Question

The user should input some file names in the command line and the program will read each file name from argv[] array. I have to perform error checking etc.

I want to read each filename. For example, if argv[2] is ‘myfile.txt’, the program should read the content of ‘myfile.txt’ and store value in char buffer[BUFSIZ] and then write the content of buffer into another file.

However before the content is written, the program should also write the name of the file and the size. Such that the file can be easily extracted later. A bit like the tar function.

The file I write the content of buffer, depending on the number of files added by user, should be a string like:

myfile.txt256Thisisfilecontentmyfile2.txt156Thisisfile2content..............

My question is

1) How do I write value of argv[2] into file using write() statement, as having problems writing char array, what should I put as (sizeof(?)) inside write(). see below as I don’t know the length of the file name entered by the user.

2) Do I use the ‘&’ to write an integer value into file after name, for example write 4 bytes after file name for the size of file

Here is the code I have written,

char buffer[BUFSIZ];
int numfiles=5; //say this is no of files user entered at command
open(file.....

lseek(fdout, 0, SEEK_SET); //start begging of file and move along each file some for loop

for(i=0-; ......
//for each file write filename,filesize,data....filename,filesize,data......
int bytesread=read(argv[i],buffer,sizeof(buffer));
write(outputfile, argv[i], sizeof(argv)); //write filename size of enough to store value of filename
write(outputfile, &bytesread, sizeof(bytesread));
write(outputfile, buffer, sizeof(buffer));

But the code is not working as I expected.

Any suggestions?

Answer 1

Since argv consists of null-terminated arrays, the length you can write is strlen(argv[2])+1 to write both the argument and null terminator:

size_t sz = strlen (argv[2]);
write (fd, argv[2], sz + 1);

Alternatively, if you want the length followed by the characters, you can write the size_t itself returned from strlen followed by that many characters.

size_t sz = strlen (argv[2]);
write (fd, &sz, sizeof (size_t));
write (fd, argv[2], sz);

You probably also need to write the length of the file as well so that you can locate the next file when reading it back.