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#include <stdio.h>
#include <string.h>
 
void func(void) {
  char buf[BUFSIZ + 1];

  if (fgets(buf, sizeof(buf), stdin)) {
    if (*buf) { /* see FIO37-C */
      buf[strlen(buf) - 1] = '\0';
    }
  }
 else {
    /* Handle error condition */
  }
}

However, if the last character in buf is not a newline, this code overwrites an otherwise valid character.

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#include <stdio.h>
#include <string.h>
 
void func(void) {
  char buf[BUFSIZ + 1];
  char *p;

  if (fgets(buf, sizeof(buf), stdin)) {
    p = strchr(buf, '\n');
    if (p) {
      *p = '\0';
    }
  }
 else {
    /* Handle error condition */
  }
}

An obvious alternative is to leave room in the buffer for one more character, and when no newline is transferred, append a newline followed by a null-termination character. This approach is unsafe because it quietly accepts an input that is not what was actually intended, with unknown consequences.

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

Related Guidelines

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