文件锁定+;Fscanf/Lseek
我有一个名为“input.txt”的文件,其中包含一些值。我正在编写一个程序,该程序将在该文件中找到最小值,并将该最小值替换为作为命令行参数给出的数字——如果该命令行参数大于最小值。这些值表示室温,因此可以使用事实来确定最低温度。此外,需要锁定文件的该部分(新编号替换最小值) 示例:文件锁定+;Fscanf/Lseek,c,locking,file-descriptor,lseek,C,Locking,File Descriptor,Lseek,我有一个名为“input.txt”的文件,其中包含一些值。我正在编写一个程序,该程序将在该文件中找到最小值,并将该最小值替换为作为命令行参数给出的数字——如果该命令行参数大于最小值。这些值表示室温,因此可以使用事实来确定最低温度。此外,需要锁定文件的该部分(新编号替换最小值) 示例: $ ./prog 23 档案:2125192224 档案:2125232224 档案:2125232224 档案:2925232224 代码: #include <stdio.h> #include
$ ./prog 23
档案:2125192224
档案:2125232224
档案:2125232224
档案:2925232224
代码:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <stdint.h>
/* Function that handles errors */
void fatal_error(const char *message){
perror(message);
exit(EXIT_FAILURE);
}
int main(int argc, char **argv){
if(argc != 2)
fatal_error("Bad arguments.\n");
/* Fetching command line argument */
int temp = atoi(argv[1]);
/* Opening file and checking for errors */
FILE *file = fopen("input.txt", "r+");
if(!file)
fatal_error("Unable to open file.\n");
/* Finding minimum in the file */
int min = 200;
int value;
while(fscanf(file, "%d", &value) != EOF)
if(value < min)
min = value;
/* Exiting if nothing needs to change */
if(temp <= min)
return 0;
/* Creating file descriptor from stream and checking for errors */
int fdOpen = fileno(file);
if(fdOpen == -1)
fatal_error("Unable to open file descriptor.\n");
/* Moving offset to the beginning of the file */
off_t of = lseek(fdOpen,0,SEEK_SET);
printf("Ofset pre petlje: %jd\n", (intmax_t)of);
while(1){
/* I'm reading file all over again */
if(fscanf(file, "%d", &value) == EOF)
fatal_error("Reached end of file.\n");
/* If I reached minimum */
if(value == min){
/* I lock that part of the file - temperatures are two digit numbers*/
struct flock lock;
lock.l_type = F_WRLCK;
lock.l_whence = SEEK_CUR;
lock.l_start = -2;
lock.l_len = 2;
/* I create lock */
if(fcntl(fdOpen,F_SETLK, &lock) == -1){
if(errno == EACCES || errno == EAGAIN)
fatal_error("File is locked.\n");
}
/* Moving two positions back from current position */
off_t offset;
if((offset = lseek(fdOpen, -2, SEEK_CUR)) == -1)
fatal_error("lseek error.\n");
/* Inserting read command line value into file */
fprintf(file, "%d", temp);
/* Unlocking */
lock.l_type = F_UNLCK;
if(fcntl(fdOpen, F_SETLK, &lock) == -1)
fatal_error("Unable to destroy lock.\n");
/* Closing file descriptor, and breaking loop */
close(fdOpen);
break;
}
}
return 0;
}
正如你所看到的,这是完全相同的概念。第二个程序有效,第一个程序无效
我现在很困惑。若我从文件流创建文件描述符,然后在该文件描述符上使用
lseek()
来更改偏移量,那个么流的偏移量也会更改吗?如果您使用fscanf()
从流中读取某些内容,那么offset\u t
的更改是否与从文件中读取的内容相同?如果我将fscanf()
与格式说明符%d
和%s
一起使用,在更改偏移量方面是否有任何区别 只有当两个(源文本和替换文本)在您的case length(aa)=length(bb)中具有相同的长度时,您想要替换值的方式才会起作用。主要是您应该小心使用文件*
和int fd
描述符,并始终在退出之前关闭文件
在fclose(f)
之前调用close(fd)
将导致缓冲数据不被写入
另一个问题-相对于SEEK_CUR锁定文件区域不会锁定要修改的文件部分
在这里,您对代码进行了一些修改,可以正常工作:
int main(int argc, char **argv){
if(argc != 4)
fatal_error("You must enter exactly 3 arguments.\n");
if(strlen(argv[2])!=strlen(argv[3]))
fatal_error("src&dst words must be the length.\n");
FILE *f = fopen(argv[1], "r+");
if(!f)
fatal_error("Unable to open file for reading and writing.\n");
int fd = fileno(f);
if(fd == -1)
fatal_error("Unable to fetch file descriptor for file.\n");
char word[MAX_LEN + 1];
int word_len = strlen(argv[2]);
while(fscanf(f,"%s",word) != EOF){
printf("%jd\n", ftell(f));
if(!strcmp(argv[2],word)){
struct flock lock;
lock.l_type = F_WRLCK;
lock.l_whence = SEEK_SET;
lock.l_start = ftell(f)-word_len;
lock.l_len = word_len;
if(fcntl(fd, F_SETLKW, &lock) == -1)
fatal_error("File locking failed.\n");
fseek(f,-word_len,SEEK_CUR); //FILE* based seek
fprintf(f, "%s", argv[3]);
fflush(f); //sync output
lock.l_type = F_UNLCK;
if(fcntl(fd, F_SETLK, &lock) == -1)
fatal_error("Failed to release lock.\n");
}
}
fclose(f); // close the file
}
Update1:文件
接口有自己的缓冲区,与int fd
