Logical error with writing to a file in C - c

Question

I have a project I am working on, basically it requires me to read switches from the command line and reads and writes some output to the same file overwriting its previous content. If the file is not specified it reads from stdin and writes to stdout, here is a function to number all output lines:
int main(int argc,char *argv[]) {
FILE *fp;
if((fp = fopen(argv[2]),"a+")) == 0) {
fp = stdin; /* if we cant read the file then it is read from stdin*/
}
if((strcmp("-e",argv[1])) == 0){
if(fp == stdin) {
display_dollar(fp);
}
}
fclose(fp);
}
and this is my display_dollar function (the one that actually prints to stdout or fp)
void display_dollar(FILE *fp) {
char line[LINESIZE];
char lines[LINESIZE];
while(fgets(line,LINESIZE,fp)) {
sscanf(line,"%s\n",lines);
if(fp == stdin) {
printf("%s%c\n",lines,'$');
lines[0] = '\0'; /* "clear" buffer */
} else {
fprintf(fp,"%s%c\n",lines,'$');
lines[0] = '\0'; /* "clear" buffer */
}
}
}
This works perfectly for stdout but I am having trouble printing anything to the file specified by fp, I dont understand what I am doing wrong. Thanks!
Some suggested i take out the parantheses when I compare fp but I get an compiler warning when I compile it with
gcc -ansi -W -Wall -pedantic
The warning as follows:
assign2.c: In function ‘main’:
assign2.c:20:9: warning: assignment makes pointer from integer without a cast [enabled by default]
if(fp = fopen(argv[argc - 1],"a+") == 0) {
^
assign2.c:20:3: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
if(fp = fopen(argv[argc - 1],"a+") == 0) {
^

Answer 1

Original code
Look very carefully at this line — it doesn't do what you want:
if((fp = fopen(argv[2]),"a+") == 0) {
^ ^ ^ ^
1 2 2 1
The parentheses aren't paired up as you intend. You're not passing enough arguments to fopen(). And you're comparing "a+" with 0.
You really want:
if ((fp = fopen(argv[2], "a+")) == 0) {
Revised code
In the revised code, you seem to have:
if(fp = fopen(argv[argc - 1],"a+") == 0) {
This is parsed as:
if (fp = (fopen(argv[argc - 1], "a+") == 0)) {
It is assigning the 0 or 1 result of comparing the return value from fopen() with 0, and that isn't a pointer, hence the complaint about assignment makes pointer from integer without a cast.
The second warning about suggest parentheses around assignment used as truth value is indicative of the problem; the result of the assignment is used as the condition, which is only sometimes correct (and this is one of the times when it isn't). It more usually appears in a context such as:
if (i = j)
where you probably intended:
if (i == j)
but if you really wanted to test the assignment, then you should write:
if ((i = j) != 0)
or (perish the thought — I hate it when people do this!):
if ((i = j))
Still having problems…
but it still won't write to the file,
You have a single read/write position in a file. You are reading from and writing to the same file, fp. If that is stdin, you've go one set of problems; if it is a regular text file, you have another set.
First of all, between a read and a write (or a write and a read) on a file opened for update, you must do a positioning operation — fseek() or rewind(), mainly. Secondly, because you opened the file in append mode, all writes occur at the end of the file. That means that when you try to read after a write, the position is at the end of the file (aka EOF).
If the file is standard input, that is often not seekable, which has implications, too. Standard input (or, at least, the underlying file descriptor 0 on Unix) is very often writable, but that's not something you're supposed (ab)use like that.
You either need to have an input file and an output file, or you need to be seeking appropriately.
And maybe you want "r+" instead of "a+" mode.

Answer 2

this code:
void display_dollar(FILE *fp) {
char line[LINESIZE];
char lines[LINESIZE];
while(fgets(line,LINESIZE,fp)) {
sscanf(line,"%s\n",lines);
if(fp == stdin) {
printf("%s%c\n",lines,'$');
lines[0] = '\0'; /* "clear" buffer */
} else {
fprintf(fp,"%s%c\n",lines,'$');
lines[0] = '\0'; /* "clear" buffer */
}
}
}
1) changes the number of characters in the next line, so the output file will be corrupted.
2) each read/write to the file moves the file-position-pointer.
you could use ftell() to see this happening.
3) the code must not corrupt the file, so it will have to write a new file.
4) this has a logic error, the %s will stop when any white space is encountered.
This includes newlines (of which there will (almost) always be one
or any tab or any space.
then appending a '$' will overlay something in the line.
therefore, the line to output will be corrupted.
the corruption could be a space or a NUL or a tab or a newline being overlayed
Suggestions:
1) When outputting to a file, always create a new file
2) always find the newline with strstr() (or similar), overlay the newline with "$\0",
then strcat() "\n\0"
this will mean modifying the display_dollar() declaration to include both the input and output file pointers

Answer 3

Alright, long story short I rewrote the code to have an input and output file, because things would get more complicated overwriting the current file in place without using a buffer.
Please let me know if there are any issues, or if things need to be explained.
#include <stddef.h>
#include <stdio.h>
#include <string.h>
#define LINESIZE 1024
void display_dollar(FILE *, FILE *);
int main(int argc, char *argv[]) {
FILE *infp = stdin;
FILE *outfp = stdout;
for (int i = 4; i < argc; ++i)
fprintf(stderr, "Unknown argument: %s\n", argv[i]);
if (argc >= 4 && (outfp = fopen(argv[3], "w")) == NULL)
outfp = stdout;
if (argc >= 3 && (infp = fopen(argv[2], "r")) == NULL)
infp = stdin;
if (argc >= 2 && strcmp("-e", argv[1]) == 0)
display_dollar(infp, outfp);
else
fprintf(stderr, "Usage: %s -e [infile [outfile]]\n", argc ? argv[0] : "program");
fclose(infp);
fclose(outfp);
return 0;
}
void remove_newlines(char *str) {
for(long i = strlen(str)-1; i >= 0 && (str[i] == '\n' || str[i] == '\r'); --i)
str[i] = '\0';
}
void display_dollar(FILE *infp, FILE *outfp) {
char line[LINESIZE];
while (fgets(line, LINESIZE, infp)) {
remove_newlines(line);
fprintf(outfp, "%s%c\n", line, '$');
}
}