This document lists and describes 143 C functions. It groups the functions into categories such as string manipulation, preprocessor directives, mathematical functions, file handling, and more. Some key functions include printf() for formatted output, scanf() for formatted input, strcpy() to copy strings, atoi() to convert strings to integers, and exit() to terminate a program.

1. All C Functions
1. #, ## manipulate strings
2. #define define variables
3. #error display an error message
4.
#if, #ifdef, #ifndef, #else, #elif,
#endif
conditional operators
5. #include insert the contents of another file
6. #line set line and file information
7. #pragma implementation specific command
8. #undef used to undefine variables
9.
Predefined preprocessor
variables
miscellaneous preprocessor variables
10. abort stops the program
11. abs absolute value
12. acos arc cosine
13. asctime a textual version of the time
14. asin arc sine
15. assert stops the program if an expression isn't true
16. atan arc tangent
17. atan2 arc tangent, using signs to determine quadrants
18. atexit sets a function to be called when the program exits
19. atof converts a string to a double
20. atoi converts a string to an integer
21. atol converts a string to a long
22. bsearch perform a binary search
23. calloc allocates and clears a two-dimensional chunk of memory
24. ceil the smallest integer not less than a certain value
25. clearerr clears errors
26. clock returns the amount of time that the program has been running
27. cos cosine
28. cosh hyperbolic cosine

2. 29. ctime returns a specifically formatted version of the time
30. difftime the difference between two times
31. div returns the quotient and remainder of a division
32. exit stop the program
33. exp returns "e" raised to a given power
34. fabs absolute value for floating-point numbers
35. fclose close a file
36. feof true if at the end-of-file
37. ferror checks for a file error
38. fflush writes the contents of the output buffer
39. fgetc get a character from a stream
40. fgetpos get the file position indicator
41. fgets get a string of characters from a stream
42. floor returns the largest integer not greater than a given value
43. fmod returns the remainder of a division
44. fopen open a file
45. fprintf print formatted output to a file
46. fputc write a character to a file
47. fputs write a string to a file
48. fread read from a file
49. free returns previously allocated memory to the operating system
50. freopen open an existing stream with a different name
51. frexp decomposes a number into scientific notation
52. fscanf read formatted input from a file
53. fseek move to a specific location in a file
54. fsetpos move to a specific location in a file
55. ftell returns the current file position indicator
56. fwrite write to a file
57. getc read a character from a file
58. getchar read a character from STDIN
59. getenv get enviornment information about a variable
60. gets read a string from STDIN

3. 61. gmtime returns a pointer to the current Greenwich Mean Time
62. isalnum true if a character is alphanumeric
63. isalpha true if a character is alphabetic
64. iscntrl true if a character is a control character
65. isdigit true if a character is a digit
66. isgraph true if a character is a graphical character
67. islower true if a character is lowercase
68. isprint true if a character is a printing character
69. ispunct true if a character is punctuation
70. isspace true if a character is a space character
71. isupper true if a character is an uppercase character
72. isxdigit true if a character is a hexidecimal character
73. labs absolute value for long integers
74. ldexp computes a number in scientific notation
75. ldiv
returns the quotient and remainder of a division, in long
integer form
76. localtime returns a pointer to the current time
77. log natural logarithm
78. log10 natural logarithm, in base 10
79. longjmp start execution at a certain point in the program
80. malloc allocates memory
81. memchr searches an array for the first occurance of a character
82. memcmp compares two buffers
83. memcpy copies one buffer to another
84. memmove moves one buffer to another
85. memset fills a buffer with a character
86. mktime returns the calendar version of a given time
87. modf decomposes a number into integer and fractional parts
88. perror displays a string version of the current error to STDERR
89. pow returns a given number raised to another number
90. printf write formatted output to STDOUT
91. putc write a character to a stream

4. 92. putchar write a character to STDOUT
93. puts write a string to STDOUT
94. qsort perform a quicksort
95. raise send a signal to the program
96. rand returns a pseudorandom number
97. realloc changes the size of previously allocated memory
98. remove erase a file
99. rename rename a file
100. rewind move the file position indicator to the beginning of a file
101. scanf read formatted input from STDIN
102. setbuf set the buffer for a specific stream
103. setjmp set execution to start at a certain point
104. setlocale sets the current locale
105. setvbuf set the buffer and size for a specific stream
106. signal register a function as a signal handler
107. sin sine
108. sinh hyperbolic sine
109. sprintf write formatted output to a buffer
110. sqrt square root
111. srand initialize the random number generator
112. sscanf read formatted input from a buffer
113. strcat concatenates two strings
114. strchr finds the first occurance of a character in a string
115. strcmp compares two strings
116. strcoll compares two strings in accordance to the current locale
117. strcpy copies one string to another
118. strcspn searches one string for any characters in another
119. strerror returns a text version of a given error code
120. strftime returns individual elements of the date and time
121. strlen returns the length of a given string
122. strncat concatenates a certain amount of characters of two strings
123. strncmp compares a certain amount of characters of two strings

5. 124. strncpy
copies a certain amount of characters from one string to
another
125. strpbrk
finds the first location of any character in one string, in
another string
126. strrchr finds the last occurance of a character in a string
127. strspn returns the length of a substring of characters of a string
128. strstr finds the first occurance of a substring of characters
129. strtod converts a string to a double
130. strtok finds the next token in a string
131. strtol converts a string to a long
132. strtoul converts a string to an unsigned long
133. strxfrm
converts a substring so that it can be used by string
comparison functions
134. system perform a system call
135. tan tangent
136. tanh hyperbolic tangent
137. time returns the current calendar time of the system
138. tmpfile return a pointer to a temporary file
139. tmpnam return a unique filename
140. tolower converts a character to lowercase
141. toupper converts a character to uppercase
142. ungetc puts a character back into a stream
143. va_arg use variable length parameter lists
144. vprintf, vfprintf, and vsprintf write formatted output with variable argument lists