我正在编写一些C代码来处理文件中的某些数据，但我刚刚了解到该文件将不断添加到（大约1次/秒，可能更快）。所以我想知道如何在添加文件时继续读取文件。然后当我到达最后，等到下一行被添加然后处理它。然后再等一下再进行处理，依此类推。我有类似的东西：
while(1){
fgets(line, sizeof(line), file);
while(line == NULL){
//wait ? then try to read again?
}
//tokenize line and do my stuff here
}
我以为我可以使用inotify，但我无处可去。有没有人有任何建议？
答案 0 :(得分：3)
最有效的方法是使用inotify，直接方法是直接使用read()系统调用。
inotify 以下代码可能会给你一些帮助，它适用于Debian 7.0，GCC 4.7：
/*This is the sample program to notify us for the file creation and file deletion takes place in “/tmp/test_inotify” file*/
// Modified from: http://www.thegeekstuff.com/2010/04/inotify-c-program-example/
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/inotify.h>
#define EVENT_SIZE ( sizeof (struct inotify_event) )
#define EVENT_BUF_LEN ( 1024 * ( EVENT_SIZE + 16 ) )
int main( )
{
int length, i = 0;
int fd;
int wd;
char buffer[EVENT_BUF_LEN];
/*creating the INOTIFY instance*/
fd = inotify_init();
/*checking for error*/
if ( fd < 0 ) {
perror( "inotify_init error" );
}
/* adding the “/tmp/test_inotify” test into watch list. Here,
* the suggestion is to validate the existence of the
* directory before adding into monitoring list.
*/
wd = inotify_add_watch( fd, "/tmp/test_inotify", IN_CREATE | IN_DELETE | IN_ACCESS | IN_MODIFY | IN_OPEN );
/* read to determine the event change happens on “/tmp/test_inotify” file.
* Actually this read blocks until the change event occurs
*/
length = read( fd, buffer, EVENT_BUF_LEN );
/* checking for error */
if ( length < 0 ) {
perror( "read" );
}
/* actually read return the list of change events happens.
* Here, read the change event one by one and process it accordingly.
*/
while ( i < length ) {
struct inotify_event *event = ( struct inotify_event * ) &buffer[ i ];
if( event->len == 0) {
// For a single file watching, the event->name is empty, and event->len = 0
printf(" Single file watching event happened\n");
} else if ( event->len ) {
if ( event->mask & IN_CREATE ) {
if ( event->mask & IN_ISDIR ) {
printf( "New directory %s created.\n", event->name );
} else {
printf( "New file %s created.\n", event->name );
}
} else if ( event->mask & IN_DELETE ) {
if ( event->mask & IN_ISDIR ) {
printf( "Directory %s deleted.\n", event->name );
} else {
printf( "File %s deleted.\n", event->name );
}
} else if( event->mask & IN_ACCESS ) {
if ( event->mask & IN_ISDIR ) {
printf( "Directory %s accessed.\n", event->name );
} else {
printf(" File %s accessed. \n", event->name );
}
} else if( event->mask & IN_MODIFY ) {
if ( event->mask & IN_ISDIR ) {
printf( "Directory %s modified.\n", event->name );
} else {
printf(" File %s modified. \n", event->name );
}
} else if( event->mask & IN_OPEN ) {
if ( event->mask & IN_ISDIR ) {
printf( "Directory %s opened.\n", event->name );
} else {
printf(" File %s opened. \n", event->name );
}
} else {
printf( "Directory or File is accessed by other mode\n");
}
}
i += EVENT_SIZE + event->len;
}
/* removing the “/tmp/test_inotify” directory from the watch list. */
inotify_rm_watch( fd, wd );
/* closing the INOTIFY instance */
close( fd );
}
运行上述程序时。您可以通过创建名为/tmp/test_inotify的文件或指南来测试它。
可以找到详细解释here
read系统调用如果文件已打开，并且已读取到当前文件大小的末尾。 read()系统调用将返回0。如果某个作者稍后将N字节写入此文件，则read()将返回min(N, buffersize)。
因此它适用于您的情况。以下是代码示例。
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
typedef int FD ;
int main() {
FD filed = open("/tmp/test_inotify", O_RDWR );
char buf[128];
if( !filed ) {
printf("Openfile error\n");
exit(-1);
}
int nbytes;
while(1) {
nbytes = read(filed, buf, 16);
printf("read %d bytes from file.\n", nbytes);
if(nbytes > 0) {
split_buffer_by_newline(buf); // split buffer by new line.
}
sleep(1);
}
return 0;
}
答案 1 :(得分：0)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
int
main()
{
char ch;
FILE *fp;
long int nbytes_read = 0;
char str [128];
int j = 0;
int first_time = 1;
memset(str, '\0', 128);
fp = fopen("file.txt", "r");
while (1) {
if (first_time != 1) {
fp = fopen("file.txt", "r");
fseek(fp, nbytes_read, SEEK_SET);
sleep(10);
}
if (fp != NULL) {
while ((ch = fgetc(fp)) != EOF) {
if (ch == '\n') {
str[j++] = ch;
printf("%s", str);
memset(str, '\0', 128);
j = 0;
} else {
str[j++] = ch;
}
nbytes_read++;
}
//printf("%ld\n", nbytes_read);
first_time = 0;
}
fclose(fp);
}
return 0;
}
答案 2 :(得分：-1)
您可以将select()与fileno(file)一起用作文件描述符。 select将返回超时（如果您设置超时）或者您可以从文件中读取。
答案 3 :(得分：-1)
使用select可能是一个不错的选择，但如果您不想使用它，您可以在读取值之前添加一个小睡眠时间。