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

Answer 1

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 <stdio.h>  int main(int argc, char* argv[]) {     char const* const fileName = argv[1]; /* should check that argc > 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; } 

Answer 2

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.