Going through a text file line by line in C

Question

I have been working on a small exercise for my CIS class and am very confused by the methods C uses to read from a file  All that I really need to do is read through a file line by line and use the information gathered from each line to do a few manipulations  I tried using the getline method and others with no luck   My code is currently as follows   int main char  argc  char  argv           const char  filename   argv 0         FILE  file   fopen filename   r          char  line   NULL         while  feof file            sscanf line  filename    s            printf   s n   line               return 1      Right now I am getting a seg fault with the sscanf method and I am not sure why  I am a total C noob and just wondering if there was some big picture thing that I was missing   Thanks

User · Answer

To read a line from a file  you should use the fgets function  It reads a string from the specified file up to either a newline character or EOF   The use of sscanf in your code would not work at all  as you use filename as your format string for reading from line into a constant string literal  s   The reason for SEGV is that you write into the non-allocated memory pointed to by line

User · Answer

So many problems in so few lines  I probably forget some    argv 0  is the program name  not the first argument  if you want to read in a variable  you have to allocate its memory one never loops on feof  one loops on an IO function until it fails  feof then serves to determinate the reason of failure  sscanf is there to parse a line  if you want to parse a file  use fscanf    s  will stop at the first space as a format for the  scanf family to read a line  the standard function is fgets  returning 1 from main means failure   So   include  lt stdio h gt   int main int argc  char  argv          char const  const fileName   argv 1      should check that argc  gt  1        FILE  file   fopen fileName   r       should check the result        char line 256        while  fgets line  sizeof line   file                note that fgets don t strip the terminating  n  checking its            presence would allow to handle lines longer that sizeof line             printf   s   line                 may check feof here to make a difference between eof and io failure -- network        timeout for instance         fclose file        return 0

User · Answer

Say you re dealing with some other delimiter  such as a  t tab  instead of a  n newline   A more general approach to delimiters is the use of getc    which grabs one character at a time    Note that getc   returns an int  so that we can test for equality with EOF   Secondly  we define an array line BUFFER MAX LENGTH  of type char  in order to store up to BUFFER MAX LENGTH-1 characters on the stack  we have to save that last character for a  0 terminator character    Use of an array avoids the need to use malloc and free to create a character pointer of the right length on the heap    define BUFFER MAX LENGTH 1024  int main int argc  char  argv          FILE  file   NULL      char line BUFFER MAX LENGTH       int tempChar      unsigned int tempCharIdx   0U       if  argc    2           file   fopen argv 1    r        else            fprintf stderr   error  wrong number of arguments n                            usage   s textfile n   argv 0             return EXIT FAILURE             if   file             fprintf stderr   error  could not open textfile   s n   argv 1             return EXIT FAILURE                get a character from the file pointer        while tempChar   fgetc file                    avoid buffer overflow error            if  tempCharIdx    BUFFER MAX LENGTH                fprintf stderr   error  line is too long  increase BUFFER MAX LENGTH  n                return EXIT FAILURE                        test character value            if  tempChar    EOF                line tempCharIdx      0               fprintf stdout    s n   line               break                    else if  tempChar      n                 line tempCharIdx      0               tempCharIdx   0U              fprintf stdout    s n   line               continue                    else             line tempCharIdx       char tempChar             return EXIT SUCCESS      If you must use a char    then you can still use this code  but you strdup   the line   array  once it is filled up with a line s worth of input  You must free this duplicated string once you re done with it  or you ll get a memory leak    define BUFFER MAX LENGTH 1024  int main int argc  char  argv          FILE  file   NULL      char line BUFFER MAX LENGTH       int tempChar      unsigned int tempCharIdx   0U      char  dynamicLine   NULL       if  argc    2           file   fopen argv 1    r        else            fprintf stderr   error  wrong number of arguments n                            usage   s textfile n   argv 0             return EXIT FAILURE             if   file             fprintf stderr   error  could not open textfile   s n   argv 1             return EXIT FAILURE             while tempChar   fgetc file                    avoid buffer overflow error            if  tempCharIdx    BUFFER MAX LENGTH                fprintf stderr   error  line is too long  increase BUFFER MAX LENGTH  n                return EXIT FAILURE                        test character value            if  tempChar    EOF                line tempCharIdx      0               dynamicLine   strdup line               fprintf stdout    s n   dynamicLine               free dynamicLine               dynamicLine   NULL              break                    else if  tempChar      n                 line tempCharIdx      0               tempCharIdx   0U              dynamicLine   strdup line               fprintf stdout    s n   dynamicLine               free dynamicLine               dynamicLine   NULL              continue                    else             line tempCharIdx       char tempChar             return EXIT SUCCESS

User · Answer

In addition to the other answers  on a recent C library  Posix 2008 compliant   you could use getline  See this answer  to a related question

[c] Going through a text file line by line in C

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