How to read string from keyboard using C

Question

I want to read a string entered by the user  I don t know the length of the string  As there are no strings in C I declared a pointer   char   word    and used scanf to read input from the keyboard   scanf   s    word      but I got a segmentation fault   How can I read input from the keyboard in C when the length is unknown

User · Answer

The following code can be used to read the input string from a user  But it s space is limited to 64        char word 64        0        initialize all elements with   0  int i   0  while   word i       n   amp  amp   i lt 64         scanf s   c    amp word i     1

User · Answer

include lt stdio h gt   int main         char str 100       scanf      n s  str       printf   s  str       return 0         input  read the string   ouput  print the string   This code prints the string with gaps as shown above

User · Answer

I cannot see why there is a recommendation to use scanf   here  scanf   is safe only if you add restriction parameters to the format string - such as  64s or so   A much better way is to use char   fgets   char   str  int num  FILE   stream      int main         char data 64       if  fgets data  sizeof data  stdin                input has worked  do something with data            untested

User · Answer

When reading input from any file  stdin included  where you do not know the length  it is often better to use getline rather than scanf or fgets because getline will handle memory allocation for your string automatically so long as you provide a null pointer to receive the string entered  This example will illustrate    include  lt stdio h gt   include  lt stdlib h gt   int main  int argc  char  argv           char  line   NULL      forces getline to allocate with malloc        size t len   0         ignored when line   NULL        ssize t read       printf    nEnter string below  ctrl   d  to quit n         while   read   getline  amp line   amp len  stdin      -1             if  read  gt  0              printf    n  read  zd chars from stdin  allocated  zd bytes for line    s n   read  len  line            printf   Enter string below  ctrl   d  to quit n               free  line       free memory allocated by getline         return 0      The relevant parts being   char  line   NULL      forces getline to allocate with malloc    size t len   0         ignored when line   NULL       snip    read   getline   amp line   amp len  stdin     Setting line to NULL causes getline to allocate memory automatically  Example output       getline example  Enter string below  ctrl   d  to quit A short string to test getline     read 32 chars from stdin  allocated 120 bytes for line   A short string to test getline   Enter string below  ctrl   d  to quit A little bit longer string to show that getline will allocated again without resetting line   NULL    read 99 chars from stdin  allocated 120 bytes for line   A little bit longer string to show that getline will allocated again without resetting line   NULL  Enter string below  ctrl   d  to quit   So with getline you do not need to guess how long your user s string will be

User · Answer

You need to have the pointer to point somewhere to use it   Try this code   char word 64   scanf   s   word     This creates a character array of lenth 64 and reads input to it  Note that if the input is longer than 64 bytes the word array overflows and your program becomes unreliable   As Jens pointed out  it would be better to not use scanf for reading strings  This would be safe solution   char word 64  fgets word  63  stdin   word 63    0

User · Answer

You have no storage allocated for word - it s just a dangling pointer   Change   char   word    to   char word 256     Note that 256 is an arbitrary choice here - the size of this buffer needs to be greater than the largest possible string that you might encounter   Note also that fgets is a better  safer  option then scanf for reading arbitrary length strings  in that it takes a size argument  which in turn helps to prevent buffer overflows    fgets word  sizeof word   stdin

[c] How to read string from keyboard using C?

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