如何打印当前行？

Question

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()

{
    int i, f=0;
    int c;
    char file_name[100];
    char  search[10];

    printf("Enter the file name:");
    scanf("%s", file_name);
    printf("Search word:");
    scanf("%s", search);

    FILE *f = fopen((strcat(file_name, ".txt")), "rb");
    fseek(f, 0, SEEK_END);
    long pos = ftell(f); 
    fseek(f, 0, SEEK_SET);

    char *bytes = malloc(pos);
    fread(bytes, pos, 1, f);
    fclose(f);

/*search*/

    if (strstr(bytes, search) != NULL){
      printf("found\n");
       f = 1;}
      else{
    printf("Not found\n");

    f=0;}

    if (f==1){    /* if found...print the whole line */
     ....}
  free(bytes);

}

Above stated is my program for searching a string from .txt file. When found, it prints "found", else it prints "Not found". Now I want to print the complete line of which the string was a part. I was thinking of using 'f==1' as the condition for 'if found' print the whole line, not really sure what is the best way to proceed.

Answer 1

First you need to fix your read to leave the data you read from the file NUL-terminated:

char *bytes = malloc(pos + 1);
fread(bytes, pos, 1, f);
bytes[ pos ] = '\0';

Add some error checking too - Check the return from malloc() and fread() . it's a good habit to get into.

Then, if you find your string, split what you read at that point:

char *found = strstr( bytes, search );
if ( found != NULL )
{
    *found = '\0';
    char *lineStart = strrchr( bytes, '\n' );
    char *lineEnd = strchr( found + 1, '\n' );
        .
        .

I'll leave it up to you to figure out what it means if either or both of those are NULL.

Also, using fseek() to figure out how many bytes are in a file is technically wrong as ftell() doesn't return a byte offset but only a value that can be used by fseek() to return to that same spot in the file. There are some architectures out there where ftell() returns a meaningless number.

If you want to know how big a file is, use stat() - or fstat() on an open file:

struct stat sb;
FILE *f = fopen(...)
fstat( fileno( f ), &sb );
off_t bytesInFile = sb.st_size;

Note also that I didn't use long - I used off_t . Using long to store the number of bytes in a file is a recipe for serious bugs when file sizes get over 2 GB for 32-bit programs.