This demistifies the internals and removes outdated information and needless glibc guts Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@xxxxxxxxxxxxxxxxxx> --- man3/alloca.3 | 66 ++++++++++++++++----------------------------------- 1 file changed, 21 insertions(+), 45 deletions(-) diff --git a/man3/alloca.3 b/man3/alloca.3 index 5bceeabe1..133ca6ab3 100644 --- a/man3/alloca.3 +++ b/man3/alloca.3 @@ -84,20 +84,14 @@ T} Thread safety MT-Safe .SH CONFORMING TO This function is not in POSIX.1. .PP -There is evidence that the .BR alloca () -function appeared in 32V, PWB, PWB.2, 3BSD, and 4BSD. -There is a man page for it in 4.3BSD. -Linux uses the GNU version. +originates from PWB and 32V, and appears in all their derivatives. .SH NOTES The .BR alloca () function is machine- and compiler-dependent. -For certain applications, -its use can improve efficiency compared to the use of -.BR malloc (3) -plus -.BR free (3). +Because it allocates from the stack, it's always faster than +.BR malloc (3)/ free (3). In certain cases, it can also simplify memory deallocation in applications that use .BR longjmp (3) @@ -125,51 +119,33 @@ Do not attempt to .BR free (3) space allocated by .BR alloca ()! -.SS Notes on the GNU version -Normally, -.BR gcc (1) -translates calls to +.PP +By necessity, +.BR alloca () +is a compiler built-in, also known as +.BR __builtin_alloca (). +By default, modern compilers automatically translate plain .BR alloca () -with inlined code. -This is not done when either the +calls, but this is forbidden if .IR "\-ansi" , .IR "\-std=c89" , .IR "\-std=c99" , -or the +or .IR "\-std=c11" -option is given -.BR and -the header -.I <alloca.h> -is not included. -Otherwise, (without an \-ansi or \-std=c* option) the glibc version of -.I <stdlib.h> -includes +are specified, in which case .I <alloca.h> -and that contains the lines: +is required, lest an actual symbol dependency is emitted. .PP -.in +4n -.EX -#ifdef __GNUC__ -#define alloca(size) __builtin_alloca (size) -#endif -.EE -.in -.PP -with messy consequences if one has a private version of this function. -.PP -The fact that the code is inlined means that it is impossible -to take the address of this function, or to change its behavior -by linking with a different library. -.PP -The inlined code often consists of a single instruction adjusting -the stack pointer, and does not check for stack overflow. -Thus, there is no NULL error return. +The fact that +.BR alloca () +is a built-in means it is impossible to take its address +or to change its behavior by linking with a different library. .SH BUGS -There is no error indication if the stack frame cannot be extended. -(However, after a failed allocation, the program is likely to receive a +As it's untestable, there is no error indication if the allocation +would overflow the space available for the stack. +(However, the program is likely to receive a .B SIGSEGV -signal if it attempts to access the unallocated space.) +signal if it attempts to access that space.) .PP On many systems .BR alloca () -- 2.20.1
Attachment:
signature.asc
Description: PGP signature
