如何从c中的文件读取一行？

Question

I am having a requirement to read a file line by line through ac program. I know I can do it very straight by using a FILE pointer and using fgets (if by fixed length string) or fgetc (if by character by character). The only point here to note is that the length of a line in the file can be different. So , if i am using

char *fgets(char *__restrict s, int n, FILE *__restrict stream); 

I do not know the value of "int n" here since it can vary from line to line in the file.

I searched for an answer for this query but i got to know the usage of getline function which is c++ function. The only way for me is to read each character until i encounter a '\\n' and get that copied to a string.

Is there any other way to do this straight in ac program ?

Answer 1

Check this -> explanation (possible duplicate question) The getline function is documented -> here

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>

int
main(void)
{
    FILE *fp;
    char *line = NULL;
    size_t len = 0;
    ssize_t myread;

   fp = fopen("/etc/motd", "r");
    if (fp == NULL)
        exit(EXIT_FAILURE);

   while ((myread = getline(&line, &len, fp)) != -1) {
        printf("Retrieved line of length %zu :\n", myread);
        printf("%s", line);
    }

   free(line);
    exit(EXIT_SUCCESS);
}

Answer 2

I do not know the value of "int n" here since it can vary from line to line in the file.

The above and other discussion implies OP wants code that auto-sizes the buffer to whatever the line length may be. fgets() does the job fine if you are willing to accept a max length. Read the line and then copy in to a right-sized buffer.

  #define BUF_SIZE 1000
  char buf[BUF_SIZE];

  while (fgets(buf, sizeof buf, stream)) {

    // Add long line detection here if desired

    char *s = strdup(buf);
    foo(s);
  }

If code does not want to impose any maximum length, IMO, that is a weak design. Certainly lines can be very long and BUF_SIZE above should be a very generous value, but to allow code to handle lines of any length is to give control of the program to external forces be it a file or user input. That is the basis of a hacker exploit. The given application should have a liberal upper bound of the acceptable line length. fgets() provides that - although it does have other weaknesses. The common *nix getline() allows external forces to cause your program to consume up to SIZE_MAX byte in one call - not defensive code.