C read file line by line

Question

I wrote this function to read a line from a file   const char  readLine FILE  file         if  file    NULL            printf  Error  file pointer is null             exit 1              int maximumLineLength   128      char  lineBuffer    char   malloc sizeof char    maximumLineLength        if  lineBuffer    NULL            printf  Error allocating memory for line buffer             exit 1              char ch   getc file       int count   0       while   ch      n    amp  amp   ch    EOF             if  count    maximumLineLength                maximumLineLength    128              lineBuffer   realloc lineBuffer  maximumLineLength               if  lineBuffer    NULL                    printf  Error reallocating space for line buffer                     exit 1                                   lineBuffer count    ch          count             ch   getc file              lineBuffer count      0       char line count   1       strncpy line  lineBuffer   count   1        free lineBuffer       const char  constLine   line      return constLine      The function reads the file correctly  and using printf I see that the constLine string did get read correctly as well   However  if I use the function e g  like this   while   feof myFile         const char  line   readLine myFile       printf   s n   line       printf outputs gibberish  Why

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My implement from scratch   FILE  pFile   fopen your file path   r    int nbytes   1024  char  line    char    malloc nbytes   char  buf    char    malloc nbytes    size t bytes read  int linesize   0  while  fgets buf  nbytes  pFile     NULL        bytes read   strlen buf          if line length larger than size of line buffer     if  linesize   bytes read  gt  nbytes            char  tmp   line          nbytes    nbytes   2          line    char    malloc nbytes           memcpy line  tmp  linesize           free tmp             memcpy line   linesize  buf  bytes read       linesize    bytes read       if  feof pFile     buf bytes read-1       n             handle line line           linesize   0          memset line    0   nbytes            free buf   free line

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readLine   returns pointer to local variable  which causes undefined behaviour   To get around you can    Create variable in caller function and pass its address to readLine   Allocate memory for line using malloc   - in this case line will be persistent Use global variable  although it is generally a bad practice

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A complete  fgets   solution   include  lt stdio h gt   include  lt string h gt    define MAX LEN 256  int main void        FILE  fp      fp   fopen  quot file txt quot    quot r quot        if  fp    NULL          perror  quot Failed   quot          return 1             char buffer MAX LEN          -1 to allow room for NULL terminator for really long string     while  fgets buffer  MAX LEN - 1  fp                    Remove trailing newline         buffer strcspn buffer   quot  n quot      0          printf  quot  s n quot   buffer              fclose fp       return 0     Output  First line of file Second line of file Third  and also last  line of file  Remember  if you want to read from Standard Input  rather than a file as in this case   then all you have to do is pass stdin as the third parameter of fgets   method  like this  while fgets buffer  MAX LEN - 1  stdin     Appendix Removing trailing newline character from fgets   input how to detect a file is opened or not in c

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Implement method to read  and get content from a file  input1 txt    include  lt stdio h gt   include  lt stdlib h gt   void testGetFile            open file     FILE  fp   fopen  input1 txt    r        size t len   255         need malloc memory for line  if not  segmentation fault error will occurred      char  line   malloc sizeof char    len          check if file exist  and you can open it  or not     if  fp    NULL            printf  can open file input1 txt             return            while fgets line  len  fp     NULL            printf   s n   line             free line       Hope this help  Happy coding

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You make the mistake of returning a pointer to an automatic variable  The variable line is allocated in the stack and only lives as long as the function lives  You are not allowed to return a pointer to it  because as soon as it returns the memory will be given elsewhere   const char  func x        char line 100       return  const char   line    illegal     To avoid this  you either return a pointer to memory which resides on the heap eg  lineBuffer and it should be the user s responsibility to call free   when he is done with it  Alternatively you can ask the user to pass you as an argument a memory address on which to write the line contents at

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open and get the file handle FILE  fh  fopen s  amp fh  filename   r       check if file exists if  fh    NULL       printf  file does not exists  s   filename       return 0        read line by line const size t line size   300  char  line   malloc line size   while  fgets line  line size  fh     NULL         printf line     free line         dont forget to free heap memory

