Impara Buffer Overflows and Memory Safety | Writing Safe and Predictable Code

Buffer overflows happen when you write more data to a buffer, such as an array, than it can hold. In C, arrays do not automatically check their bounds, so it is easy to accidentally overwrite memory beyond the end of an array. This can occur when using unsafe functions like strcpy or by writing to an array index that is outside its defined range.

main.c


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#include <stdio.h>
#include <string.h>

// Function that causes a buffer overflow
void unsafe_copy(const char *input) {
    char buffer[8];
    // Unsafe: does not check input length
    strcpy(buffer, input);
    printf("Buffer contents: %s\n", buffer);
}

// Corrected version that checks bounds
void safe_copy(const char *input) {
    char buffer[8];
    // Safe: copies only up to buffer size - 1 and null-terminates
    strncpy(buffer, input, sizeof(buffer) - 1);
    buffer[sizeof(buffer) - 1] = '\0';
    printf("Buffer contents: %s\n", buffer);
}

int main() {
    printf("Unsafe copy (may overflow):\n");
    unsafe_copy("This input is too long for the buffer!");
    printf("\nSafe copy (prevents overflow):\n");
    safe_copy("This input is too long for the buffer!");
    return 0;
}

The consequences of a buffer overflow can be serious. It may cause program crashes, unpredictable behavior, or security vulnerabilities. Attackers can exploit them to run malicious code or access sensitive data. These are some of the most dangerous bugs in C, as they are often hard to detect and fix.

To prevent them, always check buffer sizes and avoid writing past their limits. Use safer functions like strncpy instead of strcpy, and make sure to null-terminate strings. Validate input lengths and avoid hard-coded buffer sizes to keep your programs safe and reliable.

Study More

To deepen your understanding of secure coding in C, consult resources such as the CERT C Coding Standard for secure coding.

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