Printffamily

Printffamily refers to a group of functions in the C standard library that output formatted text to streams or strings. The core members are printf, fprintf, sprintf, and snprintf, along with their variadic counterparts vprintf, vfprintf, vsprintf, and vsnprintf. In many environments, GNU extensions provide asprintf, which allocates a buffer large enough to hold the resulting string.

These functions rely on a format string that specifies how subsequent arguments are converted to text. Common format specifiers include %d, %s, %f, %x, %p, and %c, with optional flags, width, precision, and length modifiers to control alignment, padding, and the size of values. The v- versions take a va_list instead of a variable number of arguments, enabling programmatic formatting.

Output targets differ among the family. printf writes to standard output, fprintf writes to a specified FILE, and sprintf and vsprintf write to a user-provided character buffer. snprintf and vsnprintf perform length-limited writes to prevent buffer overflow by limiting the number of characters written, including the terminating null byte.

Safety and usage considerations are important. Because these functions rely on format strings and variadic arguments, mismatches between format specifiers and argument types can cause undefined behavior. Avoid constructing format strings from untrusted input, prefer snprintf to avoid overflows, and ensure adequate buffer size. In addition, some printf-style functions can introduce format-string vulnerabilities if used unsafely.

The printf family is a central feature of C's standard library and has influenced similar facilities in other languages, though not all languages categorize their formatting tools as part of a single “printf family.” See also: printf, fprintf, sprintf, snprintf.