Seg Fault when working with strings C Program / Popen - c

Question

I have looked for an answer to my question for almost two days and tried every solution suggested to no avail.
I am trying to access a file through a linux terminal using my C Program.
I want to run popen() to do this.
The command I want to run in popen() is : grep -o %s /usr/share/dict/words
Where %s is a variable word that changes each iteration. I have tried using pointers, arrays, and alternative functions such as asprintf() / snprintf()
Here is the code I have right now:
char *message = (char *)malloc(500);
strcpy(message, "grep -n");
printf("%s", message);
strcat(message, "hello");
printf("%s", message);
strcat(message, " /usr/share/dict/words"); // SEG FAULT OCCURS HERE
printf("%s", message);
I would then pass this to popen.
I have also tried initializing as: char message[500] and this returns the same error in the same spot.
Here is my full code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "caeserheader.h"
int main( int argc, char *argv[]){
char *inputfile;
int n = 0;
int shiftamount = 0;
//Determine amount of arguments
if(argc == 2){
inputfile = argv[1];
}
else if(argc == 3){
inputfile = argv[1];
n = atoi(argv[2]);
shiftamount = n * (-1) ;
}
else{
printf("Please enter a proper number of arguments.");
return -1;
}
//OPENS INPUT FILE
FILE *input = fopen(inputfile, "r");
if(input == NULL){
printf("\n FILE NOT FOUND.");
perror("fopen");
return -1;
}
//RESERVES MEMORY AND GRABS STRING
fseek(input, 0L, SEEK_END);
long Tsize = ftell(input);
rewind(input);
char *inputtext;
inputtext = calloc( 1, Tsize+1);
//ERROR CHECKING
if(!inputtext){
fclose(input), printf("MEMORY FAILED.");
}
if(1!=fread( inputtext, Tsize, 1, input)){
fclose(input), free(inputtext), printf("READ FAIL.");
}
//CREATES DECRYPTED STRING
char newletter;
char *newstring;
int i;
//WITH GIVEN NUMBER OF SHIFTS
if(argc == 3){
newstring = malloc(Tsize + 1);
for(i=0; i<Tsize; i++){
newletter = shift(inputtext[i], shiftamount);
newstring[i] = newletter;
}
}
//WITHOUT GIVEN NUMBER OF SHIFTS
if(argc == 2){
char *message = (char *)malloc(500); //SEG FAULT SOMEWHERE HERE?
// strcpy(message, "grep -n");
// printf("%s", message);
//strcat(message, "hello");
// printf("%s", message);
// strcat(message, "/usr/share/dict/words");
//printf("%s", message);
// word = strtok(inputtext," ,.-!?\n");
// int i;
//for(i=0; i<10; i++){
//word = strtok(NULL," ,.-!?\n");
//printf("\n%s", word);
//}
// if(( fp = popen(message, "r")) == NULL){
//perror("No file stream found.");
//return -1;
// }
// else {
// pclose(fp);
// printf("FOUND.");
// }
}
// PUTS DECRYPTED STRING IN NEW FILE
char copiedname[100];
strcpy(copiedname, inputfile);
strcat(copiedname, ".dec");
FILE *newfile = fopen(copiedname, "w");
fputs(newstring, newfile);
// free(newstring);
fclose(input);
fclose(newfile);
return 0;
}

Answer 1

You have set inputfile to argv[1] and later you have used strcat to append to it. Don't do this. You don't own argv.
The strcat function appends a copy of the source string to the destination string, and then returns a pointer to the destination string. It does not "add two strings and return the result" which is how you seem to be using it.