struct flock 结构体详解及用法

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功能：定义一些文件的锁的选项

Description

The flock structure in the /usr/include/sys/flock.h file, which describes a lock, contains the following fields:

`l_type`	Describes the type of lock. If the value of the Command parameter to the fcntl subroutine is F_SETLK orF_SETLKW, the `l_type` field indicates the type of lock to be created. Possible values are: F_RDLCK A read lock is requested. F_WRLCK A write lock is requested. F_UNLCK Unlock. An existing lock is to be removed. If the value of the Command parameter to the fcntl subroutine is F_GETLK, the `l_type`field describes an existing lock. Possible values are: F_RDLCK A conflicting read lock exists. F_WRLCK A conflicting write lock exists. F_UNLCK No conflicting lock exists.
`l_whence`	Defines the starting offset. The value of this field indicates the point from which the relative offset, the `l_start`field, is measured. Possible values are: SEEK_SET The relative offset is measured from the start of the file. SEEK_CUR The relative offset is measured from the current position. SEEK_END The relative offset is measured from the end of the file. These values are defined in the unistd.h file.
`l_start`	Defines the relative offset in bytes, measured from the starting point in the `l_whence` field.
`l_len`	Specifies the number of consecutive bytes to be locked.
`l_sysid`	Contains the ID of the node that already has a lock placed on the area defined by the fcntlsubroutine. This field is returned only when the value of the Command parameter isF_GETLK.
`l_pid`	Contains the ID of a process that already has a lock placed on the area defined by thefcntl subroutine. This field is returned only when the value of the Command parameter isF_GETLK.
`l_vfs`	Specifies the file system type of the node identified in the `l_sysid` field.

举例说明：在嵌入式编程中经常会遇到写配置文件的问题，这个时候由于多进程操作就需要跟配置文件加写锁操作。

int cfg_save()
{int fd = -1;struct flock lock;struct stat stat_buf;char *p = NULL;char description[200] = {0};fd = open(cfg_file, O_WRONLY | O_CREAT | O_TRUNC, 0666);if (fd == -1){sprintf(description,"open error %s.", cfg_file);write_log(time(NULL), SER_ERR, CFG_LOG, "cfg_func", "cfg_save",description);return -1;}/* Set a write lock */lock.l_type = F_WRLCK;lock.l_start = 0;lock.l_whence = SEEK_SET;lock.l_len = 0;if (fcntl(fd, F_SETLKW, &lock) == -1 || fstat(fd, &stat_buf) == -1) {close(fd);fprintf(stderr,"lock cfg_file error\n");return -1;}save_to_tree();if(tree == NULL){fprintf(stderr,"cfg_save  error\n");write_log(time(NULL), SER_ERR, CFG_LOG, "cfg_func", "cfg_save","cfg_save  error.");close(fd);return -1;}mxmlSaveFd(tree, fd, whitespace_cb);//把值写入配置文件if (tree!=NULL) {mxmlDelete(tree);tree = NULL;}close(fd);return 0;
}

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