[英]reading strings from a file in c
I am reading a string from a file in C
. 我正在从
C
的文件读取字符串。 The string is supposed to have a specific length and start with thisisnumbr
. 该字符串应具有特定长度,并以
thisisnumbr
。 If both requirements are met, then something else is supposed to happen. 如果同时满足这两个要求,那么应该发生其他事情。 Furthermore, I want to prevent that anything unexpected in the file might cause a crash.
此外,我想防止文件中任何意外内容都可能导致崩溃。
My code: 我的代码:
#define MYSTRING "thisisnumb-"
void read_mystring()
{
int c, i = 0, len =0 ;
char input[sizeof( MYSTRING)+2] ;
char check[] = MYSTRING ;
FILE *file ;
file = fopen("/pathto/myfile", "r") ;
if (file) {
while ((c = getc(file)) != EOF)
{
input[i] = c ;
i++ ;
if (i > sizeof(input))
{
len = 1 ;
break ;
}
}
fclose(file) ;
}
if(strncmp(input,check,sizeof(check)-1) == 0 && len == 0)
{
//do something
}
}
So input
has the size of MYSTRING
plus 2 more characters (supposed to be 2 digits. 因此,
input
的大小为MYSTRING
另加2个字符(假定为2位数字)。
In the while
loop I am reading myfile
and storing it in input
. 在
while
循环中,我正在读取myfile
并将其存储在input
。 With 同
if (i > sizeof(input))
{
len = 1 ;
break ;
}
I make sure that the string reading stops if the string in the file appears to be longer than expected. 如果文件中的字符串看起来比预期的长,我确保字符串读取停止。
Then I compare the beginning of the string with strncmp
and check if len==0
to make sure the string starts with MYSTRING
AND also has the correct length. 然后,我将字符串的开头与
strncmp
进行比较,并检查len==0
是否确保字符串以MYSTRING
开头并且长度也正确。
If so, something else happens. 如果是这样,还会发生其他情况。
This works, meaning that I don't get an Segmentation fault if there is no file, the string in file is too long, or the string in the file doesn't start with MYSTRING
. 这行得通,这意味着如果没有文件,文件中的字符串太长或文件中的字符串不是以
MYSTRING
开头,则不会出现MYSTRING
。
I was wondering, if there is anything else that might break my program? 我在想,是否还有其他事情可能会破坏我的程序?
And also, when I do printf("input=%s\\n",input)
at the end of my function, I get my string but also an additional line with garbage? 而且,当我在
printf("input=%s\\n",input)
末尾执行printf("input=%s\\n",input)
时,我得到了我的字符串,而且还有一行带有垃圾的内容?
Any ideas? 有任何想法吗?
There are a number of things you need to look at. 您需要看很多东西。 Foremost
sizeof MYSTRING
includes the storage size required for the nul-byte . sizeof MYSTRING
最主要sizeof MYSTRING
包括nul-byte所需的存储大小。 It is strlen + 1
. 它是
strlen + 1
。 You must be very careful mixing sizeof string
(on a char array) and string length . 您必须非常小心地将
sizeof string
(在char数组上)和string length混合使用 。
Next, if you call this function more than once throughout your code, it may be better to fopen
the file in the caller and pass a FILE*
parameter to your function. 接下来,如果您在整个代码中多次调用此函数,则最好在调用程序中
fopen
文件,然后将FILE*
参数传递给函数。 (it's up to you) I would do: (由您决定)我会做:
/* open file in caller to prevent repeatedly opening and closing file */
FILE *fp = fopen (fname, "r");
if (!fp) { /* validate file open for reading */
fprintf (stderr, "error: file open failed '%s'.\n", fname);
exit (EXIT_FAILURE);
}
Next, there are many ways to handle your function itself. 接下来,有很多方法可以处理您的函数本身。 As mentioned in the comment, you are better served by providing a buffer large enough to handle long strings in the file (and even then you need to validate a complete line read occurred) Reading with
fgets
the '\\n'
is read and included in the resulting buffer, so you will need to remove the trailing '\\n'
by overwriting with a nul-byte , eg 如评论中所述,最好提供一个足够大的缓冲区来处理文件中的长字符串(即使这样,您仍需要验证是否发生了完整的行读取),才能更好地为您服务。用
fgets
读取并读入'\\n'
结果缓冲区,因此您需要通过用nul-byte覆盖来删除结尾的'\\n'
,例如
char buf[BUFSZ] = "";
while (fgets (buf, BUFSZ, fp)) {
size_t len = strlen (buf);
if (len > 0 && buf[len - 1] == '\n')
buf[--len] = 0;
else {
/* handle more chars remain in line than buf can hold */
}
After validating your line read, you simply need to check the length against your requirement and then check that the last two characters are digits , eg 验证行读取之后,您只需要根据需要检查长度,然后检查最后两个字符是否为数字 ,例如
if (len != sizeof MYSTRING + 1) {
/* not right length - handle error */
}
if (strncmp (buf, MYSTRING, sizeof MYSTRING - 1) == 0 &&
isdigit (buf[sizeof MYSTRING - 1]) &&
isdigit (buf[sizeof MYSTRING]))
{
/* string matches criteria -- do something */
}
else {
/* doesn't meet conditon -- handle error */
}
Putting it altogether, and adding a moretoread
flag to read until the end of a long line if it exceeds BUFSZ
, you would have something similar to the following: 放在
BUFSZ
,如果超过BUFSZ
,则添加一个moretoread
标志以读取直到长行的BUFSZ
,您将具有类似于以下内容:
void read_mystring (FILE *fp)
{
char buf[BUFSZ] = "";
int moretoread = 0;
while (fgets (buf, BUFSZ, fp)) {
size_t len = strlen (buf);
if (len > 0 && buf[len - 1] == '\n') { /* check for newline */
buf[--len] = 0; /* overwrite with nul-byte */
moretoread = 0; /* reset moretoread flag */
}
else {
/* handle more chars remain in line than buf can hold */
moretoread = 1;
}
if (moretoread) /* you are way over your wanted length */
continue; /* just read until newline encountered */
if (len != sizeof MYSTRING + 1) {
/* not right length - handle error */
}
/* check prefix followed by two digits */
if (strncmp (buf, MYSTRING, sizeof MYSTRING - 1) == 0 &&
isdigit (buf[sizeof MYSTRING - 1]) &&
isdigit (buf[sizeof MYSTRING]))
{
/* string matches criteria -- do something */
}
else {
/* doesn't meet conditon -- handle error */
}
}
}
Include ctype.h
for isdigit()
. 为
isdigit()
包括ctype.h
。
Like I said, there are many, many different approaches you can take, these are just thoughts based on your conditions and one way of doing it. 就像我说的,您可以采用许多不同的方法,这些只是基于您的条件和一种实现方式的想法。
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