不同步。
因此,主要的问题是使用lseek,而应该使用fseek
Update2:使用循环查找最小值的代码
int main(int argc, char **argv){
if(argc != 3)
fatal_error("You must enter exactly 2 arguments.\n");
if(strlen(argv[2]) != 2)
fatal_error("replace num must have 2 digits.\n");
FILE *f = fopen(argv[1], "r+");
if(!f)
fatal_error("Unable to open file for reading and writing.\n");
int fd = fileno(f);
if(fd == -1)
fatal_error("Unable to fetch file descriptor for file.\n");
// search for minimum
int word_len = strlen(argv[2]);
int value, minValue;
long minOffs=-1;
while(fscanf(f,"%d",&value) == 1){ //compare number of parsed items
printf("test value %d\n", value);
if (minValue > value) {
minValue = value;
minOffs = ftell(f) - word_len;
}
}
// replace if found
if (minOffs >= 0) {
printf("replacing v=%d at %ld\n", minValue, minOffs);
struct flock lock;
memset(&lock, 0, sizeof(lock));
lock.l_type = F_WRLCK;
lock.l_whence = SEEK_SET;
lock.l_start = minOffs;
lock.l_len = word_len;
fseek(f,minOffs,SEEK_SET);
if(fcntl(fd, F_SETLK, &lock) == -1)
fatal_error("File locking failed.\n");
fprintf(f, "%s", argv[2]);
fflush(f); //sync output
lock.l_type = F_UNLCK;
if(fcntl(fd, F_SETLK, &lock) == -1)
fatal_error("Failed to release lock.\n");
}
fclose(f);
}
您修改了已经运行的代码。第一个代码就是问题所在。我不明白为什么它不起作用,而第二个却起作用。“只有当两个(源文本和替换文本)的长度相同时,替换值的方法才会起作用。”-这根本不是真的,我可以锁定整个文件(如果我想的话)并替换其中的一小部分。如果文件内容“ab cd ef”将“cd”替换为“xyz”,会发生什么?我的意思是内容将是腐败者。如果替换代码的长度不是源代码,那么您必须将文件重写到最后。正如我所说的,我了解第二个代码的作用,以及它何时起作用,何时不起作用。假设您可以用另一个两个字母的单词替换两个字母的单词。第二个代码可以工作,因为您没有关闭
fdOpen
。当您添加这样的数据时(如您在第一个代码中所做的),行为将是相同的-文件不会更改,因为使用fprintf
写入的数据实际上在fflush
或fclose
之前不会写入。请指定数字(室温)的约束条件,它们是否始终为2位正整数?这可以大大简化问题。文件中只有一个最小值。如果存在多个最小值,请指定场景。是否用fseek替换了第一个lseek?在那里使用lseek时,不清除第一个循环中的EOF似乎是个问题。
int main(int argc, char **argv){
if(argc != 4)
fatal_error("You must enter exactly 3 arguments.\n");
if(strlen(argv[2])!=strlen(argv[3]))
fatal_error("src&dst words must be the length.\n");
FILE *f = fopen(argv[1], "r+");
if(!f)
fatal_error("Unable to open file for reading and writing.\n");
int fd = fileno(f);
if(fd == -1)
fatal_error("Unable to fetch file descriptor for file.\n");
char word[MAX_LEN + 1];
int word_len = strlen(argv[2]);
while(fscanf(f,"%s",word) != EOF){
printf("%jd\n", ftell(f));
if(!strcmp(argv[2],word)){
struct flock lock;
lock.l_type = F_WRLCK;
lock.l_whence = SEEK_SET;
lock.l_start = ftell(f)-word_len;
lock.l_len = word_len;
if(fcntl(fd, F_SETLKW, &lock) == -1)
fatal_error("File locking failed.\n");
fseek(f,-word_len,SEEK_CUR); //FILE* based seek
fprintf(f, "%s", argv[3]);
fflush(f); //sync output
lock.l_type = F_UNLCK;
if(fcntl(fd, F_SETLK, &lock) == -1)
fatal_error("Failed to release lock.\n");
}
}
fclose(f); // close the file
}
int main(int argc, char **argv){
if(argc != 3)
fatal_error("You must enter exactly 2 arguments.\n");
if(strlen(argv[2]) != 2)
fatal_error("replace num must have 2 digits.\n");
FILE *f = fopen(argv[1], "r+");
if(!f)
fatal_error("Unable to open file for reading and writing.\n");
int fd = fileno(f);
if(fd == -1)
fatal_error("Unable to fetch file descriptor for file.\n");
// search for minimum
int word_len = strlen(argv[2]);
int value, minValue;
long minOffs=-1;
while(fscanf(f,"%d",&value) == 1){ //compare number of parsed items
printf("test value %d\n", value);
if (minValue > value) {
minValue = value;
minOffs = ftell(f) - word_len;
}
}
// replace if found
if (minOffs >= 0) {
printf("replacing v=%d at %ld\n", minValue, minOffs);
struct flock lock;
memset(&lock, 0, sizeof(lock));
lock.l_type = F_WRLCK;
lock.l_whence = SEEK_SET;
lock.l_start = minOffs;
lock.l_len = word_len;
fseek(f,minOffs,SEEK_SET);
if(fcntl(fd, F_SETLK, &lock) == -1)
fatal_error("File locking failed.\n");
fprintf(f, "%s", argv[2]);
fflush(f); //sync output
lock.l_type = F_UNLCK;
if(fcntl(fd, F_SETLK, &lock) == -1)
fatal_error("Failed to release lock.\n");
}
fclose(f);
}