read length of string from stdin [duplicate]

问题:

This question already has an answer here:

• How to read a line from the console in C? 13 answers

I want to take a string from stdin but I don't want a static array of fixed size

I know that scanf need something where save the stdin input, but I can't do something like this:

char string[10]
scanf("%s",string);

becouse I need to know before how long will be the string in order to allocate the right memory space.

Can you help me to resolve this problem?

---

woooooooo

i'm still locked with this problem... I'm going mad

can you please give me the working code?

回答1:

The only way to be sure is to do a loop; read one character at a time and store. If your allocated buffer becomes full, grow it by some suitable amount (more than one byte at a time is recommended for performance, a classic rule-of-thumb is to double it).

Stop when you consider the string to end, perhaps at line feed or EOF.

回答2:

Do not use scanf, use fgets, which prevents too many characters from being read in.

char tmp[256]={0x0};
while(fgets(tmp, sizeof(tmp), stdin)!=NULL)
{
    printf("%s", tmp);
}

fgets will return a '\n' at the end of the line, NULL when stdin closes or encounters an error.

Play with it first to see what it does. If you need to break the input line into fields you can use sscanf() on tmp, sscanf works just like scanf, but on strings.

回答3:

If you use fgets() then you would most likely have a '\n' character at the end in the buffer if it is sufficienly large. Some may orefer to remove it via strtok() function but it is nit safe.

I suggest the following safe way to remove that extra special character.

char *get_line (char *s, size_t n, FILE *f)
{
  char *p = fgets (s, n, f);

  if (p != NULL) strtok (s, "\n");
  return p;
}

回答4:

You could process the input data char by char using:

int getc(FILE *stream);

or

int getchar(void);

回答5:

You may use the following format string:

char string[10];
scanf("%10s" string);

This forces the buffer boundaries to be respected, but requires the format string to be aware of the size of the buffer. I usually overcome this by declaring both as constants:

#define BUFSIZE 10
#define BUFFMT "%10s"

If you are working on a GNU system, you can also take advantage from the GNU extensions: the getline(3) function can avoid you a lot of headaches! Have a look here.

回答6:

You can ensure you don't over-run your buffer like this:

   char buffer[128];
   printf("Input Text: ");
   fgets(buffer,127,stdin);

Then you just keep getting the same fixed size amount of input if you need variable size of input

回答7:

If you don't want a static array of fixed size, the consider using a dynamically allocated array which grows as needed.

If you work on Linux or other systems with POSIX 2008 support, then you can use the (newer) getline() function. If you don't have access to such a function, consider rolling your own using the same interface.