Member Avatar for yuzhang

Codes:

#define MAX_BUFFER_SIZE 1000
int ch = 0, alpha [25] = {0}, buffer[MAX_BUFFER_SIZE] = {0} ,count = 0;

....

while ((ch = (fgetc(stream)))!=EOF)
    {
        buffer [count] = ch;
        count++;
    }

printf("%d", alpha [25]);

.....

The problem is: alpha [25] contains buffer [0], (alpha[25] is supposed to be 0). which suggests that the memory space allocated for alpha[25] is also the starting memory space of buffer[0]!!! How could this be? I am ultimately confused. Is there anything wrong with my computer?

Any help is deeply thanked.

Edited by TrustyTony because: fixed formatting
  • 3 Contributors
  • 2 Replies
  • 105 Views
  • 6 Days Discussion Span
  • Latest Post Latest Post by ivanCeras

Dani AI

Generated

This is a textbook case of undefined behavior from an out-of-bounds access. correctly identified the immediate mistake: asking for the element one past the last valid index is invalid. When code reads or writes outside an array, the C standard makes no guarantees — in practice local arrays are often laid out next to each other on the stack, so overrunning one array can appear to corrupt the next (which is why alpha’s “missing” value looks like buffer[0]).

To diagnose and catch this reliably, enable compiler warnings and runtime sanitizers and re-run the program. For example, compile with AddressSanitizer and debug symbols:

gcc -std=c11 -Wall -Wextra -Wpedantic -g -O0 -fsanitize=address source.c -o prog
./prog

Sanitizers will flag invalid reads/writes and show the offending line. Valgrind can also help on platforms it supports.

When using values read from the file as indexes (as hinted), always validate them against the actual array length. A compact, portable pattern is:

#define COUNT_OF(x) (sizeof(x) / sizeof((x)[0]))

size_t len = COUNT_OF(alpha);
int idx = /* parsed/obtained value */;
if (idx >= 0 && idx < (int)len) {
    /* safe to use alpha[idx] */
} else {
    /* handle out-of-range value */
}

Also review the loop that fills the buffer: ensure you check the buffer capacity before writing and stop or reallocate when full. Do not rely on observed memory layouts to justify out-of-bounds reads; fix the index arithmetic or bounds checks instead. Running with sanitizers, turning on warnings, and adding explicit bounds checks will both explain the current behavior and prevent this class of bugs going forward.

Recommended Answers

All 2 Replies

Member Avatar for Dave Sinkula

alpha [25] is off the end of the array. Arrays are indexed from 0 to N-1, which means an array declared as

int alpha[25];

may be indexed from 0 to 24.

Member Avatar for ivanCeras

im confused with your explaination of your problem.....
maybe you mean ... the index of alpha will be the value of buffer[0]..
then use..

x=buffer[0];
printf("%d",apha[x]);
Be a part of the DaniWeb community

We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts meeting, networking, learning, and sharing knowledge.