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Here is my several hours    Reading whole file line by line   char   readline FILE  fp  char  buffer        int ch      int i   0      size t buff len   0       buffer   malloc buff len   1       if   buffer  return NULL      Out of memory      while   ch   fgetc fp        n   amp  amp  ch    EOF                buff len            void  tmp   realloc buffer  buff len   1           if  tmp    NULL                        free buffer               return NULL     Out of memory                   buffer   tmp           buffer i     char  ch          i              buffer i      0           Detect end     if  ch    EOF  amp  amp   i    0    ferror fp                  free buffer           return NULL            return buffer     void lineByline FILE   file   char  s  while   s   readline file  0      NULL        puts s       free s       printf   n         int main         char  fileName    input-1 txt       FILE  file   fopen fileName   r        lineByline file       return 0

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You should use the ANSI functions for reading a line  eg  fgets  After calling you need free   in calling context  eg       const char  entirecontent readLine myFile   puts entirecontent   free entirecontent        const char  readLine FILE  file      char  lineBuffer calloc 1 1   line 128      if    file     lineBuffer           fprintf stderr  an ErrorNo 1             exit 1          for   fgets line sizeof line file    strcat lineBuffer line            if  strchr line   n      strchr line   n   0      lineBuffer realloc lineBuffer strlen lineBuffer  strlen line  1       if   lineBuffer               fprintf stderr  an ErrorNo 2               exit 2               return lineBuffer

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FILE  filePointer  int bufferLength   255  char buffer bufferLength    filePointer   fopen  file txt    r     while fgets buffer  bufferLength  filePointer         printf   s n   buffer      fclose filePointer

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Provide a portable and generic getdelim function  test passed via msvc  clang  gcc    include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   ssize t portabl getdelim char    restrict linep                   size t   restrict linecapp                   int delimiter                   FILE   restrict stream        if  0     linep             linecapp   8           linep   malloc  linecapp           if  0     linep                return EOF                       ssize t linelen   0      int c   0      char  p    linep       while  EOF     c   fgetc stream              if  linelen     ssize t   linecapp - 1                 linecapp  lt  lt   1              char  p1   realloc  linep   linecapp               if  0     p1                    return EOF                            p   p1   linelen                     p     c          linelen             if  delimiter    c                 p   0              return linelen                      return EOF    c   EOF   linelen      int main int argc  char   argv        const char  filename     a b c c       FILE  file   fopen filename   r        if   file            perror filename           return 1             char  line   0      size t linecap   0      ssize t linelen       while  0  lt   linelen   portabl getdelim  amp line   amp linecap    n   file              fwrite line  linelen  1  stdout             if  line            free line             fclose file           return 0

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Some things wrong with the example    you forgot to add  n to your printfs   Also error messages should go to stderr i e  fprintf stderr        not a biggy but  consider using fgetc   rather than getc     getc   is a macro  fgetc   is a proper function getc   returns an int so ch should be declared as an int   This is important since the comparison with EOF will be handled correctly   Some 8 bit character sets use 0xFF as a valid character  ISO-LATIN-1 would be an example  and EOF which is -1  will be 0xFF if assigned to a char  There is a potential buffer overflow at the line  lineBuffer count      0     If the line is exactly 128 characters long  count is 128 at the point that gets executed  As others have pointed out  line is a locally declared array   You can t return a pointer to it  strncpy count   1  will copy at most count   1 characters but will terminate if it hits   0   Because you set lineBuffer count  to   0  you know it will never get to count   1   However  if it did  it would not put a terminating   0  on  so you need to do it   You often see something like the following   char buffer  BUFFER SIZE   strncpy buffer  sourceString  BUFFER SIZE - 1   buffer BUFFER SIZE - 1      0    if you malloc   a line to return  in place of your local char array   your return type should be char  - drop the const

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Use fgets   to read a line from a file handle

