How to write text containing newline given as command line arguments in C? - c

Question

I want to create a text file with mulitple lines using system calls in C and populate it with the text provided as command line arguments.
This is what I wrote:
#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#define MAX_SZ 1024
int main(int argc, char *argv[]) {
if (argc != 3) {
printf("Invalid Number of arguments\n");
printf("USAGE: ./a.out file_name \"msg\"\n");
} else {
int fd_creat, fd_open, fd_write;
char file_name[MAX_SZ];
char *msg = (char *)malloc(strlen(argv[2]) * sizeof(char));
strcpy(file_name, argv[1]);
fd_creat = creat(file_name, 0777);
if (fd_creat < 2) {
printf("ERROR: File could not be created\n");
} else {
fd_open = open(file_name, O_WRONLY);
strcpy(msg, argv[2]);
fd_write = write(fd_open, msg, strlen(msg));
close(fd_open);
}
}
return 0;
}
If I execute this program as:
./a.out test.txt "Foo\nBar"
It writes the whole thing into test.txt as it is. Basically, I want 'Foo' and 'Bar' in their separate lines.

Answer 1

There's two problems here:
The way you're handling arguments and failing to allocate enough memory for the data involved,
Interpreting escape sequences like \n correctly since the shell will give them to you as-is, raw.
#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
// This moves overlapping strings from src -> dest but only
// if dest is before src
void cc_str_drag(char* dest, char* src) {
while (*dest) {
*dest = *src;
++dest;
++src;
}
}
// This interprets the \n sequence and can be extended to handle others, like
// \t, \\, or even \g.
void cc_interpret(char* str) {
for (;*str; ++str) {
// If this is a sequence start...
if (*str == '\\') {
// ...find out which one...
switch (str[1]) {
case 'n':
// Shift back...
cc_str_drag(str, &str[1]);
// ...and replace it.
*str = '\n';
break;
}
}
}
}
int main(int argc, char *argv[]) {
if (argc != 3) {
printf("Invalid Number of arguments\n");
// Remember argv[0] is the name of the program
printf("USAGE: %s file_name \"msg\"\n", argv[0]);
return -1;
}
// Since it's not the 1970s, use fopen() and FILE*
FILE* output = fopen(argv[1], "w");
if (!output) {
printf("ERROR: File could not be created\n");
return -2;
}
// Copying here to avoid tampering with argv
char* str = strdup(argv[2]);
// Replace any escape sequences
cc_interpret(str);
// Then just dump it directly into the file
fwrite(str, 1, strlen(str), output);
fclose(output);
return 0;
}
Note the tools used here:
strdup is a way quicker method of copying a C string than malloc(strlen(s)) and then copying it. That's asking for dreaded off-by-one errors.
FILE* performs much better because it's buffered. open() is used for low-level operations that can't be buffered. Know when to use which tool.
Don't be afraid to write functions that manipulate string contents. C strings are really important to understand, not fear.