我正在写一些C代码来处理文件中的数据，但我刚刚得知该文件将不断地添加内容(大约每秒钟1次，也可能更快)。因此，我想知道如何在文件被添加时继续读取它。当我读到文件结尾时，等待下一行被添加，然后再处理它。然后再等待并处理，如此往复。我的代码类似于：
while(1){
fgets(line, sizeof(line), file);
while(line == NULL){
//wait ? then try to read again?
}
//tokenize line and do my stuff here
}
我曾尝试使用inotify，但是一无所获。有没有人能提供一些建议？
read()系统调用。inotify
以下代码可能会对您有所帮助，在Debian 7.0，GCC 4.7上运行良好：/*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 );
}
/tmp/test_inotify的文件或目录来测试它。read系统调用read()系统调用将返回0。如果某个写入者随后向此文件写入了N字节，那么read()将返回min(N, buffersize)。#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;
}
read()的问题在于当读取位置到达文件末尾时，它会立即返回0。因此，在尝试新的读取之前，您必须进行某种等待。这与在管道、套接字或FIFO上的读取形成对比，后者将阻塞等待更多数据到达。inotify信息可能是处理它的最佳方式。或者循环和睡眠... - Jonathan Leffler#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;
}
select()的定义是：与常规文件相关联的文件描述符应始终选择准备好读取、准备好写入和错误条件。 - Jonathan Leffler使用 select 可以是一个不错的选择，但如果您不想使用它，可以在读取值之前添加一小段毫秒级的延迟。
select()的定义是：与常规文件相关联的文件描述符应始终选择准备好读取、准备好写入和错误条件。 - Jonathan Leffler
while (fgets(line, sizeof(line), file) != 0) { process(line); } ...nanosleep?...; clearerr(file);（然后它将循环执行while(1)循环以尝试进行下一次读取）。 - Jonathan Lefflerclearerr(file);会清除流中的EOF和错误位，以便它将尝试从文件中读取更多数据（而无需关闭、重新打开和重新定位文件中的当前位置）。 - Jonathan Leffler