sockets/read_line.c

This is sockets/read_line.c (Listing 59-1, page 1201), an example from the book, The Linux Programming Interface.

The source code file is copyright 2024, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

This page shows the "distribution" or "book" version of the file (why are there two versions?), or the differences between the two versions. You can switch between the views using the tabs below.

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

  Cover of The Linux Programming Interface
+/* read_line.c
+
+   Implementation of readLine().
+*/
 #include <unistd.h>
 #include <errno.h>
 #include "read_line.h"                  /* Declaration of readLine() */
 
+/* Read characters from 'fd' until a newline is encountered. If a newline
+  character is not encountered in the first (n - 1) bytes, then the excess
+  characters are discarded. The returned string placed in 'buf' is
+  null-terminated and includes the newline character if it was read in the
+  first (n - 1) bytes. The function return value is the number of bytes
+  placed in buffer (which includes the newline character if encountered,
+  but excludes the terminating null byte). */
+
 ssize_t
 readLine(int fd, void *buffer, size_t n)
 {
     ssize_t numRead;                    /* # of bytes fetched by last read() */
     size_t totRead;                     /* Total bytes read so far */
     char *buf;
     char ch;
 
     if (n <= 0 || buffer == NULL) {
         errno = EINVAL;
         return -1;
     }
 
     buf = buffer;                       /* No pointer arithmetic on "void *" */
 
     totRead = 0;
     for (;;) {
         numRead = read(fd, &ch, 1);
 
         if (numRead == -1) {
             if (errno == EINTR)         /* Interrupted --> restart read() */
                 continue;
             else
                 return -1;              /* Some other error */
 
         } else if (numRead == 0) {      /* EOF */
             if (totRead == 0)           /* No bytes read; return 0 */
                 return 0;
             else                        /* Some bytes read; add '\0' */
                 break;
 
         } else {                        /* 'numRead' must be 1 if we get here */
             if (totRead < n - 1) {      /* Discard > (n - 1) bytes */
                 totRead++;
                 *buf++ = ch;
             }
 
             if (ch == '\n')
                 break;
         }
     }
 
     *buf = '\0';
     return totRead;
 }

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