Write a C program, analogous to the Unix/Linux cat command, that uses mmap() and

Question

Write a C program, analogous to the Unix/Linux cat command, that uses mmap() and write() system calls (instead of read() or write() ) to copy a source file to the screen. Use fstat() to obtain the size of the input file which can be used to size the required memory mapping.

The statement to write the file contents to STDOUT is

         write (STDOUT_FILENO, addr, sb.st_size);

where

addr is the start of the mapping, gotten from mmap() and,

                  sb.st_size is the size of the mapping returned by fstat() in the

                 sb struct.

Please show the command line and the screenshots

Explanation / Answer

#include #include #include #include #include #include #define handle_error(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0) int main(int argc, char *argv[]) { char *addr; int fd; struct stat sb; off_t offset, pa_offset; size_t length; ssize_t s; if (argc < 3 || argc > 4) { fprintf(stderr, "%s file offset [length] ", argv[0]); exit(EXIT_FAILURE); } fd = open(argv[1], O_RDONLY); if (fd == 1) handle_error("open"); if (fstat(fd, &sb) == 1) /* To obtain file size */ handle_error("fstat"); offset = atoi(argv[2]); pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) 1); /* offset for mmap() must be page aligned */ if (offset >= sb.st_size) { fprintf(stderr, "offset is past end of file "); exit(EXIT_FAILURE); } if (argc == 4) { length = atoi(argv[3]); if (offset + length > sb.st_size) length = sb.st_size offset; /* Can't display bytes past end of file */ } else { /* No length arg ==> display to end of file */ length = sb.st_size offset; } addr = mmap(NULL, length + offset pa_offset, PROT_READ, MAP_PRIVATE, fd, pa_offset); if (addr == MAP_FAILED) handle_error("mmap"); s = write(STDOUT_FILENO, addr + offset pa_offset, length); if (s != length) { if (s == 1) handle_error("write"); fprintf(stderr, "partial write"); exit(EXIT_FAILURE); } munmap(addr, length + offset pa_offset); close(fd); exit(EXIT_SUCCESS); }

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