C unistd.h write() command write extra character - c

Question

I've been trying to do c programming implementation of cp command in unix/linux by using system calls (read(), write(), open(), close()).
But when I run my program through terminal by copying my source code of this program to the same directory with name change (the source code is about 300 lines)
and when I open that output file , it has more character than the original file.
The extra line is the same as 200ish line. Where does it came from?
here's the screenshot when compile
argp.c
123.c
This is the source code near the end of the original file(argp.c). You will see how I use the read write method.
while (count - ind > 1) {
strcpy(cdir, argv[argc-1]);
if (!isFile(cdir)) {
strcat(cdir, basename(arguments.argv[ind]));
}
if (arguments.update) {
stat(cdir,&stDest);
stat(arguments.argv[ind],&stSrc);
if (difftime(stDest.st_mtim.tv_sec, stSrc.st_mtim.tv_sec) > 0 ) {
printf("Destination file is newer\n");
exit(EXIT_FAILURE);
}
}
//open source file
src = open(arguments.argv[ind],O_RDONLY);
//if source file can't be opened
if (src == -1) {
printf("\nError opening file %s errno = %d\n",arguments.argv[ind],errno);
exit(EXIT_FAILURE);
}
//open target file
if (arguments.force) {
//with -f option(default)
tgt = open(cdir, O_WRONLY | O_CREAT | O_TRUNC , S_IRUSR | S_IRGRP | S_IROTH | S_IWUSR | S_IWGRP | S_IWOTH);
} else {
//with -n option
tgt = open(cdir, O_WRONLY | O_CREAT | O_EXCL , S_IRUSR | S_IRGRP | S_IROTH | S_IWUSR | S_IWGRP | S_IWOTH);
}
//if the target file cannot be read or already exist(with -n option)
if (tgt == -1) {
printf("File exist or it's not a file.\nCan't copy.\n");
exit(EXIT_FAILURE);
}
//read source file
//write target file
while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
if (write(tgt, buffer, BUFFERSIZE) != pos) {
exit(EXIT_FAILURE);
}
}
//if the source file cannot be read
if (pos == -1) {
printf("\nError in reading data from %s\n",arguments.argv[ind]);
}
//close source file
if (close(src) == -1) {
printf("\nError in closing file %s\n",arguments.argv[ind]);
}
//close target file
if (close(tgt) == -1) {
printf("\nError in closing file %s\n",cdir);
}
ind++;
}
if (arguments.verbose) {
printf("Copy successfully!\n");
}
exit(EXIT_SUCCESS);
}
This is the source code near the end of the copy file(123.c)
while (count - ind > 1) {
strcpy(cdir, argv[argc-1]);
if (!isFile(cdir)) {
strcat(cdir, basename(arguments.argv[ind]));
}
if (arguments.update) {
stat(cdir,&stDest);
stat(arguments.argv[ind],&stSrc);
if (difftime(stDest.st_mtim.tv_sec, stSrc.st_mtim.tv_sec) > 0 ) {
printf("Destination file is newer\n");
exit(EXIT_FAILURE);
}
}
//open source file
src = open(arguments.argv[ind],O_RDONLY);
//if source file can't be opened
if (src == -1) {
printf("\nError opening file %s errno = %d\n",arguments.argv[ind],errno);
exit(EXIT_FAILURE);
}
//open target file
if (arguments.force) {
//with -f option(default)
tgt = open(cdir, O_WRONLY | O_CREAT | O_TRUNC , S_IRUSR | S_IRGRP | S_IROTH | S_IWUSR | S_IWGRP | S_IWOTH);
} else {
//with -n option
tgt = open(cdir, O_WRONLY | O_CREAT | O_EXCL , S_IRUSR | S_IRGRP | S_IROTH | S_IWUSR | S_IWGRP | S_IWOTH);
}
//if the target file cannot be read or already exist(with -n option)
if (tgt == -1) {
printf("File exist or it's not a file.\nCan't copy.\n");
exit(EXIT_FAILURE);
}
//read source file
//write target file
while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
if (write(tgt, buffer, BUFFERSIZE) != pos) {
exit(EXIT_FAILURE);
}
}
//if the source file cannot be read
if (pos == -1) {
printf("\nError in reading data from %s\n",arguments.argv[ind]);
}
//close source file
if (close(src) == -1) {
printf("\nError in closing file %s\n",arguments.argv[ind]);
}
//close target file
if (close(tgt) == -1) {
printf("\nError in closing file %s\n",cdir);
}
ind++;
}
if (arguments.verbose) {
printf("Copy successfully!\n");
}
exit(EXIT_SUCCESS);
}
it(EXIT_FAILURE);
}
//read source file
//write target file
while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
if (write(tgt, buffer, BUFFERSIZE) != pos) {
exit(EXIT_FAILURE);
}
}
//if the source file cannot be read
if (pos == -1) {
printf("\nError in reading data from %s\n",arguments.argv[ind]);
}
//close source file
if (close(src) == -1) {
printf("\nError in closing file %s\n",arguments.argv[ind]);
}
//close target file
if (close(tgt) == -1) {
printf("\nError in closing file %s\n",cdir);
}
ind++;
}
if (arguments.verbose) {
printf("Copy successfully!\n");
}
exit(EXIT_SUCCESS);
}

Answer 1

You always try to read/write BUFFERSIZE bytes. But what happens if the file you want to copy has a size multiple of BUFFERSIZE? You write what have been read last time.
read return the number of bytes read, so each write should try to write this number of bytes:
Instead of:
pos = read(src, buffer, BUFFERSIZE);
//write target file
while (pos > 0) {
write(tgt, buffer, BUFFERSIZE);
pos = read(src, buffer, BUFFERSIZE);
}
Use:
pos = read(src, buffer, BUFFERSIZE);
//write target file
while (pos > 0) {
write(tgt, buffer, pos);
pos = read(src, buffer, BUFFERSIZE);
}
Same here, instead of:
//write target file
while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
if (write(tgt, buffer, BUFFERSIZE) != pos) {
exit(EXIT_FAILURE);
}
}
Use:
//write target file
while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
if (write(tgt, buffer, pos) != pos) {
exit(EXIT_FAILURE);
}
}

Answer 2

You unconditionally write a full buffer, regardless of how much was read:
//write target file
while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
if (write(tgt, buffer, BUFFERSIZE) != pos) {
exit(EXIT_FAILURE);
}
}
should be:
//write target file
while ((pos = read(src, buffer, BUFFERSIZE)) > 0) {
if (write(tgt, buffer, pos) != pos) {
exit(EXIT_FAILURE);
}
}