Can a character in C be both a newline character and a NULL terminator?

• What is the purpose of if (cmdline[-1] == '\n' && cmdline[-1] == '\0')
  
  the following code example?
  • when will it ever be judged true?
• How does the negative index work here cmdline[-1]
  
  ? I've seen similar questions on SO, but there is no pointer arithmetic in advance here.

Here's the pseudocode:

parseInfo *parse (char *cmdline) {
  parseInfo *Result;
  char command[MAXLINE];
  int com_pos = -1;

  if (cmdline[-1] == '\n' && cmdline[-1] == '\0')
    return NULL;

  Result = malloc(sizeof(parseInfo));
  init_info(Result);
  com_pos=0;
  /*  while (cmdline[i] != '\n' && cmdline[i] != '\0') { */

  command[com_pos]='\0';
  parse_command(command, 0); /* &Result->CommArray[Result->pipeNum]);*/

  return Result;
}

      

        
        
        
      

    

EDIT # 1: Why "When is this NOT printed?" printed in the following situation: EDIT # 2: I made a cmdline

valid string by increasing the length by 1 (including the null character) and copying the length

bytes into length

malloc'd.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void parse(char *cmdline) {
  printf("'%c'\n", cmdline[-1]);
  if (cmdline[-1] == '\0' || cmdline[-1] == '\n') {
    printf("When does this NOT get printed?\n");
  }
}

int main() {
  char str[100] = "hello";
  const int length = strlen(str) + 1;
  char *cmdline = malloc(length * sizeof(char));
  strncpy(cmdline, str, length);
  parse(cmdline);
  return 0;
}

      

        
        
        
      

    

With the appropriate output:

$ gcc -v
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 6.1.0 (clang-602.0.49) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.3.0
Thread model: posix
$ gcc negative_index.c
$ ./a.out
''
When does this NOT get printed?