How can I read an input string of unknown length

Question

If I don t know how long the word is  I cannot write char m 6    The length of the word is maybe ten or twenty long  How can I use scanf to get input from the keyboard    include  lt stdio h gt  int main void        char  m 6       printf  please input a string with length 5 n        scanf   s   amp m       printf  this is the string   s n   m       return 0      please input a string with lenght 5 hello this is the string  hello

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There is a new function in C standard for getting a line without specifying its size  getline function allocates string with required size automatically so there is no need to guess about string s size  The following code demonstrate usage    include  lt stdio h gt   include  lt stdlib h gt    int main void        char  line   NULL      size t len   0      ssize t read       while   read   getline  amp line   amp len  stdin      -1            printf  Retrieved line of length  zu   n   read           printf   s   line              if  ferror stdin                handle error               free line       return 0

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i also have a solution with standard inputs and outputs   include lt stdio h gt   include lt malloc h gt  int main         char  str ch      int size 10 len 0      str realloc NULL sizeof char  size       if  str return str      while EOF  scanf   c   amp ch   amp  amp  ch    n                 str len    ch          if len  size                        str   realloc str sizeof char   size  10                if  str return str                      str len      0       printf   s n  str       free str

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If I may suggest a safer approach   Declare a  buffer big enough to hold the string   char user input 255    Get the user input in a safe way   fgets user input  255  stdin    A safe way to get the input  the first argument being a pointer to a buffer where the input will be stored  the second the maximum input the function should read and the third is a pointer to the standard input - i e  where the user input comes from   Safety in particular comes from the second argument limiting how much will be read which prevents buffer overruns  Also  fgets takes care of null-terminating the processed string   More info on that function here   EDIT  If you need to do any formatting  e g  convert a string to a number   you can use atoi once you have the input

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With the computers of today  you can get away with allocating very large strings  hundreds of thousands of characters  while hardly making a dent in the computer s RAM usage  So I wouldn t worry too much   However  in the old days  when memory was at a premium  the common practice was to read strings in chunks  fgets reads up to a maximum number of chars from the input  but leaves the rest of the input buffer intact  so you can read the rest from it however you like   in this example  I read in chunks of 200 chars  but you can use whatever chunk size you want of course    include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   char  readinput      define CHUNK 200    char  input   NULL     char tempbuf CHUNK      size t inputlen   0  templen   0     do          fgets tempbuf  CHUNK  stdin          templen   strlen tempbuf          input   realloc input  inputlen templen 1          strcpy input inputlen  tempbuf          inputlen    templen        while  templen  CHUNK-1  amp  amp  tempbuf CHUNK-2     n        return input     int main         char  result   readinput        printf  And the result is   s  n   result       free result       return 0      Note that this is a simplified example with no error checking  in real life you will have to make sure the input is OK by verifying the return value of fgets   Also note that at the end if the readinput routine  no bytes are wasted  the string has the exact memory size it needs to have

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Take a character pointer to store required string If you have some idea about possible size of string then use function  char  fgets  char  str  int size  FILE  file      else you can allocate memory on runtime too using malloc   function which dynamically provides requested memory

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Read directly into allocated space with fgets     Special care is need to distinguish a successful read  end-of-file  input error and out-of memory   Proper memory management needed on EOF   This method retains a line s   n     include  lt stdio h gt   include  lt stdlib h gt    define FGETS ALLOC N 128  char  fgets alloc FILE  istream      char  buf   NULL    size t size   0    size t used   0    do       size    FGETS ALLOC N      char  buf new   realloc buf  size       if  buf new    NULL             Out-of-memory       free buf         return NULL            buf   buf new      if  fgets  amp buf used    int   size - used   istream     NULL             feof or ferror       if  used    0    ferror istream             free buf           buf   NULL                return buf            size t length   strlen  amp buf used        if  length   1    size - used  break      used    length      while  buf used - 1       n      return buf      Sample usage  int main void      FILE  istream   stdin    char  s    while   s   fgets alloc istream      NULL        printf    s    s       free s       fflush stdout         if  ferror istream         puts  Input error        else if  feof istream         puts  End of file        else       puts  Out of memory          return 0

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I ve seen only one simple way of reading an arbitrarily long string  but I ve never used it   I think it goes like this   char  m   NULL  printf  please input a string n    scanf   ms   amp m   if  m    NULL      fprintf stderr   That string was too long  n    else       printf  this is the string  s n  m              any other use of m        free m       The m between   and s tells scanf   to measure the string and allocate memory for it and copy the string into that  and to store the address of that allocated memory in the corresponding argument   Once you re done with it you have to free   it   This isn t supported on every implementation of scanf    though   As others have pointed out  the easiest solution is to set a limit on the length of the input   If you still want to use scanf   then you can do so this way   char m 100   scanf   99s   amp m     Note that the size of m   must be at least one byte larger than the number between   and s   If the string entered is longer than 99  then the remaining characters will wait to be read by another call or by the rest of the format string passed to scanf     Generally scanf   is not recommended for handling user input   It s best applied to basic structured text files that were created by another application   Even then  you must be aware that the input might not be formatted as you expect  as somebody might have interfered with it to try to break your program

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I know that I have arrived after 4 years and am too late but I think I have another way that someone can use  I had used getchar   Function like this -   include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt     I had putten the main Function Bellow this function    d for asking string f is pointer to the string pointer void GetStr char  d char   f        printf   s  d        for int i  0 1 i                      if i   I e if i  0              f    char  realloc   f  i 1           else              f    char  malloc i 1             f  i  getchar            if   f  i       n                           f  i     0               break                        int main         char  s  NULL      GetStr  Enter the String -    amp s       printf  Your String -  s  nAnd It s length -  lu n  s  strlen s         free s       here is the sample run for this program -  Enter the String - I am Using Linux Mint XFCE 18 2   eclispe CDT and GCC7 2 compiler   Your String - I am Using Linux Mint XFCE 18 2   eclispe CDT and GCC7 2 compiler    And It s length - 67

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Enter while securing an area dynamically E G   include  lt stdio h gt   include  lt stdlib h gt   char  inputString FILE  fp  size t size     The size is extended by the input with the value of the provisional     char  str      int ch      size t len   0      str   realloc NULL  sizeof  str  size    size is start size     if  str return str      while EOF   ch fgetc fp    amp  amp  ch      n            str len    ch          if len  size               str   realloc str  sizeof  str   size  16                if  str return str                      str len      0        return realloc str  sizeof  str  len      int main void       char  m       printf  quot input string    quot        m   inputString stdin  10       printf  quot  s n quot   m        free m       return 0

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Safer and faster  doubling capacity  version   char  readline char  prompt      size t size   80    char  str   malloc sizeof char    size     int c    size t len   0    printf   s   prompt     while  EOF     c   getchar     amp  amp  c      r   amp  amp  c      n         str len      c      if len    size  str   realloc str  sizeof char     size    2          str len      0     return realloc str  sizeof char    len

[c] How can I read an input string of unknown length?

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