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The getline() function is similar to the fgets() function but can dynamically allocate memory for the input buffer. If passed a null pointer, getline() dynamically allocates a buffer of sufficient size to hold the input. If passed a pointer to dynamically allocated storage that is too small to hold the contents of the string, the getline() function resizes the buffer, using realloc(), rather than truncating the input. If successful, the getline() function returns the number of characters read, which can be used to determine if the input has any null characters before the newline. The getline() function works only with dynamically allocated buffers. Allocated memory must be explicitly deallocated by the caller to avoid memory leaks (see MEM31-C. Free dynamically allocated memory when no longer needed).

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
void func(void) {
  int ch;
  size_t buffer_size = 32;
  char *buffer = malloc(buffer_size);
 
  if (!buffer) {
    /* Handle error */
    return;
  }

  if ((ssize_t size = getline(&buffer, &buffer_size, stdin))
        == -1) {
    /* Handle error */
  } else {
    char *p = strchr(buffer, '\n');
    if (p) {
      *p = '\0';
    } else {
      /* Newline not found; flush stdin to end of line */
      while ((ch = getchar()) != '\n' && ch != EOF)
	    ;
	  if (ch == EOF && !feof(stdin) && !ferror(stdin)) {
         /* Character resembles EOF; handle error */
      }
    }
  }
  free (buffer);
}

Note that the getline() function uses an in-band error indicator, in violation of ERR02-C. Avoid in-band error indicators.

Noncompliant Code Example (getchar())

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Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

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