Looking for a string of characters copied to a buffer - c

Question

I have an assignment that has asked me to copy a file using buffered i/o. It has multiple requirements:
Take one parameter and an optional second
Open the first parameter for reading
Open the second for writing
If there is no second parameter make a new file called prog1.out
Use a buffer size of 20 bytes
When copying the file, print any buffer starting with the characters "rwxr"
close all opened files before exiting.
The problem I'm having is with number six, I've looked around and can't figure this out. I've tried memchr but I don't think I'm on the right track. If anyone can help push me in the right direction I'd be grateful.
This is my code:
# include <stdlib.h>
# include <stdio.h>
int main(int argc, char *argv[])
{
FILE *readfile, *writefile;
char buffer[1024];
int fileSize;
int readResult;
int writeResult;
// making sure arguments exist
if (argc < 2|| argc > 3){
printf("This program takes either 1 or 2 arguments.\n");
exit(1);
}
//Opening file for reading
readfile = fopen(argv[1], "r");
if (!readfile) {
printf("Unable to open file %s.\n", argv[1]);
exit(1);
}
//finding the file size
fseek (readfile, 0, SEEK_END);
fileSize = ftell (readfile);
fseek (readfile, 0, SEEK_SET);
// read the file
readResult = fread(buffer, 20, fileSize/20, readfile);
if (readResult == 0) {
printf("A read error occured.\n");
exit(1);
}
//check to see if there is a second parameter (argument)
if (argc == 3) {
writefile = fopen(argv[2], "w");
if (!writefile) {
printf("Unable to open file %s.\n", argv[2]);
exit(1);
}
writeResult = fwrite(buffer, 20, fileSize/20, writefile);
if (writeResult != readResult) {
printf("A write error occured.\n");
exit(1);
}
printf("File %s successfully copied to %s.\n", argv[1], argv[2]);
}
else {
writefile = fopen("program1.out", "w");
if (!writefile) {
printf("Unable to open file program1.out\n");
exit(1);
}
writeResult = fwrite(buffer, 20, fileSize/20, writefile);
if (writeResult != readResult) {
printf("A write error occured.\n");
exit(1);
}
printf("File %s successfully copied to %s.\n", argv[1], "program1.out
}
fclose(readfile);
fclose(writefile);
exit(0);
}

Answer 1

There's the naive way:
if(buffer[0] == 'r' && buffer[1] == 'w'
&& buffer[2] == 'x' && buffer[3] == 'r') {
//do something
}
But take a look at strncmp(), which you can use for comparing parts of a string.
remember to first check if you have read at least 4 chars into the buffer. e.g. if the file is 21 bytes long, your 2. fread might only read 1 character, and you shouldn't compare against the other 3 chars in the buffer.
If you print out the buffer with e.g. printf or puts or any other function that expects a string, the buffer needs to end with a '\0' byte, otherwise the string functions doesn't know when to stop.

Answer 2

I'll first answer the question you actually asked: memcmp is a good way to compare two buffers. Some caveats:
You also have to make sure the size of the buffer is at least as large as the size of the target string
memcmp returns 0 if the two buffers match, which can be counter-intuitive.
So for example, if you wanted to see if a buffer is equal to the string "rwxw", you could write
if (readresult >= strlen("rwxw") && !memcmp(buffer, "rwxw", strlen("rwxw"))) {
// buffer match occurred!
}
Personally I would use a "#define" or const char to ensure that the three places where that string appear are actually the same string. For example:
#define MATCH_STRING "rwxw"
if (readresult >= strlen(MATCH_STRING) && !memcmp(buffer, MATCH_STRING, strlen(MATCH_STRING))) {
// buffer match occurred!
}
However there are a couple of other problems with your code. One is that you need a loop that continually reads from the input file and write from the output file until the input is exhausted. For example, along the lines of
while (true) {
readResult = fread(buffer, 20, 1, readfile);
if (readResult == 0) {
// end of file
break;
}
// put your check for the "rwxr" string here!
writeResult = fwrite(buffer, readResult, 1, writefile);
if (writeResult != readREsult) {
printf("error\n");
}
}
Finally, you have what might be called a "stylistic" bug. There are two cases in your program: a specified filename and a default filename. Those two cases share a lot of code in common, but you did a cut and paste. This makes the code harder to understand, and more prone to bugs if it's changed in the future. If you are cutting-and-pasting code you're doing something wrong! Consider instead something like this, which maximizes shared code paths:
char *outFileName;
if (argc == 3) {
outFileName = argv[2];
} else {
outFileName = "prog1.out";
}
writefile = fopen(outFileName, "w");
if (!writefile) {
printf("Unable to open file %s.\n", writeFileName);
exit(1);
}