create new file with system calls - c

Question

Im trying to create a new file / overwrite an existing file using systemcalls , but for some reason I have two problems:
1. When I'm first running the program it exits with value 0, so it seems like it created the file successfully, but I can't see anything in my project directory.
then when I secondly running the program the file is created, but an error message is printed on the screen.
2. Also after the first iteration of the program, I can't see the prinf message at the end of the main function.
Thanks for helping.
int readFileDesc = 0, writeFiledesc = 0;
int sourceFile = 1, destFile = 2, bufferSize = 3, isOverwrite;
if (argc != 4 && argc != 5) {
printf("Invalid number of arguments\n");
printf("Usage:\n");
printf(" ex1 [-f] SOURCE DEST BUFFER_SIZE");
exit(EXIT_FAILURE);
}
//Checking if -f [OP] is activated.
isOverwrite = (strcmp(argv[1], "-f") == 0);
if (isOverwrite) {
sourceFile++;
destFile++;
bufferSize++;
}
//Opening the source file
readFileDesc = open(argv[sourceFile], O_RDONLY);
if (readFileDesc < 0) {
perror("Unable to open source file for reading: ");
exit(EXIT_FAILURE);
}
//opening the destination file
if (!isOverwrite) {
//Case we dont have the -f [op] so we create the file.
writeFiledesc = open(argv[destFile],
O_CREAT | O_EXCL | O_WRONLY ,
S_IRUSR | S_IWUSR);
if (writeFiledesc < 0) {
perror("Unable to open destination file for reading: ");
exit(EXIT_FAILURE);
}
} else {
//Case we have the -f [op] so we override existing file.
writeFiledesc = open(argv[destFile], O_RDONLY | O_WRONLY | O_TRUNC);
if (writeFiledesc < 0) {
perror("Unable to open destination file for writing: ");
exit(EXIT_FAILURE);
}
}
//Assume the buffersize is legal.
bufferSize = atoi(argv[bufferSize]);
char data[bufferSize];
int nread, nwrite;
while ((nread = read(readFileDesc, data, bufferSize)) > 0) {
if ((nwrite = write(writeFiledesc, data, nread)) != nread) {
printf("write problem: ");
}
}
// cant see this!
printf("File %s was copied to %s" , argv[sourceFile] , argv[destFile]);
//handling errors
close(sourceFile);
close(destFile);
return EXIT_SUCCESS;
}

Answer 1

This is wrong:
writeFiledesc = open(argv[destFile], O_RDONLY | O_WRONLY | O_TRUNC);
Using both O_RDONLY and O_WRONLY is wrong. You need to use O_RDWR.
Per the POSIX standard for open():
SYNOPSIS
#include <sys/stat.h> #include <fcntl.h>
int open(const char *path, int oflag, ...);
...
Values for oflag are constructed by a bitwise-inclusive OR of flags
from the following list, defined in . Applications shall
specify exactly one of the first five values (file access modes)
below in the value of oflag:
O_EXEC
Open for execute only (non-directory files). The result is unspecified if this flag is applied to a directory.
O_RDONLY
Open for reading only.
O_RDWR
Open for reading and writing. The result is undefined if this flag is applied to a FIFO.
O_SEARCH
Open directory for search only. The result is unspecified if this flag is applied to a non-directory file.
O_WRONLY
Open for writing only.
Any combination of the following may be used:
...
Also, read() and write() return ssize_t, not int.