Question

I’m new to C, and I’ve been trying to figure out pointers.

This program works with -i but segfaults after a few lines and -f segfaults right away.

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

void search_and_print ( char pattern[], FILE* search_file );

int main ( int argc, char *argv[] ) {
        const char TOO_MANY_VARIABLES[] = "Too many arguments from the command line!";
        const char NOT_ENOUGH_VARIABLES[] = "\nUSAGE: a.out [-i] [-f filename] (Search Pattern)\n";

        if (argc < 2) { printf(NOT_ENOUGH_VARIABLES); return(1);}
        // If input
        if (strcmp(argv[1],"-i") == 0) {
                char *pattern = argv[2];
                search_and_print(pattern, stdin);
        }


        // If file
        if (strcmp(argv[1],"-f") == 0) {
                char *pattern = argv[3];
                // Check if file exists
                // Open file
                FILE *file = fopen( argv[2], "r" );
                search_and_print(pattern, file);
                fclose( file );
        }

}

void search_and_print ( char pattern[], FILE* search_file ) {
        // Read through file
        const int MAX_CHARACTERS_PER_LINE = 1000;
        char* line[MAX_CHARACTERS_PER_LINE];
        while  ( fgets(*line, MAX_CHARACTERS_PER_LINE, search_file) != NULL )
                if  ( strstr(*line, pattern) != NULL )
                    printf(*line);
}

Answer 1

You have quite a few bugs here.

char* line[MAX_CHARACTERS_PER_LINE];

defines an array of 1000 pointers, not characters. fgets(*line, ... passes the first of those pointers, which is uninitialized, to fgets, most likely causing a segvio.

printf(*line);

The first argument to printf is a format. Never ever pass user input as the format, as this opens a huge security hole in your program … see http://en.wikipedia.org/wiki/Uncontrolled_format_string

You should use fputs(line) or printf("%s", line) (once you fix the declaration of line).

int main

You don’t return a value (except in the error case) … that results undefined behavior.

FILE *file = fopen( argv[2], "r" );

You should check whether this succeeds. If the file can’t be opened (e.g., it doesn’t exist), passing it to fgets results in undefined behavior.

if (argc < 2) { printf(NOT_ENOUGH_VARIABLES); return(1);}

This test isn’t sufficient for your -f case.