What is a segmentation fault in C?

A segmentation fault (segfault) happens when a program reads or writes memory that it should not touch. The operating system stops the program.

Common causes:

• Dereferencing a NULL or uninitialized pointer.
• Accessing an array outside its limits.
• Using memory after free().
• Very deep recursion (stack overflow).
• Writing to a string literal, like char *s = "abc"; s[0] = 'x';.

To find it, compile with -g and run with a debugger like GDB, or use -fsanitize=address to see the exact line.

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