implicit declaration of function ‘write’; did you mean ‘fwrite’? - c

Question

I compiled an example of a simple note-taking program that uses file descriptors from a book, and I've been getting some compiler errors related to a "write" and a "close" function, along with the use of the "strncat" function. Here is the code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <sys/stat.h>
void usage(char *prog_name, char *filename) {
printf("Usage: %s <data to add to %s>\n", prog_name, filename);
exit(0);
}
void fatal(char *); // A function for fatar errors
void *ec_malloc(unsigned int); // An error-checked malloc() wrapper
int main(int argc, char *argv[]) {
int fd; // file descriptor
char *buffer, *datafile;
buffer = (char *) ec_malloc(100);
datafile = (char *) ec_malloc(20);
strcpy(datafile, "/tmp/notes");
if (argc < 2) // If there aren't command-line arguments,
usage(argv[0], datafile); // display usage message and exit.
strcpy(buffer, argv[1]); // Copy into buffer.
printf("[DEBUG] buffer # %p: \'%s\'\n", buffer, buffer);
printf("[DEBUG] datafile # %p: \'%s\'\n", datafile, datafile);
strncat(buffer, "\n", 1); // Add a newline on the end.
// Opening file
fd = open(datafile, O_WRONLY|O_CREAT|O_APPEND, S_IRUSR|S_IWUSR);
if(fd == -1)
fatal("in main() while opening file");
printf("[DEBUG] file descriptor is %d\n", fd);
//Writing data
if(write(fd, buffer, strlen(buffer)) == -1)
fatal ("in main() while writing buffer to file");
//Closing file
if(close(fd) == -1)
fatal ("in main() while closing file");
printf("Note has been saved.\n");
free(buffer);
free(datafile);
}
// A function to display and error message and then exit
void fatal(char *message) {
char error_message[100];
strcpy(error_message, "[!!] Fatal Error ");
strncat(error_message, message, 83);
perror(error_message);
exit(-1);
}
// An error-checked malloc() wrapper function
void *ec_malloc(unsigned int size) {
void *ptr;
ptr = malloc(size);
if(ptr == NULL) {
fatal("in ec_malloc() on memory allocation");
exit(-1);
}
return ptr;
}
And this is the compiler error:
implenote.c: In function ‘main’:
simplenote.c:39:5: warning: implicit declaration of function ‘write’; did you mean ‘fwrite’? [-Wimplicit-function-declaration]
39 | if(write(fd, buffer, strlen(buffer)) == -1)
| ^~~~~
| fwrite
simplenote.c:42:5: warning: implicit declaration of function ‘close’; did you mean ‘pclose’? [-Wimplicit-function-declaration]
42 | if(close(fd) == -1)
| ^~~~~
| pclose
simplenote.c:31:2: warning: ‘strncat’ specified bound 1 equals source length [-Wstringop-overflow=]
31 | strncat(buffer, "\n", 1); // Add a newline on the end.
The code was copied from the book without any alteration, i would like to know why this is happening and what i can do to make the code run. Note that the book (Hacking: The Art of Exploitation, Second Edition) is a bit dated and was released in 2008.

Answer 1

To access the Posix low level file interface such as open, read, write and close, you should include <unistd.h>.
Note however that your program does not seem to require such low level interface, you might consider creating your output file using standard streams declared in <stdio.h>, using fopen, fputs or fprintf and fclose.