如果参数不是文件，则从标准输入中读取

Question

I have a program that takes command line arguments, checks if they are files, if they are, then do something, if not, do something else.

The basic structure I have looks like this

for (i=1;i<argc-1;i++){
    if ((fp=open (argv[i],"r"))==NULL){
        #Do blah blah
    }
    else{
        #do blah blah
    }
}

But for some reason, when my argument isn't a file, instead of doing blah blah, it returns an error saying that it isn't a directory or file. Is there something wrong with my if statements?

Answer 1

The open() system call will return a negative value if there was a problem, so fp technically won't be NULL .

Also, as stated in @Kninnug's comment, open expects an int as its second param, so you would want O_RDONLY instead of "r"

Try chaning it to

if((fp=open(argv[i], O_RDONLY))==-1)

Or, go with @Kninnug's suggestion and use fopen() . If you do, you will need a FILE * pointer instead of an int .

Answer 2

Your file name would be in argv[1]. So instead of for (i=1;i<argc-1;i++){ you should have for (i=1;i<argc;i++){ `

Answer 3

Change

if ((fp=open (argv[i],"r"))==NULL)

To

if((fp=open(argv[i],"r"))==-1)

Answer 4

Checking if argument is a path to a file and attempting to open a file are 2 different things. There are lots of reasons why fopen() fails, but the argument is a legitimate file name.

To see if an argument is a file, use stat() .

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h> 

int isfile(const char *path) {
  struct stat buf;
  int retval = stat(path, &buf);

  // A pedantic approach would also check the `errno`.
  if (retval) return 0;

  switch (buf.st_mode & S_IFMT) {
    case S_IFREG: return 1;
    // Maybe other cases could return 1 depending on OP's needs.
  }
  return 0;
}