scanf (scanf, fscanf, sscanf) - glibc

Split from the chapter glibc: stdio and Formatted I/O into the per-function layout; see the method page and the glibc ledger for coverage status.

Signature – int scanf(const char *fmt, ...); int fscanf(FILE *stream, const char *fmt, ...); int sscanf(const char *s, const char *fmt, ...); plus the ISO C99 vscanf/vfscanf/vsscanf va_list forms.

Conversions (manual, Table of Input Conversions): %d signed decimal; %i signed with C-constant prefix detection (0x, 0); %b binary (ISO C23); %o, %u, %x unsigned radix; %e %f %g float; %s non-whitespace string; %[ scanset string; %c raw characters (width-bounded); %p pointer in printf("%p") format; %n stores characters consumed so far, converts nothing; %% a literal percent. Whitespace in the template matches any amount; ordinary characters must match exactly.

Return semantics – number of successful assignments; a matching failure stops the scan and returns the count so far; EOF only when an input failure (EOF or read error) happens before any conversion, including whitespace and literal matching (scanf(3), manual, Formatted Input Basics). Distinguish all three states at the call site.

Security – %s, %[, %c without an explicit maximum field width is a buffer overflow; always bound (%63s). Arguments must be pointers of matching type; wrong types or missing arguments are undefined. %n-bearing untrusted formats are dangerous as with printf.

References – man3/scanf.3, man3/fscanf.3, man3/sscanf.3.