Why do I mysteriously segfault when writing to file? - c

Question

I wish I could i could give more information, but I genuinely have no idea what is going on here. This code opens files for appension or overwriting, depending on user arguments (appension by default). It gets as far as the fgets for user input, and then as soon as the input is entered, it segfaults and dumps core. This is strange, because before I implemented arguments (i.e. it was just ./a.out file) it worked fine, so I guess it has something to do with the new stuff about arguments...
#include <stdio.h>
#include <string.h>
int printhelp(void);
int main(int argc, char *argv[])
{
char input[256];
int loopstat = 0;
FILE *inputfile;
if (argc < 2) /* Check argc for minimum 2 arguments (i.e. cw FILE) */
{
printf("ca: missing file operand\n");
return 0;
}
else if (argc > 2) /* Check argc for more than 2 arguments (i.e. cw -o FILE) */
{
if (strncmp(argv[1], "-", 1) == 0) /* if first argument begins with "-", it must be an option, so descend into option checking */
{
if (strcmp(argv[1], "-a") == 0) /* If -a option is given, open for appending */
{
FILE *inputfile = fopen(argv[2], "a");
}
else if (strcmp(argv[1], "-o") == 0) /* If -o option is given, open for overwriting */
{
FILE *inputfile = fopen(argv[2], "w");
}
else if (strcmp(argv[1], "--help") == 0) /* If --help option is given, print help and quit */
{
printhelp();
return 0;
}
else
{
printf("cw: invalid option\n"); /* If invalid option is given, print the fact and quit*/
return 0;
}
}
}
else /* if argc is equal to 2 (i.e. "cw FILE" or "cw -o")...*/
{
if (strncmp(argv[1], "-", 1) == 0) /* Check if user has specified an option but no file (i.e. "cw -o") */
{
printf("cw: missing file operand\n"); /* If they have, print that no file is spec'd and quit */
return 0;
}
else /* If not, it's a legit file with no other arguments (e.g. "cw FILE") so open it in append mode by default */
{
FILE *inputfile = fopen(argv[2], "a");
}
}
/* Writing loop */
printf("Enter input...\n");
while (loopstat == 0) /* Get user input and write to file until they give exit command */
{
fgets(input, 256, stdin); /* Get user input */
if (strcmp(input, ":x\n") == 0) /* If input == exit command, quit */
{
printf("co: exit received, terminating...\n");
loopstat++;
}
else /* Write to file */
{
fprintf(inputfile, "%s", input);
}
}
fclose(inputfile);
}
int printhelp(void) /* Print help on --help command */
{
printf(
"Usage: ca FILE\nContinuously append input to the FILE\nca does not currently support multiple file appension.\nReport bugs to scamp#lavabit.com\n");
return 0;
}
P.S. sorry if I messed up the indentation, it's really confusing to have to add four spaces before everything in this much code.

Answer 1

Here you are shadowing your variable:
else /* If not, it's a legit file with no other arguments (e.g. "cw FILE") so open it in append mode by default */
{
FILE *inputfile = fopen(argv[2], "a");
}
should be
else /* If not, it's a legit file with no other arguments (e.g. "cw FILE") so open it in append mode by default */
{
inputfile = fopen(argv[2], "a");
}
You have several instances like this, so remove the declaration there as well.

Answer 2

You are declaring the identifier inputfile several times, but it won't be the same object at each time.
See the behavior of this program for instance (from Wikipedia):
#include <stdio.h>
int main(void)
{
char x = 'm';
printf("%c\n", x);
{
printf("%c\n", x);
char x = 'b';
printf("%c\n", x);
}
printf("%c\n", x);
}
You have to declare inputfile just once, and then assign it in if statements.
FILE *inputfile;
if (/* ... */)
inputfile = /* ... */
else if (/* ... */)
inputfile = /* ... */
else
inputfile = /* ... */

Answer 3

int main(int argc, char *argv[])
{
char input[256];
int loopstat = 0;
FILE *inputfile;
then you have:
if (strcmp(argv[1], "-a") == 0)
{
FILE *inputfile = fopen(argv[2], "a");
}
You should not redeclare a new object inputfile but reuse your inputfile declared at the top of your function.
Example:
inputfile = fopen(argv[2], "a");

Answer 4

The problem is there:
FILE *inputfile;
....
if (strcmp(argv[1], "-a") == 0)
{
FILE *inputfile = fopen(argv[2], "a");
}
By writing this way you are hiding the variable inputfile from the compiler. Thus the variable defined in the beginning stays uninitialized.
You should write it following way:
if (strcmp(argv[1], "-a") == 0)
{
inputfile = fopen(argv[2], "a");
}
So now you will use the variable defined in the top of the function.
You should read about the variable scope.