Simple Program Segmentation Faults - c

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.

Answer 2

you don't need
char* line[MAX_CHARACTERS_PER_LINE];
this is a pointer to arrays (maybe useful if you want to store the file or input line by line) and you haven't allocated it first. so the seg fault is quite obvious.
change your search_and_print to this:
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("%s\n", line);
}
additional to Jim Balter's very good advices, also I would suggest using getopt for parsing your parameters.

Answer 3

For future reference for anyone, here is the code with the suggested changes that seems to work. Thanks again for the help!
#include <stdio.h>
#include <string.h>
void search_and_print ( char pattern[], FILE* search_file );
int usage(const char* err);
const char USAGE[] =
"\nUSAGE: a.out [-i] [-f filename] (Search Pattern)";
int main ( int argc, char *argv[] ) {
const char TOO_MANY_VARIABLES[] = "Too many arguments from the command line!";
if (argc < 2) return usage("Not enough options");
if (argc > 4) return usage("Too many arguments from the command line!");
// If input
if (strcmp(argv[1],"-i") == 0) {
if (argc > 2) {
char *pattern = argv[2];
search_and_print(pattern, stdin);
}
else {
printf("Need a pattern to search by!");
return 1;
}
}
// If file
if (strcmp(argv[1],"-f") == 0) {
if (argc > 3) {
char *pattern = argv[3];
// Open file
FILE *file = fopen( argv[2], "r" );
// Check if file exists
if ( file != NULL) {
search_and_print(pattern, file);
fclose( file );
} else {
printf("File not found!");
return 1;
}
} else {
printf("Need a pattern to search by!");
return 1;
}
}
return 0;
}
int usage(const char* err) {
fprintf(stderr, "%s\n%s\n", err, USAGE);
return 1;
}
void search_and_print ( char pattern[], FILE* search_file ) {
const int MAX_CHARACTERS_PER_LINE = 1000;
char line[MAX_CHARACTERS_PER_LINE];
// Read through file
while ( fgets(line, MAX_CHARACTERS_PER_LINE, search_file) != NULL )
if ( strstr(line, pattern) != NULL )
printf("%s", line);
}