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If your task is not to invent the line-by-line reading function  but just to read the file line-by-line  you may use a typical code snippet involving the getline   function  see the manual page here     define  GNU SOURCE  include  lt stdio h gt   include  lt stdlib h gt   int main void        FILE   fp      char   line   NULL      size t len   0      ssize t read       fp   fopen   etc motd    r        if  fp    NULL          exit EXIT FAILURE        while   read   getline  amp line   amp len  fp      -1            printf  Retrieved line of length  zu  n   read           printf   s   line              fclose fp       if  line          free line       exit EXIT SUCCESS

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void readLine FILE  file  char  line  int limit        int i      int read       read   fread line  sizeof char   limit  file       line read      0        for i   0  i  lt   read i                  if   0     line i       n     line i       r     line i                         line i      0               break                       if i    read                fseek file  i - read   1  SEEK CUR             what about this one

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I want a code from ground 0 so i did this to  read the content of dictionary s word line by line   char temp str 20       you can change the buffer size according to your requirements And A single line s length in a File   Note I ve initialized the buffer With Null character each time I read line This function can be Automated  But Since I need A proof of Concept and want to design a programme Byte By Byte    include lt stdio h gt   int main     int i  char temp ch  FILE  fp fopen  data txt   r    while temp ch  EOF     i 0    char temp str 20     0    0    0    0    0    0    0    0    0    0    0    0    0    0    0    0    0    0    0    while temp ch    n       temp ch fgetc fp     temp str i  temp ch    i      if temp ch    n     temp ch fgetc fp   temp str i  temp ch    printf   s  temp str     return 0

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const char  readLine FILE  file  char  line         if  file    NULL            printf  Error  file pointer is null             exit 1              int maximumLineLength   128      char  lineBuffer    char   malloc sizeof char    maximumLineLength        if  lineBuffer    NULL            printf  Error allocating memory for line buffer             exit 1              char ch   getc file       int count   0       while   ch      n    amp  amp   ch    EOF             if  count    maximumLineLength                maximumLineLength    128              lineBuffer   realloc lineBuffer  maximumLineLength               if  lineBuffer    NULL                    printf  Error reallocating space for line buffer                     exit 1                                   lineBuffer count    ch          count             ch   getc file              lineBuffer count      0       char line count   1       strncpy line  lineBuffer   count   1        free lineBuffer       return line       char linebuffer 256   while   feof myFile         const char  line   readLine myFile  linebuffer       printf   s n   line       note that the  line  variable is declared in calling function and then passed  so your readLine function fills predefined buffer and just returns it  This is the way most of C libraries work   There are other ways  which I m aware of     defining the char line   as static  static char line MAX LINE LENGTH  -  it will hold it s value AFTER returning from the function   -  bad  the function is not reentrant  and race condition can occur -  if you call it twice from two threads  it will overwrite it s results malloc  ing the char line    and freeing it in calling functions -  too many expensive mallocs  and  delegating the responsibility to free the buffer to another function  the most elegant solution is to call malloc and free on any buffers in same function    btw   explicit  casting  from char  to const char  is redundant   btw2  there is no need to malloc   the lineBuffer  just define it char lineBuffer 128   so you don t need to free it  btw3 do not use  dynamic sized stack arrays   defining the array as char arrayName some nonconstant variable    if you don t exactly know what are you doing  it works only in C99

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In your readLine function  you return a pointer to the line array  Strictly speaking  a pointer to its first character  but the difference is irrelevant here   Since it s an automatic variable  i e   it s    on the stack      the memory is reclaimed when the function returns  You see gibberish because printf has put its own stuff on the stack   You need to return a dynamically allocated buffer from the function  You already have one  it s lineBuffer  all you have to do is truncate it to the desired length       lineBuffer count      0       realloc lineBuffer  count   1       return lineBuffer      ADDED  response to follow-up question in comment   readLine returns a pointer to the characters that make up the line  This pointer is what you need to work with the contents of the line  It s also what you must pass to free when you ve finished using the memory taken by these characters  Here s how you might use the readLine function   char  line   readLine file   printf  LOG  read a line   s n   line   if  strchr line   a      puts  The line contains an a         etc     free line      After this point  the memory allocated for the line has been reclaimed     You can t use the value of  line  again  though you can assign a new value    to the  line  variable if you want

[c] C read file line by line

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