount error
errno.70 is: Communication error on send
errno.71 is: Protocol error
errno.72 is: Multihop attempted
errno.73 is: RFS specific error
errno.74 is: Bad message
errno.75 is: Value too large for defined data type
errno.76 is: Name not unique on network
errno.77 is: File descriptor in bad state
errno.78 is: Remote address changed
errno.79 is: Can not access a needed shared library
errno.80 is: Accessing a corrupted shared library
errno.81 is: .lib secion in a.out corrupted
errno.82 is: Attempting to link in too many shared libraries
errno.83 is: Cannot exec a shared library directly
errno.84 is: Invalid or incomplete multibyte or wide character
errno.85 is: Interrupted system call should be restarted
errno.86 is: Streams pipe error
errno.87 is: Too many users
errno.88 is: Socket operation on non-socket
errno.89 is: Destination address required
errno.90 is: Message too long
errno.91 is: Protocol wrong type for socket
errno.92 is: Protocol not available
errno.93 is: Protocol not supported
errno.94 is: Socket type not supported
errno.95 is: Operation not supported
errno.96 is: Protocol family not supported
errno.97 is: Address family not supported by protocol
errno.98 is: Address already in use
errno.99 is: Cannot assign requested address
errno.100 is: Network is down
errno.101 is: Network is unreachable
errno.102 is: Network dropped connection on reset
errno.103 is: Software caused connection abort
errno.104 is: Connection reset by peer
errno.105 is: No buffer space available
errno.106 is: Transport endpoint is already connected
errno.107 is: Transport endpoint is not connected
errno.108 is: Cannot send after transport endpoint shutdown
errno.109 is: Too many references: cannot splice
errno.110 is: Connection timed out
errno.111 is: Connection refused
errno.112 is: Host is down
errno.113 is: No route to host
errno.114 is: Operation already in progress
errno.115 is: Operation now in progress
errno.116 is: Stale NFS file handle
errno.117 is: Structure needs cleaning
errno.118 is: Not a XENIX named type file
errno.119 is: No XENIX semaphores available
errno.120 is: Is a named type file
errno.121 is: Remote I/O error
errno.122 is: Disk quota exceeded
errno.123 is: No medium found
errno.124 is: Wrong medium type
errno.125 is: Operation canceled
errno.126 is: Required key not available
errno.127 is: Key has expired
errno.128 is: Key has been revoked
errno.129 is: Key was rejected by service
errno.130 is: Owner died
errno.131 is: State not recoverable
errno.132 is: Unknown error 132
132-255全是Unknown error